1. Igneous rocks and processes

1.1. Hotspots and intraplate magmas: Mantle sources, magmatic processes and metasomatism.

                       ORAL PRESENTATIONS                    

The role of sublithospheric mantle sources in continental flood basalt magmatism: lessons learned from the Karoo LIP

Jussi S. Heinonen¹, Arto V. Luttinen², Teal R. Riley³ and Sanni Turunen^2
1Department of Geosciences and Geography, University of Helsinki, ²Finnish Museum of Natural History, University of Helsinki, ³British Antarctic Survey

The role of sublithospheric mantle sources is elusive in continental flood basalt (CFB) provinces. This is because on their way to the surface of the Earth, the parental magmas of CFBs interact with thick continental lithosphere and may lose their primary geochemical characteristics. Several studies have also suggested that the predominant mantle sources of continental flood basalts are within the continental lithosphere. Our studies in the Antarctic and, more recently, African parts of the Early Jurassic Karoo LIP over the past 25 years have revealed the existence of several sublithospheric mantle-derived magma types. Their sources range from depleted upper mantle (DM) to different kinds of enriched mantle (EM), and possibly even to ancient primordial plume sources originating from the core-mantle boundary. Whereas the recognized EM sources seem to be anomalous and had minor contributions to Karoo magmatism, the DM and primordial-like sources may have been very important in the generation of voluminous Karoo magmas. Energy-constrained assimilation‒fractional crystallization modeling has revealed that only a few percent of assimilation can shift the CFB trace element and radiogenic isotope signatures close to those of lithospheric wallrock materials. We suggest that, with the possible exception of very strongly enriched magma types, the continental lithosphere has in many cases acted as a ‘pepper mill’, only spicing up CFB magmas otherwise predominantly generated in the sublithospheric mantle.

Magmatism associated with Gondwanaland rifting: Evidence from Iraq Zagros Suture Zone

Sabah Ismail¹ and Shaleer Burhan^2
1Education college for pure sciences/Kirkuk University/Iraq, ²Applied Geology Department/Kirkuk University/Iraq

The opening of the southern basin of the Neo-Tethys as a result of the breakup of Gondwana during Late Triassic was accompanied by the eruption of flood basalts. This process led to the fragmentation of continental lithosphere and rifting of these fragments from the northern margin of Gondwana and created deep trench grabens which were subsequently filled by radiolarite deposits. Such events are well documented in southern Turkey, northwestern Syria and southwestern Cyprus. Fragments related to these activities also exist as thrust nappes in the Iraqi Zagros Suture Zone(Avroman platform limestone and Qulqula radiolarite)Such nappes also occur systematically beneath the obducted ophiolites from Greece to Oman. In the Iraqi Zagros Suture Zone the Late Triassic Avroman nappe was thrusted over the Early Cretaceous Qulqula radiolarite and both were thrusted over the Late Cretaceous ophiolites. The Avroman volcanics are lava flows consisting of olivine-basalts, diabases, trachytes and tuffs. Previous K-Ar dating of a diabase rock gave an age of 220 Ma ± 20. Geochemical analysis of these rocks indicates that the alkali olivine basalts are strongly enriched in light rare earth elements without Eu anomalies. Trace element data for the most primitive mafic igneous rock samples suggest that the tholeiitic basalts are derived from an enriched mantle source with OIB signature formed within plate activity. This alkaline magmatism may have taken place as continental flood basalts erupted during rifting of Gondwana at Norian with evidences of mantle plume interaction in the early rifting phase related to the opening of the Neo-Tethys.

References
Jassim, S.Z., Goff, C., 2006. Geology of Iraq, Prague and Moravian Museum, Brno, 341p.

Analogous Late Paleoproterozoic basement in Pomerania (N. Poland) and Blekinge (S. Sweden) – the isotopic evidences

KRZEMINSKA, E.1* KRZEMINSKI, L1, WISZNIEWSKA, J. 1, JOHANSSON, Å. 2, WILLIAMS I.S.3

1 Polish Geological Institute, National Research Institute, Warsaw, Poland; (* correspondence:ewa [dot] krzeminska [at] pgi [dot] gov [dot] pl) 2 Swedish Museum of Natural History, Stockholm, Sweden; 3 Research School of Earth Sciences, The Australian National University, Canberra, Australia

Geological investigations of the sediment-covered Fennoscandian basement in Poland have been performed on crystalline rocks sampled from ten deep drillings (depths 2275 – 5111m). Only these samples provide insight into the basement beneath the southern Baltic Sea and its southern coastal area in Pomerania (N. Poland).

Whole rock geochemistry has been accompanied by U-Pb SHRIMP geochronology on zircons and whole rock Sm-Nd isotopic analyses. The upper part of the basement is dominated by slightly deformed grano- to monzodiorites. The new analyses reveal calc-alkaline, magnesian compositions, with I-type characteristics and volcanic arc affinity. Geochronology on zircons yield magmatic U-Pb ages between 1799±10 and 1746±9 Ma, with an episode of synorogenic deformation recorded in thin overgrowths on zircons. Measurements of cores and rims however yield similar ages. Whole rock initial εNd values show a narrow range from +1.0 to -0.6. Moreover T(Nd)DM model ages show variation mostly from 1.99 to 1.95 Ga and demonstrate relative homogeneity of the igneous protolith.

Such ages and isotopic characteristics correlate well with genetic features of gneissic granitoids from Blekinge in SE Sweden (Johansson et al. 2006, Johansson 2016) where the crust was formed and deformed at 1.78-1.74 Ga. Comparison of whole rock signatures suggesting a subduction environment together with U–Pb zircon ages along the southern edge of Fennoscandia, provides a basis for straightforward correlation between Pomerania and Blekinge, linking the opposite sides of the southern Baltic Sea, where generation of crust occurred between 1.80 and 1.74 Ga.

References

Johansson, Å., Bogdanova, S & Cecys, A., 2006: A revised geochronology for the Blekinge Province, southern Sweden. GFF 128, 287-302.
Johansson, Å., 2016: U-Pb SIMS dating of some granitoids from eastern Blekinge, southern Sweden. GFF 138, 430-444.

Tracing of mantle heterogeneity in Karoo flood basalts using Nb-Zr-Ti-Y ratios

Arto Luttinen¹, Sanni Turunen¹ and Jussi Heinonen^2
1Finnish Museum of Natural History, P.O. Box 44, 00014 University of Helsinki, Finland, ²Department of Geosciences and Geography, P.O. Box 64, 00014 University of Helsinki, Finla

The study of mantle sources of continental flood basalt provinces is often compromised by crustal contamination overprinting of the incompatible element and isotopic ratios of the predominant rock types. Geochemical tracers which are unaffected by contamination help to: (1) identify co-genetic types of flood basalts, (2) establish parent-daughter relationships between flood basalt types and rare uncontaminated picrites, and (3) constrain mantle source heterogeneity even when picrite types are unknown. Identification of such tracers has turned out to be difficult, however.

We have examined variations of Nb/Y, Nb/Zr, and Nb/Ti at given Zr/Y (quantified using ΔNb_Y, ΔNb_Zr and ΔNb_Ti) in geochemically diverse Karoo flood basalts. Importantly, the delta-Nb values are only mildly affected by melting, crystallisation, and contamination processes and can be used as tracers of mantle source heterogeneity.

In the Karoo province, the variations in ΔNb_Y, ΔNb_Zr, and ΔNb_Ti and initial Nd and Sr isotopic ratios call for at least three mantle source end-members: Two have Nb-depleted compositions relative to primitive mantle, but markedly different depleted and enriched isotopic compositions. The third end-member is Nb-undepleted and shows isotopic affinity to bulk silicate Earth. Three Karoo picrite suites can be associated with these end-members, whereas other picrites probably represent subordinate mantle components.

In the Karoo province, the delta-Nb tracers pave the way for: (1) provincial scale mapping of mantle heterogeneity using variably contaminated flood basalts and (2) identification of picrites best suited for geochemical characterisation of the major mantle components.

Assessing primary vs. secondary geochemical fingerprints of Icelandic basalts using high-Fo# olivine crystals

Maja B. Rasmussen¹, Sæmundur A. Halldórsson¹, Martin J. Whitehouse², Sally A. Gibson³ and David R. Hilton^4
1Nordic Volcanological Center, University of Iceland, Reykjavik, Iceland, ²Swedish Museum of Natural History, Stockholm, Sweden, ³University of Cambridge, Cambridge, UK, ⁴Scripps Institution of Oceanography, UCSD, La Jolla, USA

Oxygen isotope ratios (d¹⁸O) of Icelandic basalts are notably distinct from MORB-sourced basalts. Two prevailing hypotheses have been put forward to account for this: interaction with low-d¹⁸O crust or mantle heterogeneity [1]. High-Fo# olivine crystals are widely used as a proxy for primary melt composition prior to crustal modification [2]. Therefore, their geochemical characteristics can be useful to determine the source of low-d¹⁸O in Icelandic basalts. We couple in-situ d¹⁸O of high-Fo#  (>80) olivine crystals from the neovolcanic rift- and flank zones as well as older Tertiary units with major and trace elements using SIMS, EMPA and LA ICPMS as tools to identify primary vs. secondary controls on some notable geochemical characteristics of Icelandic basalts.

The olivine crystals show limited intra-grain variability but, collectively, they display a variation in d¹⁸O_(Ol) of >3 ‰ across Iceland, with most values falling below the expected depleted mantle-value (~5.1 ±0.2‰ [3]). The lowest d¹⁸O_(Ol) (+2.77 ‰), is measured in crystals from central Iceland, close to the inferred plume head [4]. Trace element ratios of these olivine crystals (e.g., Mn/Fe) suggest a peridotitic mantle source, implying a shallow (likely crustal) origin of the low d¹⁸O_(Ol). In contrast, crystals from the South Iceland Volcanic Zone display trace element ratios indicative of greater amount of pyroxenite in their source region while their d¹⁸O_(Ol) vary significantly (from +3.45 to +4.98 ‰). These results, together with previously published ³He/⁴He values for these same samples [4], imply a regional shift in the dominating mantle lithology and plumbing conditions beneath Iceland.

References
[1] Muehlenbachs, K., Anderson, A. T., & Sigvaldason, G. E. (1974). Low-O18 basalts from Iceland. Geochimica et Cosmochimica Acta, 38(4), 577-588.

[2] Sobolev, A. V., Hofmann, A. W., Kuzmin, D. V., Yaxley, G. M., Arndt, N. T., Chung, S. L., Danyushevsky, L. V., Elliott, T., Frey, F. A., Garcia, M. O., Gurenko, A. A., Kamenetsky, V. S., Kerr, A. C., Krivolutskaya, N. A., Matvienkov, V. V., Nikogosian, I. K., Rocholl, A., Sigurdsson, I. A., Sushchevskaya, N. M., & Teklay, M. (2007). The amount of recycled crust in sources of mantle-derived melts. Science, 316(5823), 412-417.

[3] Eiler, J., Grönvold, K., & Kitchen, N. (2000). Oxygen isotope evidence for the origin of chemical variations in lavas from Theistareykir volcano in Iceland’s northern volcanic zone. Earth and Planetary Science Letters, 184(1), 269-286.

[4] Harðardóttir, S., Halldórsson, S. A., & Hilton, D. R. (2017). Spatial distribution of helium isotopes in Icelandic geothermal fluids and volcanic materials with implications for location, upwelling and evolution of the Icelandic mantle plume. Chemical Geology, (in press)

Luenha picrites reveal a primitive mantle type plume source in the Karoo large igneous province?

Sanni Turunen¹, Arto Luttinen¹ and Jussi Heinonen^2
1Finnish Museum of Natural History, P.O. Box 44, 00014 University of Helsinki, Finland, ²Department of Geosciences and Geography, P.O. Box 64, 00014 University of Helsinki, Finland

Two distinctive picrite suites have been previously considered to represent important parental magma types in the Karoo large igneous province: (1) the Mwenezi picrites indicate an enriched mantle source (initial εNd ca. -10) of lithospheric origin for enriched high-Ti basalts, whereas (2) the Vestfjella ferropicrites reveal a depleted convective upper mantle source (εNd +8) for low-Ti basalts. Both picrite suites are characterised by negative ΔNb values (-0.9 to -0.1) indicative of relative Nb-depletion. Our geochemical data on recently discovered picritic lavas in the Luenha river area, Central Mozambique, reveal a third significant mantle source for Karoo basalts.

The Luenha picrites differ from other Karoo picrites based on their high positive ΔNb values (+0.5 to +0.6), low Ti contents, and low Zr/Y. Variable initial ⁸⁷Sr/⁸⁶Sr and incompatible trace element ratios likely result from mild crustal contamination. A single picrite sample lacks indications of contamination and indicates a mantle source with ⁸⁷Sr/⁸⁶Sr of 0.7041 and εNd of +2 and an overall geochemical affinity to a primitive or mildly depleted mantle. We maintain that this magma source may represent the previously unknown mantle component which is required to explain the high abundance of Karoo low-Ti basalts with positive ΔNb. Bearing in mind that several large igneous provinces (e.g. Ontong Java) have been recently associated with melting of geochemically primitive mantle type plume heads, the Luenha picrites may provide a unique geochemical insight into a major plume source previously inferred from geophysical studies of the Karoo province.

The important distinction between ancient HIMU and young HIMU-like sources in intraplate magmatism

Quinten van der Meer¹, Tod Waight², James Scott³ and Carsten Münker^4
1Natural History Museum, University of Copenhagen, ²Department of Geosciences and Natural Resource Management (Geology Section), Copenhagen University, ³Department of Geology, University of Otago, ⁴Institut für geologie und Mineralogie, Universität zu Köln

Continental intraplate magmas with isotopic affinities similar to HIMU are identified worldwide. Involvement of an asthenospheric HIMU or HIMU-like source is contested because the characteristic radiogenic Pb compositions at unradiogenic Sr and moderate Nd and Hf compositions can also result from in-situ ingrowth in metasomatised lithospheric mantle. Sr-Nd-Pb-Hf isotopic composition of late Cretaceous lamprophyre dikes from Westland, New Zealand provide new insight into the formation of a HIMU-like alkaline intraplate magmatic province under the Zealandia microcontinent and former contiguous regions of Gondwana. The oldest (102-100 Ma) calc-alkaline lamprophyres are compositionally similar to the preceding arc-magmatism and represent melts originating from subduction-modified lithosphere. From ~98 Ma, alkaline intraplate magmatism emerged throughout Zealandia that would remain active up to the present day. We break with the classical grouping of this magmatism into a single suite and show that distinct ancient and young enriched sources controlled melt isotopic compositions. The distribution of these sources in the Cretaceous was strongly coupled to lithosphere terrane structure and emphasizes the influence of enriched metasomes in the lithospheric mantle as a melt source.

Tracing pyroxenite in the mantle source at Heard Island in the Indian Ocean

Abigail K Barker¹, Inês G Nobre Silva², Jane Barling³ and Naomi J Saunders^3
1Mineralogy, Petrology, Tectonics, Uppsala University, ²Memorial University of Newfoundland, ³Department of Earth Sciences, University of Oxford

Heard Island hosts the largest active volcano of the Kerguelen Oceanic Plateau, where whole rock geochemistry suggests an important contribution from recycled ocean crust. We present minor element compositions for olivine from Heard Island, as a tracer of the initial magma, in order to investigate the connection between source lithology and mantle source components.

Olivine crystals in the lavas from Heard Island have 72.5 to 87.5 Fo mol% and form two groups distinguished by Ni and Mn compositions. The main group of olivine crystals contain Ni* of 540 to 870 (Ni*=Ni x FeO/MgO; n = 14 of 17 samples) and Mn* of 90 to 110 (Mn* = Mn/FeO), which are similar to the global array. The second group has high Ni* of 1060 to 1820 and high Mn* of 150 to 175(n=3 of 17 samples), elevated in Ni* and Mn*with respect to the global array. The olivine mineral chemistry is independent of whole rock trace element and isotope geochemistry that reflects mixing between the mantle source components.

Olivine crystals from Heard Island that plot along the global array are consistent with derivation from a pyroxenite rich source (60 to 90%). Furthermore, a pyroxenite rich source lithology is attributed to all geochemical end members. However, the wide range implies incomplete hybridisation or heterogeneous supply of eclogite in the mantle below Heard Island.

The olivine crystals that display elevated Ni* and Mn* potentially reflect melting of the oceanic lithospheric mantle, which has previously been metasomatised by melts generated by the mantle plume.

                       POSTER PRESENTATIONS                    

Hydrothermal alteration of the ultramafic rocks at the Kemi intrusion

Fabian Mauricio Botello¹, Holger Paulick¹, Timo Huhtelin² and Sari Grönholm^3
1Oulu Mining School, University of Oulu, Finland, ²Outokumpu Chrome Oy, Kemi Mine, ³Geological Survey of Finland-GTK

The Kemi chromite deposit is located close to the town of Tornio (northern Finland). The mineralization is hosted within the mafic-ultramafic Kemi intrusion that is part of the Tornio – Näränkäväärä belt. This belt consist of about 20 individual intrusive igneous complexes within the Fennoscandian Shield that have been emplaced at 2.4 to 2.5 Ga. The Kemi intrusion has a lenticular shape and is about 15 km long and 2 km wide in the middle section (Alapieti et al., 1989). The main chromite units are located in the basal part of the intrusion and have an average thickness of 40 m, but vary from a few meters to 160m. The chromitite units are enveloped and intercalated with variably altered peridotite and bronzites.

Hydrothermal alteration has pervasively affected the ultramafic sequence mainly in the lower and upper segments, leaving the middle partly unaltered (Alapieti et al., 1989). These lithologies can still be identified due to the preservation of their primary textures. Bronzites and peridotites surrounding the chromitite layers have been altered to serpentine, talc, tremolite, chlorite and carbonates (Gronhölm, 1994). Alteration has also affected chromite grain composition and texture, finding some grains with inclusions of altered silicate minerals. The occurrence of the mentioned alteration assemblages requires the influx of fluids rich in SiO_2aq (Paulick et al., 2006). The study of the hydrothermal alteration permits interpretation of its origin and makes it possible to establish spatial distribution of the altered assemblage as well as associations with the ore and possible exploration vectors.

References
Alapieti, T. T., Kujanpaa, J., Lahtinen, J. J., & Papunen, H., 1989. The Kemi stratiform chromitite deposit, northern Finland. Economic Geology, 84, 1057-1077.

Grönholm, S., 1994. Influence of mineral composition and microstructures on the mechanical properties of host rocks of the Kemi (Elijärvi) chromite deposit, Finland. Geological Survey of Finland-GTK, Report 126, 36 pp.

Paulic, H., Bach, W., Godard, M., De Hoog, J. C. M., Suhr, G., & Harvey, J. (2006). Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15 20’ N, ODP Leg 2019): implications for fluid/rock interaction in slow spreading environments. Chemical Geology, 234, 179-210.

Birthing an ocean island rift: mantle fingers or lateral flow?

Frances Deegan¹, Valentin Troll¹, Harri Geiger¹, Lina Barke¹, Juan Carlos Carracedo² and Francisco José Perez-Torrado^2
1Uppsala University, Uppsala, Sweden, ²Universidad de Las Palmas de Gran Canaria, Gran Canaria, Spain

Rift zones play a key role in modulating the growth and destruction of ocean islands. However, it is not always clear whether magma feeds into rifts from the mantle or from beneath the island’s central complex before being diverted into rift zones. Seismic monitoring of the 2011- 2012 eruption on El Hierro showed that it was fed by magma that underwent lateral transport at the base of the ocean crust before final ascent. If this is generally true for Canary Island rifts, then there may be large implications for rift zone magma evolution and the preservation (or crustal modification) of mantle isotope signatures. To test this concept, we carried out mineral-melt thermobarometric modelling on dykes of the Miocene-Pliocene Northeast Rift Zone (NERZ) on Tenerife, which is a superbly exposed example of a feeder system to a major ocean island volcanic rift. Our initial results point to sub-MOHO level crystallization of feldspar and pyroxene, in stark contrast to the latest rift eruption on El Hierro. Our preliminary data thus support a model of rift growth on Tenerife through injection of mantle-derived hot fingers. This conclusion is in line with anisotropy of magnetic susceptibility (AMS) data, which indicate non-unidirectional emplacement of magma along the rift. Moreover, the mantle finger model is consistent with plume-like (HIMU) radiogenic isotope ratios preserved in many of the NERZ dykes. Ocean island rift formation thus appears to differ between islands in the Canary archipelago, which must be taken into consideration when assessing mantle versus crustal geochemical indices.

Geochemical characteristics of an enriched Icelandic tholeiitic magma suite: the case of the Kverkfjöll volcanic system

Eemu Ranta¹, Sæmundur A. Halldórsson¹, Gu∂mundur H. Gu∂finsson¹, Enikö Bali¹, Vesa Nykänen², Karl Grönvold¹ and Riina Kaikkonen^3
1Nordic Volcanological Center, University of Iceland, Reykjavik, ICELAND, ²Geological Survey of Finland, Rovaniemi, FINLAND, ³Oulu Mining School, University of Oulu, FINLAND

The active Kverkfjöll central volcano and its associated fissure swarm are situated at the northern margin of the Vatnajökull glacier, Iceland. The volcanic system is characterized by subglacial pillow basalt ridges and post-glacial lava flows, erupted on c. 35 km thick crust at the flank of the Northern Rift Zone (NRZ). Published petrochemical data from Kverkfjöll are very limited, in contrast to its neighboring volcanoes Bárdarbunga, Grímsvötn and Askja. We present preliminary geochemical and Hf-Pb isotope data largely acquired from subglacial pillow rim glasses, focusing further on their dissolved volatile contents (EPMA, FTIR) as well as trace elements (LA-ICP-MS).

The Kverkfjöll basalts are noticeably enriched in FeO (12-16 wt.%), with relatively uniform compositions (MgO = 4-8 wt.%) compared to other tholeiitic magma suites in Iceland. They are anomalously enriched in incompatible elements, closely resembling transitional and alkali basalts from flank zone areas in Iceland. Such enrichments are also evident in the dissolved volatile contents of the Kverkfjöll glasses, which are some of the highest in Icelandic basalts (e.g. Cl = 350 ppm, H2O = 0.9 wt.%). Moreover, the Pb-Pb and Pb-Hf isotopic signatures of Kverkfjöll falls in between the well-defined trend of the NRZ basalts and the EM1-like compositions of the flank volcano Öræfajökull, unique among Icelandic basalts [1].

The aim of this in-progress study is to address if the Kverkfjöll basalts sample a distinct, previously unrecognized component of the Iceland mantle, or if their volatile enrichment and atypical isotopic signatures can be explained by other means.

References

[1] Manning, C.J. & Thirlwall, M.F. 2014: Isotopic evidence for interaction between Öræfajökull mantle and the Eastern Rift Zone, Iceland. Contributions to Mineralogy and Petrology, 167(1), 959.

Alkaline OIBs derived by pyroxenite-melting of low-T mantle

Nina Søager¹, Maxim Portnyagin², Paul Martin Holm¹ and Dieter Garbe-Schönberg^3
1Department of Geosciences and Natural Resource Management, University of Copenhagen, ²Geomar, Helmholtz Centre for Ocean Research, Kiel, Germany, ³Christian-Albrechts-Universität zu Kiel, Kiel, Germany

High precision microprobe and laser ablation analyses of forsteritic olivine phenocrysts from the alkaline EM1-type ocean islands of Gough and Tristan have high ratios of Fe/Mn, Zn/Fe, Ga/Sc and Ni*FeO/MgO suggesting a large pyroxenite melt component in the magmas. Crystallization temperatures calculated for the magmas range between 1140 and 1227°C and overlap the temperatures of MORBs confirming that the melts are derived from the asthenosphere. Applying correction along the olivine-liquidus slope to sub-lithospheric pressures suggests a mantle segregation temperature of max. ~1300-1350°C which overlaps the range estimated for MORB. Therefore the high Fe/Mn and Ni*FeO/MgO cannot have been caused by melting of very high temperature peridotitic mantle at large depths as has been suggested for some plume basalts by various authors. Results for trace element modeling in the OBS1 pyroxenite melting simulator (Kimura & Kawabata, 2015) on primitive samples from Gough Island reproduce expected pressures and temperatures of melting and suggest 8-13% pyroxenite in the mantle source and ~80% pyroxenite melt in the magmas in agreement with the olivine compositions. For comparison, olivine compositions from two Cape Verde Islands indicate a plume melt end-member derived by peridotite melting at temperatures comparable to those for Gough Island despite very similar pressures and extents of melting as indicated by the La/Sm and Dy/Yb of the basalts. This shows that the Sobolev et al. (2007) olivine model is still viable for moderate temperature OIBs.

References
Kimura, J.-I., Kawabata, H. 2015: Ocean Basalt Simulator version 1 (OBS1): Trace element mass balance in adiabatic melting of a pyroxenite-bearing peridotite. Geochemistry, Geophysics, Geosystems 16, 267–300.

Sobolev, A.V., Hofmann, A.W., Kuzmin, D.V., et al. 2007: The amount of recycled crust in sources of mantle derived melts. Science 316, 412-417.

1.2. Volcanism in the North Atlantic, from breakup to current time

                       ORAL PRESENTATIONS                    

Anatomy of the Sørvágsfjörd volcanic complex: Late Palaeocene explosive basaltic volcanism on the Faroe Islands

Uni Árting¹, Morten S. Riishuus², Hans Amundsen³, Turid Madsen² and Nicole Schmitz
¹Jarðfeingi & University of the Faroe Islands, ²Jarðfeingi, ³Vestfonna Geophysical

We report on the basaltic Sørvágsfjörd volcanic complex (SVC) at the base of the Malinstindur Fm in the Faroe Islands Basalt Group. This ~1.5 km wide and ~90 m high structure is exposed in a sea cliff on Vágar Island. The structure rests on fluvio-lacustrine sediments of the Prestfjall Fm and it represents a positive paleo-relief onlapped by flood basalts. The SVC composite stratigraphic sequence includes from base to top: i) volcaniclastic sandstone grading into bedded tuffs, ii) agglomerates, iii) two ignimbrite units, iv) erosion surface, v) paleosol bed, in places replaced by lahar/debris flow, vi) ignimbrite unit, vii) agglomerate, viii) two ignimbrite units, ix) erosion surface. The complex is faulted and intruded by stacked sills, dykes and irregular plugs. On nearby Tindhólmur islet (1.5 km SSW) a paleo-valley incises the Prestfjall Fm, is filled with lava flows and overlain by lahars and ignimbrites. Another younger and deeper paleo-valley incises the lahar/ignimbrite sequence and is subsequently filled by lavas, mega-breccia, and lahars (and re-incised). Both valleys are buried by the Malinstindur Fm. The SVC records a significant explosive volcanic period characterized by two phases that both include near-vent agglomerates and pyroclastic density currents. The explosive volcanism was preceded by regional uplift, faulting, erosion and valley incision, followed by construction of positive volcanic relief, erosion and continued uplift due to intrusion activity, erosion and valley incision prior to burial. The SVC is unique within the NAIP and important to the tectono-volcanic evolution during the transition from pre- to syn-breakup magmatism.

Structural links between the Jan Mayen Microcontinent and the central East Greenland coast prior to, during, and after breakup.

Anett Blischke¹, Ögmundur Erlendsson², Pierpaolo Guarnieri³, Bryndís Brandsdóttir⁴ and Carmen Gaina^5
1Iceland GeoSurvey, Branch at Akureyri, Rangárvöllum, 603 Akureyri, Iceland, ²Iceland GeoSurvey, Grensásvegi 9, 108 Reykjavík, Iceland, ³Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK1350 Copenhagen K, Denmark, ⁴Institute of Earth Science, Science Institute, University of Iceland, Askja, Sturlugata 7, Reykjavik, ⁵Dynamics (CEED), University of Oslo, Sem Sælands vei 24, P.O. Box 1048, Blindern, Oslo, Norway

A detailed tectonostratigraphic framework model of the Jan Mayen microcontinent area has been correlated with its western conjugate margin of central East Greenland, spanning from the primary North Atlantic breakup (~56-55 Ma) to secondary local breakup processes (~49-21 Ma). The reconstructions of the Jan Mayen microcontinent and surrounding oceanic crustal evolution demonstrate a clear correlation to central East Greenland, with respect to major structural boundaries and unconformities within the stratigraphic record. Igneous stratigraphy was compared specifically for the plateau basalts, the alignment to the Iceland Plateau rift segments, and to the Igtertivâ Formation of Kap Dalton within the Blosseville Kyst area.

The Jan Mayen microcontinent represents the next adjacent graben system besides the Liverpool Land high domain and appears to follow the northeast-southwest trending half-graben symmetry of the Mesozoic period of the Jameson Land Basin. These observations are based on structural segmentation and trends visible in Paleozoic-Mesozoic stratigraphic thickness map series for both conjugate areas. The Blosseville Kyst adjacent area of the southern extension of the microcontinent indicate a rapid thinning of the pre-break-up section but increase in break-up and post break-up volcanic strata.

For the first time we demonstrate that the western flank of the Jan Mayen microcontinent was also a volcanic margin, and we reconstruct the newly mapped Late Eocene to Early Oligocene sub-crop and volcano-facies maps of the Jan Mayen microcontinent aligned to a newly mapped volcano-facies and structural elements map of the Blosseville Kyst – Scoresby Sund – Liverpool Land basin margin.

From Iceland to the Bight Transform Fault. Evolution of 1000 km of Volcanic Rifting Plate Boundary

Armann Hoskuldsson¹, Richard N Hey², Fernando Martinez³, Asdis Benediktsdottir¹ and Sigvaldi Thordarson^4
1University of Iceland, ²University of Hawaii, ³University of Hawaii – SOEST/HIGP, ⁴Iceland Geosurvey

Iceland is a part of the NAIP and as such forms a large island in the middle of the north Atlantic. With its continental shelf, Iceland covers about 780 kkm3. The MAR plate boundary crosses through Iceland from south to north. In this presentation, we present results from two missions that cover the southern and northern part of the Reykjanes ridge. We shall show how the Reykjanes ridge plunges into the abyss from Iceland and follow its extent some 1000 km to the south, until it hits the Bight Transform fault. This part of the MAR system is slow spreading with an average separation of 2 cm per year. Due to unprecedented detailed multibeam mapping in the area finest details in volcanic structures can be observed. However, the active plate boundary shows remarkable diversity in overall forms and individual landforms as we extend away from Iceland. Close to Iceland shallow magma storage in the crust is evident, with calderas and evolved magma composition. At the termination of the Reykjanes ridge AVR are observed at regular interval. These ridges are formed in multiple eruptions and occupy the rift valley. However, towards the north rift valleys are absent ore shallow. In between the rift valleys we observe monogenetic eruptive vents. Reorganization of the plate boundary is observed in magnetic fabric of the seafloor and numerous oceanic core complexes seem to be related to such reorganization. Off rift monogenetic eruptive vents are also more frequent as we go further away from Iceland.

Coast-parallel dolerite dykes along SE Greenland: Southernmost onshore evidence of the Tertiary North Atlantic Igneous Province?

Martin Klausen¹, Dominic Loreti¹, Christian Tegner², Charles Lesher², Thomas Ulrich² and Thomas Kokfelt^3
1Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa, ²Department of Geoscience, Aarhus University, Hoegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark, ³Geological Survey of Denmark and Greenland, GEUS, Oester Voldgade 10, DK-1350 Copenhagen K, Denmark

Pristine dolerite dykes of presumed Tertiary age have previously been mapped on the SE coast of Greenland, south of where the coast-parallel Tertiary East Greenland dyke swarm (EGDS) extends into the Atlantic Ocean (Klausen & Larsen, 2002). Between 64˚15’ and 65˚45’N, a total of six dykes constituting a more northerly located and E-W trending segment are all compositionally similar to the older tholeiites of EGDS (Hanghøj et al., 2003); whereas, twelve dykes constituting a more curved SW-NE trending southern segment resemble younger transitional-alkaline EGDS dolerites. Together with their distinct OIB-like bulk rock geochemical signatures, these two southerly located swarm segments most likely formed at the same time as the main EGDS and were primarily sourced from a ~600 km southward displaced proto-Icelandic hot spot enriched mantle source. Compared to the Icelandic Mantle array by Fitton et al. (1997) and a petrogenetic REE-grid by Tegner et al. (1998), relatively high Zr/Y, Nb/Y and (La/Sm)_N suggest that the southern segment’s transitional-alkaline primary melts formed through ~ 5% partial mantle melting and segregated at ~30 km depth. Based on these results – albeit without supporting absolute ages or cross cutting relationships – we tentatively propose that (1) early tholeiitic melts formed through decompression melting along a proto-Atlantic rift that lines up with the EGDS, but injected obliquely into the E-W trending northern dyke swarm segment; and (2) a dextrally offset off-axis rift system allowed for the generation of more alkaline melts to feed more westerly located coast-parallel dykes within the southern swarm segment.

References
Fitton, J.G., Saunders, A.D., Norry, M.J., Hardarson, B.S. & Taylor, R.N., 1997: Thermal and chemical structure of the Iceland plume. Earth and Planetary Science Letters 153, 197–208.

Hanghøj, K., Storey, M. & Stecher, O., 2003: An Isotope and Trace Element Study of the East Greenland Tertiary Dyke Swarm: Constraints on Temporal and Spatial Evolution during Continental Rifting. Journal of Petrology 44, 2081–2112.

Klausen, M.B. & Larsen, H.-C., 2002: The East Greenland coast-parallel dyke swarm and its role in continental breakup. In: Menzies et al (eds), Volcanic Rifted Margins. Geological Society of America Special Paper 362, 133–158.

Tegner, C., Lesher, C.E., Larsen, L.M., Watt, W.S., 1998: Evidence from the rare-earth-element record of mantle melting for cooling of the Tertiary Iceland plume. Nature 395, 591–594.

NE Atlantic break-up and magmatism

Erik Lundin¹, A.G. Doré² and R. Kyrkjebø
¹Statoil ASA, Research Center, Arkitekt Ebbels vei 10, 7053 Trondheim, NORWAY, ²Statoil (UK) Ltd, One Kingdom Street, London W2 6BD, UK

Northeast Atlantic (NEA) break-up in the Early Eocene has generally been viewed as the culmination of a series of episodic rifts events, spanning from Carboniferous through Paleocene. However, Paleocene rifting is conspicuously minor or absent. We propose development of a dextral Paleocene proto-NEA Transform system that allowed opening without further rifting. Several observations support this proposal: the linear, although segmented, line of break-up cutting obliquely across the Cretaceous hyperextended basin; a steeply terminated outer margin; shallowing basement and thickening continental crust toward the margin; coeval compression at Greenland’s northern leading edge, and sinistral motion along its western side.

The proto-NEA Transform linked with the De Geer Transform and further with a transform along the northern Barents and Kara Sea, effectively yielding a broken plate between Greenland’s southern tip and the Laptev Sea. Opening of this c. 6000 km long segment occurred around a pole in the Laptev Sea, with higher extension rates at the southern distal end. SDRs of the magma-rich NEA margins diminish northward and are absent in the Eurasia Basin. This architecture is well known and has typically been attributed to the radius of a plume. However, given that plates are approximately rigid, there is another option – that magmatism reflects the rate of extension (and associated decompressional melting). The NEA break-up magmatism may therefore be rooted in plate tectonics.

A relationship between extension rate and melt generation is further supported by evidence from the margins along the Aegir-Kolbeinsey Ridge pair, located on either side of Jan Mayen.

The evolution of Hekla volcano in the 20th century: Integrating remote sensing data from the past 70 years

Gro B. M. Pedersen¹, Joaquin M. C. Belart¹, Fadi Kizel², Eyjolfur Magnusson¹, Olga K. Vilmundardóttir³, Friðþór S. Sigurmundsson³, Guðrun Gisladóttir³ and Jón Atli Benidiktsson^2
1Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland,, ²Faculty of Electrical and Computer Engineering, University of Iceland, ³Institute of Life and Environmental sciences, University of Iceland

Hekla volcano is one of the most active volcanic systems in Iceland and has erupted ~23 times since the settlement of Iceland in AD 874. The historical Hekla eruptions have been studied from written records and tephra chronology. These eruption records indicate a very constant magma production, which over the last millennium have produced a total of at least 7 km³ (DRE) of eruptive material.

In the 20^th century Hekla mountain erupted five times (1947-1948; 1970, 1980-1981, 1991, 2000). These eruptions were well documented and it has been estimated that ~80% of the total erupted volume was lava. However, the lava flow thicknesses used in the volume calculations are uncertain due to coarse sampling at selected lava thickness profiles or at flow fronts. Accurate lava volume estimates are therefore crucial in order to asses the volume of eruptive material in the 20^th century.

 In the Hekla area, repeated aerial stereo-photography surveys have been conducted since 1945 allowing creation of Digital Elevation Models (DEMs) and ortho-photographs using digital photogrammetric techniques.  We selected seven photogrammetric surveys (1945, 1946, 1960, 1979, 1984, 1987, 1992) to construct historical DEMs (5m/pixel) and ortho-photos (0.5-1m/pixel). Together with modern radar-based DEMs (1998 and 2012-2013) and a lidar-based DEM (2015) these data sets provide pre- and post-eruption topography of Hekla for each of the five eruptions in the 20^th century. These DEMs allow creation of thickness maps and thereby unprecedented estimation of lava flow thickness and lava flow volume.

A hot, top-down model for the formation of the North Atlantic Igneous Province and the Iceland hot spot

Kenni Dinesen Petersen¹, Christian Schiffer² and Thorsten Nagel^1
1Aarhus University, ²Durham University

The Greenland-Iceland-Faroe Ridge (GIFR) is an area that has been magmatically hyperactive since the formation of the North Atlantic Igneous Province at ~55 Ma. The magmatic crustal thickness and its compositions is different from the average oceanic crust in a way that possibly requires elevated temperature and enriched composition of the mantle melt source. A popular explanation for this involves the convective rise of hot plumes, often postulated to originate at the lower mantle with a different temperature and composition than the upper mantle. However, since the GIFR initiated just where the embryonic North Atlantic rift crossed older Caledonian sutures, alternative “top-down” models have been proposed, opposing a “bottom-up” model in which an emerging deep mantle plume would exactly coincide with the location of lithospheric weaknesses. Here, we present a new top-down physical model where lithospheric extension led to delamination of a dense quasi-stable mantle lithosphere and lower crust. The delaminated material sank into the lower mantle and caused a plume-like return flow of hot and primordial material that reached the North Atlantic rift system and caused enhanced melt productivity. Our model predicts melt volumes that are of similar magnitude as those observed in the North Atlantic, and explains why Iceland remains magmatically active at present.

Geometry and temporal development of the Faroe Islands Basalt Group in the Faroe-Shetland Basin: Seismic mapping and well correlations by chemo/lithostratigraphy

Morten S. Riishuus¹, Uni Árting¹, Heri Ziska¹ and Jim á Horni^1
1Faroese Geological Survey

The offshore distribution of the Faroe Islands Basalt Group (FIBG) has received much debate since the early 1980’s. We present new observations and interpretations on the late Palaeocene-early Eocene volcanic history of the Faroe-Shetland Basin (FSB). These efforts are based on novel integration of i) seismic mapping of the geometry of the pre- and syn-breakup volcanic piles and volcanic facies with ii) chemo/litho-stratigraphy of the volcanic sequences in the nine exploration wells in Faroese sector of FSB and iii) the known onshore petro-chemistry of the FIBG. Contour maps of the depth to i) the base of the volcanic pile, ii) the A’-horizon, and iii) the top of the Palaeogene volcanics are presented along with isopach maps of the thickness distribution of the respective pre- and syn-breakup volcanics and associated volcanic foresets. We illustrate the offshore geometry of the FIBG with generalized geoseismic sections and its temporal development with sequence stratigraphic charts. Comparative analysis of volcanic thickness distributions and kinematic indicators reveal a number of aspects concerning the emplacement of flood volcanism into FSB; including seemingly persistent lava transport systems, south- and eastward progradational lava deltas, sub-basins at times acting as depocentres for volcanics while volcanically starved at other times. A greater volume of volcanics was delivered to FSB during the pre-breakup volcanism in comparison to the syn-breakup phase. The syn-breakup volcanic deposits are predominantly present in the northern part of FSB and along Fugloy ridge, likely reflecting eastward migration of volcanic systems and proximity of the emerging Aegir ridge.

Interaction between volcanic and sedimentary transport systems in the greater Judd Basin on the North East Atlantic Margin

Heri Ziska¹, Uni Árting¹ and Morten S. Riishuus^1
1Jarðfeingi (Faroese Geological Survey)

Understanding the transition from the volcanic units of the North Atlantic Igneous Province to the non-volcanic units in the Faroe Shetland Basin, has proven to be a challenge, partly due to inability of seismic data to resolve the difference in seismic response between volcanic and non-volcanic units, and partly due to lack of well control.

Published interpretation of well 6005/15-1 on Sjúrður High suggested a thin extrusive basalt section in the early Eocene and intrusive units in the mid-Palaeocene section. A new integrated interpretation of reprocessed seismic data and re-evaluation of well-cuttings demonstrates that the intrusive units originally classified as basaltic sills are predominantly pre-breakup extrusive basalts and hyaloclastites. This enabled mapping of discrete volcanic packages within the dominantly non-volcanic sedimentary section in the greater Judd Basin area.

Mapping of pre- and syn-breakup volcanic units around the greater Judd Basin area, demonstrates a large variation in the interaction between the volcanic and non-volcanic systems. Towards the south volcanics were fed into the Judd basin during mid-Palaeocene. Following a volcanic regression, basinal accommodation space was filled by siliciclastic sediments primarily sourced from the southeast. In the early Eocene volcanics again transgressed far into the Judd basin before volcanism eventually was restricted to the location of the incipient Northeast Atlantic further north-northwest.

One implication of mapping the event horizon of an igneous province using seismic data is that we can now achieve a more detailed understanding of the temporal and spatial evolution of volcanic and sedimentary depositional processes in basin-hosted volcanic provinces.

1.3. Understanding Large Igneous Provinces and associated rapid environmental changes: from the North Atlantic Igneous Province and beyond.

                       ORAL PRESENTATIONS                    

The Pre-Caledonian Margin of Baltica: overview and research in progress

Torgeir B. Andersen¹, Johannes Jakob¹, Hans Jørgen Kjøll¹, Fernando Corfu¹, Sverre Planke¹, Christian Tegner², Loic Labrousse³, Geoffroy Mohn⁴ and Trond H. Torsvik^1
1CEED, IG-UiO, Norway, ²Geosciences, Univ. Århus, Denmark, ³UPMC Paris6, France, ⁴Univ-Cergy-Pontoise, France
The Caledonian margin of Baltica formed by continental break-up of Rodinia in the Late Proterozoic to Ediacaran. With exception of the dike-swarm near Egersund in SW Norway, the Fennoscandian basement including the autochthonous basement windows along the axis of the mountain belt were little affected by the magmatism associated with the break-up. The distal parts, however, were strongly attenuated, hyper-extended and a 1000 km long segment, intensively intruded by a Large Igneous Province (LIP), the Pre-Caledonian LIP (PC-LIP). Here, we provide glimpses of our work in progress from the vestiges of the margin. More details on several aspects of the margin evolution are presented by co-authors. Here we present a regional model for the pre-Caledonian margin suggesting it was highly complex and included micro-continental sliver(s) and both a hyperextended, magma-poor domain with transition(s) to attenuated embryonic oceanic and magma-rich margin domains. The break-up related PC-LIP magmatism lasted from approximately 615 to 570 Ma, but the most intense activity appears to have been at ~600 Ma. Our ongoing work suggests that the impingement of a mantle plume on the Ediacaran continental lithosphere was associated with a temperature anomaly of ~100^oC, causing widespread melting of the asthenosphere and dyke-intusion of the continental crust and sediments of the margin. We suggest that the pre-Caledonian margin of the Iapetus preserved in the Scandes comprise most of the elements of passive continental margins, and that it probably represents one of the best exposed field analogue for the deeper and least known parts of passive margins.

Comparing the geophysical expression of the Seiland Igneous Province with the SW Barents Sea crust – indications for a LIP during opening of the Iapetus ocean?

Christine Fichler¹, Zeudia Pastore¹ and Suzanne A. McEnroe^1
1Norwegian University of Science and Technology

The Seiland Igneous Province (SIP) in northern Norway is expressed by large outcrops of mafic and ultramafic rocks of Neo-Proterozoic (Ediacaran) age and is hosted by the Caledonian Kalak nappe. The SIP has been suggested to be the plumbing system of a large igneous province. Unaltered ultramafic rocks in crustal settings are characterized by low magnetization and exceptional high crustal densities, the latter forming a contrast to common crustal rocks. Such rock attributes have also been detected by integrated interpretation of gravity, magnetic and seismic data on the continental shelf in the southwestern (SW) Barents Sea at Senja Ridge and Veslemøy High. Furthermore, a highly dense rock unit of large lateral extension, has been found below large parts of the SW Barents Sea, either, at a lower crustal level, or in the uppermost mantle. We investigate a possible link between the rock units with high density in the SW Barents Sea and the SIP. If both belong to the same magmatic event, they may be characterized as relicts of a LIP which developed at the time of the opening of the Iapetus Ocean. An alternative scenario for the origin of the offshore ultramafic rocks at the Veslemøy High has been published suggesting exhumed mantle developed by hyperextension during the ultraslow opening of the Cretaceous Bjørnøy Basin. Finally, obducted ophiolitic crust containing mantle rocks is another possibility which will be discussed.

Karoo LIP thermogenic degassing less important than previously estimated — new evidence from thermo-hydro-mechanical numerical simulations

Christophe Galerne¹ and Jörg Hasenclever^2
1GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, ²Institute for Geophysics, University of Hamburg, Germany

Thermogenic discharge into the atmosphere associated with the Karoo LIP event (183 Ma) is currently estimated to as much as 12,600 Gt of methane. In order to precise the degassing rates at the basin floor, we investigate the influence of the Karoo Basin stratigraphic physical properties, including a realistic initial TOC profile. Using state-of-the-art numerical models of hydrothermal fluid flow through porous rocks we investigate the formation and evolution of hydrothermal systems powered by sill intrusions. We take into account the energy consumption of the dehydration reaction and the thermogenic cracking in the sediments as well as the increasing pore space upon sediment dehydration. Permeability depends on porosity and, in addition, a simple Mohr-Coulomb rheology is used to simulate hydrofracturing. Our results indicate that fluid flow caused by a single sill intrusion emplaced in the deep organic-rich formation is insufficient to rapidly transport thermogenic methane to the basin floor. Instead the mobilized methane accumulates in shallower parts of the basin, where it may be later remobilized by shallower intrusions. Our new estimates suggest a reduction of one to two orders of magnitude in the amount of thermogenic gas emitted during the Karoo LIP event.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis

Thea Hatlen Heimdal¹, Henrik Hovland Svensen¹, Jahandar Ramezani², Karthik Iyer³, Egberto Pereira⁴, René Rodrigues⁴, Morgan Thomas Jones¹ and Sara Callegaro^1
1Centre for Earth Evolution and Dynamics (CEED), UiO, ²Massachusetts Institute of Technology (MIT), ³GeoModelling Solutions GmbH; GEOMAR, ⁴Rio de Janeiro State University

It has long been postulated that the Central Atlantic Magmatic Province (CAMP) was involved in the end-Triassic extinction (ETE), one of the largest mass extinctions of the Phanerozoic. Previous studies have mainly focused on the extrusive section of the province, however several lines of evidence point toward sill emplacement in Brazil being a key factor regarding the extinction. These sills have intruded the Amazonas and Solimões basins, which include major deposits of evaporite, carbonate and organic-rich shale, and it has been suspected that contact metamorphism of these lithologies generated thermogenic gases that were released to the end-Triassic atmosphere. Until now, detailed studies regarding the extent of the sill complex and estimates of sediment-derived volatiles have been absent, likely because outcrops and samples are extremely limited. Here we have access to samples and logs from seven deep boreholes drilled in the Amazonas and Solimões basins. The sills comprise up to 24% of the stratigraphy, and the majority have intruded evaporite-and carbonate dominated deposits. Thermal modeling demonstrates that large scale carbon generation followed sill emplacement, including as much as 116 000 Gt CO₂. We also provide new constraints on the timing of sill emplacement by high precision U-Pb zircon geochronology. Our findings demonstrate synchronicity between the sills and the ETE, and we suggest that venting of sediment-derived volatiles such as C, S and Cl played a major role in the end-Triassic crisis.

Evidence from Denmark for NE Atlantic pre-PETM volcanism

Claus Heilmann-Clausen¹, Bo Pagh Schultz², Claus Beyer³, Henrik Friis¹, Petra L. Schoon⁴, Christian Tegner¹ and Ole Bjørslev Nielsen^1
1Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark, ²Muserum, Division of Natural History, Havnevej 14, 7800 Skive, Denmark, ³CB-Magneto, Nørregade 27, 8670 Låsby, Denmark, ⁴Department of Geology, Lund University, Sölvegatan 12, 22362 Lund, Sweden

A new section on the Fur island in Denmark includes two quadruple volcanic ash layers which are older than the Paleocene-Eocene Thermal Maximum (PETM). The ash layers occur immediately below sediments dated to the PETM interval. The ash layers are of a normal basaltic type and older than hitherto recognized volcanic ash layers from the Paleocene-Eocene transition in Denmark (i.e., the negative and positive numbered ash series of Bøggild, 1918). Three other Danish PETM sections (Ny Klitgård, Ølst-Hinge, Harre-1) show 1-3 volcanic ash layers closely associated with the base of the PETM interval. The ash layers point to a significant volcanic episode and their stratigraphic position supports previous evidence for NE Atlantic volcanism as a trigger of the PETM (e.g. Svensen et al., 2004). All sections show a shift from oxic sea-floor conditions (indicated by bioturbation) below the PETM interval, to anoxic conditions (indicated by finely laminated sediment) within the PETM interval. The basal layer of the PETM interval at Fur is a 5 cm thick density flow deposit occurring immediately above the ash layers. The density flow deposit is not recognized in other Danish sections and points to local sea-floor instability.

References
Bøggild, O.B., 1918: Den vulkanske aske i moleret. Danm. Geol. Unders. II Rk., Nr. 33, 159 pp.

Svensen, H., Planke, S., Malthe-Sørenssen, A., Jamtveit, B., Myklebust, R., Eidem, T. R., & Rey, S.S., 2004: Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature 429, 542-545.

The lithostratigraphy of a hyperextended domain in the magma-rich to magma-poor transition zone in the southern Pre-Caledonian LIP, Scandinavian Caledonides, Norway

Johannes Jakob¹, Geoffroy Mohn², Pierre Closset³ and Torgeir B. Andersen^1
1The Centre for Earth Evolutions and Dynamics, Department of Geosciences, University of Oslo, Norway, ²Department of Geosciences and Environmental Sciences, Université de Cergy-Pontoise, France, ³Institut de Physique du Globe de Strasbourg, UMR 7516 CNRS, Université de Strasbourg, France

Remnants of the ancient rifted margin of Baltica are preserved in the allochthons of the Scandinavian Caledonides. The Baltica margin nappes comprise a magma-rich segment, i.e. the pre-Caledonian LIP (Tegner et al. 2016), and a magma-poor. (Andersen et al. 2012; Jakob et al. 2017). The transition zone between the magma-rich and magma-poor domains corresponds approximately to the northern termination of the Jotun Nappe Complex. Metaperidotites and metamorphosed mafic rocks abound in the metasediments of this transition zone.

North of Lesja, an original lithostratigraphic succession of the ancient Baltica hyperextended margin may be preserved. At its base is a sheet of metaperidotite that is locally characterized by a pegmatitic texture, containing pseudomorphs after decimetre-sized, metamorphic olivine. Few mafic dykes cutting the metaperidotite are also observed.

The top of the metaperidotite sheet is in contact with metamorphosed mafic rocks or in a stratigraphic contact with a monomictic metaperidotite conglomerate. The latter grades upwards into a polymictic conglomerate. The metamorphic mafic rocks and the conglomerates above the metaperidotites, but locally also the ultramafic rocks, are overlain by a metasedimentary unit that comprises a large number of mafic intrusives (and volcanics?) and locally contains ultramafic clasts. Similar successions have been described from fossil ocean-continent transition zones elsewhere (e.g. Manatschal and Müntener 2009).

We suggest that the lithostratigraphic succession north of Lesja represents a hyperextended domain in the magma-rich to magma-poor transition zone of the pre-Caledonian margin of Baltica that received abundant mafic magmas during and after the mantle was exhumed at the seafloor.

References

Andersen, T.B., Corfu, F., Labrousse, L., & Osmundsen, P.-T. 2012: Evidence for hyperextension along the pre-Caledonian margin of Baltica: Journal of the Geological Society 169, 601-612.

Jakob, J., Alsaif, M., Corfu, F., & Andersen, T.B. 2017: Age and origin of thin discontinuous gneiss sheets in the distal domain of the magma-poor hyperextended pre-Caledonian margin of Baltica, southern Norway. Journal of the Geological Society 174, 557-571.

Manatschal, M., Müntener, O. 2009: A type sequence across an ancient magma-poor ocean–continent transition: the example of the western Alpine Tethys ophiolites. Tectonophysics 473, 4–19.

Tegner, C., Andersen, T.B., Corfu, F., Planke, S., Kjøll, H.J., & Torsvik, T.H. 2016: The pre-Caledonian Large Igneous Province and the North Atlantic Wilson Cycle. In: Proceedings European Geoscience Union General Assembly, Vienna, Austria.

Types and styles of volcanism in the North Atlantic Igneous Province: Implications towards understanding their potential climatic impact.

Dougal A. Jerram¹, Henrik H. Svensen², Sverre Planke³, John M. Millett⁴, Morgan T. Jones², Lars Augland² and Mansour M. Abdelmalak^5
1CEED, University of Oslo, Norway/DougalEARTH LTD, UK, ²CEED, University of Oslo, Norway, ³VBPR, Oslo, Norway/CEED, University of Oslo, Norway, ⁴VBPR, Oslo, Norway/University of Aberdeen, Scotland, ⁵VBPR, Oslo, Norway

The North Atlantic Igneous Province (NAIP) is arguably the best studied of the Large Igneous Provinces (LIPs), but it is somewhat unusual in that its volcanism is somewhat protracted and pulsed. Two main phases are recognised with one occurring some 5 million years before the Paleocene-Eocene thermal maximum (PETM) and another which occurs at the PETM. The different styles and types of volcanism that are found during the onset, main phases and the waning parts of the NAIP, as well as the intrusive plumbing systems that go to feed them, all play a potential role into how the LIP may have contributed to the changing climate at this time. Flood volcanism is rarely restricted to simple lava flows, and here we review the relative timings and locations of the key types of eruption that occurred in the NAIP. Lava flows, hyaloclastites, major volcanic centres, and thick ash accumulations bears testament to the volcanism as well as a full range from basic to silicic compositions. It can be shown that over the PETM times, the volcanism was particularly widespread, with significant ash beds as well as a plumbing system that significantly intruded into the surrounding sedimentary basins at this time causing a multitude of gas escaped venting structures. The changes in style from early eruptions to the second phase volcanism in the NAIP need to be considered to help our understanding of the PETM crisis.

Fur Island in Denmark: A window into Paleocene-Eocene hyperthermals and North Atlantic volcanism

Morgan Jones¹, Ella Stokke¹, Bo Schultz², Lars Augland¹, Sverre Planke¹, Christian Tegner³, Tamsin Mather⁴, Lawrence Percival⁵ and Henrik Svensen^1
1Centre for Earth Evolution and Dynamics, University of Oslo, Norway, ²Museum Salling – Fur Museum, Denmark, ³Department of Geoscience, Aarhus University, Denmark, ⁴Department of Earth Sciences, University of Oxford, UK, ⁵Université de Lausanne, Switzerland

The Paleocene and Eocene epochs were typified by ice-free greenhouse conditions, punctuated by even warmer hyperthermal events driven by rapid release of carbon to the atmosphere. The largest of these, the Paleocene-Eocene Thermal Maximum (PETM), was an extreme (5-6 °C) and rapid (<20 kyr) global warming event at ~56 Ma that persisted for ~170 kyr. The PETM coincided with a major pulse of magmatism from the North Atlantic Igneous Province (NAIP), suggesting that the emplacement of the NAIP could be responsible for the climate perturbations. An excellent locality to explore this relationship is the island of Fur in Denmark, where over 180 tephra layers originating from the NAIP are preserved in a diatomitic clay sequence. Here we present the initial results of the project “Ashlantic”, which drilled a 73 m borehole through the Fur Formation in August 2017. We use a variety of detailed geochemical and stratigraphic analyses to assess the extent and duration of volcanism across the PETM. In particular, we use volcanic proxies such as Hg deposition to assess the influence of volcanism and thermogenic degassing on the surface carbon cycle. We will combine this with high precision U-Pb dating of magmatic zircons found in tephra layers and high-resolution cyclostratigraphy to place the PETM and later hyperthermals within an improved absolute timeframe. Expanded shelf sections such as the Fur Formation are fundamentally important for understanding hyperthermal events and volcano-climate interactions due to the possibility of measuring variations on a sub-millennial timescale.

A fossil magma-rich rifted margin revealed in the Scandinavian Caledonides

Hans Jørgen Kjøll¹, Torgeir Andersen¹, Christian Tegner², Loic Labrousse³, Fernando Corfu¹ and Sverre Planke^1
1Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway, ²Centre of Earth System Petrology, Department of Geoscience, Aarhus University, Denmark, ³Universite Pierre et Marie Curie, Paris, France

The Iapetus opened ~610 Ma ago possibly due to the interaction with a mantle plume at the base of the lithosphere. Stretching of the conjoined crystalline crust started prior to break-up and provided accommodation for continental and shallow marine syn-rift sediments. An early non-magmatic phase with discrete and localized deformation was followed by pervasive mafic magmatism where dyke-emplacement accounted for the bulk of the stretching. During the Caledonian orogeny, the Iapetan margin was thrusted onto Baltica as the Iapetus closed. Now, vestiges of the magma-rich margin reside within nappes from central Sweden to northern Norway. Although overprinted by Caledonian fabrics, there are localities where pre-Caledonian structures are well-preserved, thereby allowing for detailed studies of deep to intermediate processes at magma-rich rifted margins. We propose that the architecture of the magma-rich margin of Iapetus and magma-rich rifted margins, in general, can be studied in the Caledonides. The well-preserved parts of the margin comprise: 1) Parts of a lower crustal magmatic complex w/gabbros and mafic dykes intruding stretched crystalline basement; 2) Strongly stretched and attenuated crystalline basement intruded by mafic dikes; 3) Highly intruded pre- to syn-rift sediments and 4) Extrusive mafic lavas, including pillow basalts, interlayered with metasediments. Together these levels represent a nearly continuous section through a magma-rich rifted margin, with some never-before described levels, such as the lower crustal magmatic complex.

Environmental and ecosystem responses to massive volcanism during the end-Triassic mass extinction

Sofie Lindström¹, Bas van de Schootbrugge², Gunver K. Pedersen¹, Andrea Marzoli³, Joshua H.F.L. Davies⁴, Sara Callegaro⁵, Christian Tegner⁶ and Hamed Sanei^6
1GEUS, ²Utrecht University, ³University of Padova, ⁴University of Geneva, ⁵University of Oslo, ⁶University of Aarhus

The end-Triassic mass extinction (201.56 Ma; Wotzlaw et al. 2014) is generally explained by massive input of CO₂ and/or methane to the atmosphere from the Central Atlantic magmatic province (CAMP). High precision U/Pb dating of CAMP intrusives and extrusives have shown that the magmatic activity commenced c. 100.000 years prior to the ETE (Davies et al. 2017), and occurred in at least four pulses over approximately 600.000 years (Blackburn et al. 2013). Indeed, both calcareous and organic δ¹³C-records across the Triassic-Jurassic boundary (TJB) show that large scale emissions of isotopically light carbon to the atmosphere took place at that time. Physiological responses in terrestrial plant fossils indicate that this lead to intense global warming across the TJB (McElwain et al. 1999), while in the marine realm, ocean acidification from the increased pCO₂ is indicated by the loss of calcifying organisms (van de Schootbrugge et al. 2007). Recently, a new correlation of TJB successions, based on a combination of biotic (palynology and ammonites), geochemical (δ¹³C_org) and radio-isotopic (U/Pb ages of ash beds) constraints, was proposed (Lindström et al. 2017).  This new correlation has an impact on the causality and temporal development during the end-Triassic event, as it indicates that the bulk of the hitherto dated CAMP rocks preceded or was contemporaneous to the onset of the mass extinction. Here, we investigate the temporal chain of events across the ETE as constrained by the new correlation in combination with new high-resolution CAMP ages extrusives (Blackburn et al. 2013; Davies et al. 2017).

References
Blackburn, T.J., Olsen, P.E., Bowring, S.A., McLean, N-M., Kent, D.V., Puffer, J., McHone, G., Rasbury, E.T. & Et-Touhami, M. 2013: Zircon U-Pb geochronology links the end-Triassic extinction with the Central Atlantic magmatic province. Science 340, 941–945.

Davies, J.H.F.L., Marzoli, A., Bertrand, H., Youbi, N., Ernesto, M. & Schaltegger, U. 2017: End-Triassic mass extinction started by intrusive CAMP activity. Nature Communications 8, 15596.

Lindström, S., van de Schootbrugge, B., Hansen, K.H., Pedersen, G.K., Alsen, P., Thibault, N., Dybkjær, K., Bjerrum, C.J. & Nielsen, L.H. 2017: A new correlation of Triassic–Jurassic boundary successions in NW Europe, Nevada and Peru, and the Central Atlantic Magmatic province: A time-line for the end-Triassic mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology 478, 80–102.

McElwain, J.C., Beerling, D.J. & Woodward, F.I. 1999: Fossil plants and global warming at the Triassic-Jurassic boundary. Science 285, 1386–1390.

van de Schootbrugge, B., Tremolada, F., Bailey, T.R., Rosenthal, Y., Feist-Burkhardt, S., Brinkhuis, H., Pross, J., Kent, D.V. & Falkowski, P.G., 2007: End-Triassic calcification crisis and blooms of organic-walled disaster species. Palaeogeogr. Palaeoclimatol. Palaeoecol. 244, 126–141.

Wotzlaw, J.-F., Guex, J., Bartolini, A., Gallet, Y., Krystyn, L., McRoberts, C.A., Taylor, D., Schoene, B. & Schaltegger, U. 2014: Towards accurate numerical calibration of the Late Triassic: High-precision U-Pb geochronology constraints on the duration of the Rhaetian. Geology 42, 571–574.

The Relationship Between Paleogene Hydrothermal Vent Complex Diameters and Conduit Heights in the Vøring and Møre Basins Offshore Mid-Norway

Ben Manton¹, Sverre Planke¹, Hugo Thould², Sigurd Kjoberg³, Henrik Svensen⁴ and Øyvind Hammer^5
1VBPR, Oslo Science Park, Oslo, Norway, ²Department of Earth Sciences, Oxford University, Oxford, UK, ³CEED, Oslo University, Oslo, Norway, ⁴Department of Geosciences, Oslo University, Oslo, Norway, ⁵Natural History Museum, Oslo University, Oslo, Norway

Large Igneous Provinces (LIPs) emplaced within basins are frequently associated with many hydrothermal vent complexes sourced from thermal aureoles around sill intrusions. These piercement structures consist of cylindrical conduits that connect the sill aureoles with vents at the paleosurface. More energy is considered to be required to create both longer conduits, as they require higher fluid pressure to fracture long pathways, and larger vents, because more rock is mobilized near the seabed. Four datasets from the Vøring and Møre basins (Mid-Norway), consisting of 718, 213, 106 and 13 Paleogene hydrothermal vent complexes, are used to determine the relationship between vent diameters and conduit heights. The vent complexes and associated sills were mapped using regional 2D seismic data (first dataset) and 3D seismic data (three datasets). Generally sills have pipe-like vent complexes located above their margins or above sill steps or local highs, however, one shallow sill has >110 hydrothermal vent complexes distributed over its top surface. The upper part of the vent complexes display mound, crater, or eye-shaped geometries. Observed vent diameters range from ~100 m to ~11 km, with a sizable majority <2.5 km wide. The vertical conduit heights range from ~100 m to ~4 km with a sizable majority <2.5 km. There are weak positive correlations between the upper vent complex diameters and conduit heights in all four datasets. The weak correlations suggest the quantity of energy transferred from the sill aureoles, via fluid transport, to the hydrothermal vents is partially dependent on the heights of the conduits.

Release of volatiles during North Atlantic flood basalt volcanism and correlation to the Paleocene-Eocene Thermal Maximum (PETM)

Jonas Møller Pedersen¹, Thomas Ulrich¹, Adam Kent², Charles Lesher¹ and Christian Tegner^1
1Centre of Earth System Petrology, Department of Geoscience. Aarhus University., ²College of Earth, Ocean, and Atmospheric Sciences, Oregon State University.

Significant environmental changes and global warming during the PETM have been attributed to the release of CO₂ or methane gas due either to extensive melting of hydrates at the ocean floor or to interaction of mantle-derived magmas with carbon-rich sediments. Although it is well established that flood basalt volcanism associated with the opening of the North Atlantic Ocean broadly coincides with the PETM, further detailed timing constraints from stratigraphic correlations, magnetostratigraphy, and the duration of flood basalt volcanism suggest that it is also possible that the main flood basalt sequence in East Greenland postdates PETM.

Estimates suggest that a minimum of 1.8×10⁶km³ of basaltic lava erupted during North Atlantic flood basalt volcanism. Based on measurements of melt inclusions from the East Greenland flood basalts our calculations show that approximately 2300Gt of SO₂ and 600Gt HCl were released into the atmosphere. Yearly fluxes are estimated to be 8-23 Mt/y SO₂ and 2-6 Mt/y HCl if all the North Atlantic flood basalts erupted in 100.000-300.000 years. This is equivalent to ≈18600 Laki-like events, one happening every 5-16 years. The SO₂ released into to the atmosphere during flood basalt volcanism can form acid aerosols that absorb and reflect solar radiation, causing an effective cooling effect.

The climatic changes due to the release of volatiles in these amounts, and for periods extending for hundred thousand of years are likely to be significant. One consequence of the North Atlantic flood basalt volcanism may therefore have been the initiation of global cooling to end the PETM.

NE and NW Atlantic Paleogene Voluminous Magmatism and Global Crises: Seismic Observations and the Need for Future Scientific Drilling

Sverre Planke¹, Christian Berndt², Christian Tegner³, Ritske Huismans⁴, Henrik H. Svensen⁵, Jan Inge Faleide⁵, Dougal A. Jerram⁵, John Millett⁶, Mansour M. Abdelmalak⁶, Charles E. Lesher³ and Reidun Myklebust^7
1Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway, and VBPR, Oslo, Norway, ²GEOMAR, Kiel, Germany, ³Aarhus University, Denmark, ⁴University of Bergen, Norway, ⁵CEED, University of Oslo, Norway, ⁶VBPR, Oslo, Norway, ⁷TGS, Asker, Norway

The NE and NW Atlantic continental margins are characterized by massive Paleogene igneous constructions, associated with continental breakup and early seafloor spreading. We have recently completed a comprehensive interpretation of the nature and distribution of the extrusive and intrusive mafic rocks on regional 2D and selected high-quality 3D seismic data in both regions. Interpretations have been augmented by scientific and industry borehole data and on-shore field work. On the Vøring Marginal High, 3D mapping reveals a highly variable crustal structure, with large-scale pre-breakup structural highs and sedimentary basins. These structures where infilled and covered by flood basalts and volcanogenic sediments during the early stages of continental breakup in the earliest Eocene. Subsequently, rift basins developed along the continent-ocean boundary and were infilled by up to 6 km thick basalt sequences, imaged as SDRs. Intrusive sill complexes and associated hydrothermal vent complexes in the Vøring Basin have dominantly a Paleocene-Eocene boundary age based on a few high-quality U/Pb ages and seismic mapping. Similar igneous complexes, including SDRs, lava deltas, lava fields, sills, and hydrothermal vent complexes, were also mapped in the NW Atlantic and on the NE Greenland margin. The dominant hypothesis is that the massive magmatism was caused by a mantle plume originating from the core-mantle boundary, and that intense and short-lived (<100,000 years) intrusive and extrusive magmatism triggered global warming during the Paleocene-Eocene thermal maximum (PETM). However, we argue that new scientific drilling in the NE and NW Atlantic is required to substantiate these hypotheses.

Improving seismic interpretation of igneous sill complexes via seismic modelling

Ole Rabbel¹, Olivier Galland¹, Karen Mair¹, Isabelle Lecomte² and Juan Bautista Spacapan^3
1Physics of Geological Processes, Department of Geoscience, University of Oslo, Norway, ²Department of Geoscience, University of Bergen, Norway, ³Universidad Nacional de La Plata, Argentina

Seismic interpretation has been playing a key role in establishing the role of igneous sill complexes emplaced in organic-rich sediments on global climate change throughout geological history (e.g., Svensen et al. 2004, Aarnes et al. 2015): the heat brought by the sills to their organic-rich host leads to massive thermogenic generation of greenhouse gases (e.g., CO2, CH4), eventually catastrophically released to the atmosphere. Robust volume estimates of these greenhouse gases rely on robust estimates of magma volumes of the sill complexes, which require confident mapping of subsurface sill intrusions.

Large sills are easy to map in seismic data, because they create strong property contrasts, which cause high amplitude reflections. However, igneous sills vary strongly in thickness and up to 88 percent of the sills may be missing in the interpretation, since their thickness is below the seismic resolution limit (Schofield et al. 2015).

In this presentation, we present seismic modelling study designed to characterize typical seismic signatures (splitting, stepping, braided reflections) of thin intrusions, and to indicate potential ways to infer the shapes of thin intrusions in the seismic interpretation. Due to their anomalously high vp/vs ratio, intrusions cause a characteristic amplitude-vs-offset (AVO) response with high AVO intercept and gradient. Thus, we will highlight the great potential of using seismic pre-stack and offset data for the interpretation of thin intrusions. Our results indicate how seismic interpretation and thereby volume estimates of sill complexes can be significantly improved using data of relevant quality.

References
Aarnes, I., S. Planke, M. Trulsvik and H. Svensen (2015). “Contact metamorphism and thermogenic gas generation in the Vøring and Møre basins, offshore Norway, during the Paleocene–Eocene thermal maximum.” Journal of the Geological Society 172(5): 588-598.

Schofield, N., S. Holford, J. Millett, D. Brown, D. Jolley, S. R. Passey, D. Muirhead, C. Grove, C. Magee, J. Murray, M. Hole, C. A.-L. Jackson and C. Stevenson (2015). “Regional magma plumbing and emplacement mechanisms of the Faroe-Shetland Sill Complex: implications for magma transport and petroleum systems within sedimentary basins.” Basin Research: 1-23.

Svensen, H., S. Planke, A. Malthe-Sørenssen, B. Jamtveit, R. Myklebust, T. Rasmussen Eidem and S. S. Rey (2004). “Release of methane from a volcanic basin as a mechanism for initial Eocene global warming.” Nature 429(6991): 542-545.

U-Pb baddeleyite dating of multiple mafic dyke swarms in the Dharwar craton, India – evidence for a 30° internal block rotation during the Paleoproterozoic

Ulf Söderlund¹, Wouter Bleeker², Kursad Demirer¹, Rajesh K. Srivastava³, Michael Hamilton⁴, Mimmi Nilsson¹, Lauri J. Pesonen⁵, Richard E. Ernst⁶ and Amiya K. Samal^3
1Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden, ²Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada, ³Centre of Advanced Study in Geology, Institute of Science, Banaras Hindu University, Varanasi 22100, ⁴Jack Satterly Geochronology Laboratory, Dept. of Earth Sciences, University of Toronto, Toronto, ON, ⁵Laboratory for Paleomagnetism, Geological Survey of Finland, P.O. Box 96, 02151 Espoo, Finland, ⁶Department of Earth Sciences, Carleton University, Ottawa, ON K1S5B6, Canada

A significant portion of large igneous provinces (LIPs), especially those thought to be associated with mantle plumes, have been linked with breakup of continents and, therefore, provide a critical source of information in reconstructions of ancient crustal blocks. First-order crustal connections to a plume include radiating mafic dyke patterns, inferred to reflect lateral magma transport away from a plume centre located at the focal point of the swarm^1,2. The 2.37 Ga giant Bangalore-Karimnagar dyke swarm of the Dharwar craton, southern India, displays a distinct radial pattern (of at least 30º) from which a western source of magma has been inferred^3,4.  However, on the basis of paleomagnetic data and an arcuate trend to structures in the Archaean basement, an alternative interpretation of the fanning pattern of the Bangalore-Karimnagar swarm has been raised^5,6,7. Here we present 17 U-Pb baddeleyite ages, which comprise seven generations of Paleoproterozoic dykes swarms within the 2.37-1.79 Ga age interval.

By restoring a 30° counter-clockwise rotation of the northern Dharwar block relative to the southern block, we show that pre-2.08 Ga arcuate and fanning dyke swarms consistently become approximately linear, as do ancient curvilinear trending geological features, such as regional Dharwar greenstone belts and the late Archean (ca. 2.5 Ga) Closepet batholith. This finding reinforces previous suggestions that the radial pattern is apparent, and not a primary feature^{e.g. 7}. Our findings call for reassessment of previous paleoreconstructions involving the Dharwar craton based on tectonic/geologic piercing points, paleomagnetism and inferred plume centre locations.

References
1.      Fahrig, W.F., 1987. The tectonic setting of continental mafic dyke swarms: failed arm and early passive margin. In: Halls, H.C., Fahrig, W.F. (Eds.), Mafic Dyke Swarms, Special Paper – Geological Association of Canada, 34: 331-348.

2. Ernst, R.E., 2014. Large Igneous Provinces, Cambridge University Press, p. 653.
3. Halls, H.C., 1982. The importance and potential of mafic dyke swarms in studies of geodynamic processes. Geosci. Canada 9, 145-154.
4. Halls, H.C., Kumar, A., Srinivasan, R., Hamilton, M.A., 2007. Paleomagnetism and U-Pb geochronology of easterly trending dykes in the Dharwar craton, India: feldspar clouding, radiating dyke swarms and the position of India at 2.37 Ga. Precambrian Research, 155, 47-68.
5. Kumar, A., Nagaraju, E., Besse, J., Bhaskar Rao, Y.J., 2012a. New age, geochemical and paleomagnetic data on a 2.21 Ga dyke swarm from south India: Constraints on Paleoproterozoic reconstruction. Precambrian Research, 220-221, 123-138.
6. Kumar, A., Hamilton, M.A., Halls, H.C., 2012b. A Paleoproterozoic giant radiating dyke swarm in the Dharwar Craton, southern India. Geochemistry, Geophysics, Geosystems, 13(1), doi:10.1029/2011GC003926.

Bleeker, W., Demirer, K., Söderlund, U., Srivastava, R., 2013. The magmatic barcode of the Dharwar craton, India II. New insights in the giant Bangalore-Karminagar swarm (c. 2368 Ma). Report A117-119 of Industry – Government – Academia Consortium Project “Reconstruction of supercontinents back to 2.7 Ga using the large igneous province (LIP) record: With implications for mineral deposit targeting, hydrocarbon resource exploration and Earth-system evolution” CAMIRO Project 08E03, and grant NSERC CRDPJ 419503-11, www.supercontinent.org, www.camiro.org/exploration/ongoing-projects.

Volcanic causes for the PETM and other past hothouse climates

Henrik H. Svensen¹, Dougal A. Jerram², Alexander G. Polozov³, Morgan T. Jones⁴, Lars E. Augland⁴ and Sverre Planke^5
1CEED, Univ. Oslo, ²CEED and DougalEarth, ³CEED and Institute of Geology of Ore Dep., Petr., Min., and Geochem., RAS, Moscow, ⁴CEED, ⁵CEED and VBPR

Mass extinctions and transient climate events commonly coincide in time with the formation of Large igneous provinces (LIPs). Classic examples include the end-Permian event which coincides with the Siberian Traps, the end-Triassic with the Central Atlantic Magmatic Event (CAMP), the Toarcian with the Karoo LIP, and the Palaeocene-Eocene Thermal Maximum (PETM) with the North Atlantic Igneous Province. The emplacement of igneous sills into sedimentary basins, and the associated contact metamorphism of the host sedimentary rocks, has emerged as a major player in the understanding of the link between LIPs and past climatic change. We stress that for these processes to have an environment impact, the gases need to be transferred to the surface and atmosphere on a very short timescale, which is borne out by dating of the sill complexes in question. We have identified a range of different pipe structures that acted as gas transport channels during the end-Permian, the Toarcian, and the PETM, and present a classification and detailed overview of the key parameters governing their formation. We show that the potential for degassing of greenhouse gases, aerosols, and ozone destructive gases in sedimentary basins affected by volcanism is substantial, and can explain the triggering of both transient climatic events and mass extinctions.

The pre-Caledonian Scandinavian Dyke Complex and 600 Ma plate reconstructions of Baltica

Christian Tegner¹, Torgeir B. Andersen², Hans Jørgen Kjøll², Eric L. Brown¹, Graham Hagen-Peter¹, Fernando Corfu², Sverre Planke² and Trond H. Torsvik^2
1Centre of Earth System Petrology (ESP), Department of Geoscience, Aarhus University, Denmark, ²Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway

A spectacular dyke complex is surprisingly well preserved along c. 1000 km in the Caledonian nappes of central and northern Scandinavia. This dyke complex was originally emplaced into continental sedimentary basins along the rifted margin of Baltica, it is part of the Central Iapetus Magmatic Province (CIMP), and it has U-Pb ages of 615-590 Ma.

To constrain its origin and to potentially guide plate reconstructions of Baltica we: (1) re-visited the dyke complexes of the Ottfjället, Sarek, Kebnekaise, Tornetrask and Indre Troms mountains of Sweden and Norway; (2) produced new and compiled published geochemical data; (3) modeled mantle sources and melting dynamics; and (4) extended reconstructions of the paleo-position of Baltica back to 600 Ma. The compiled dataset includes c. 600 analyses that forms a coherent suite dominated by tholeiitic ferrobasalt, but including alkali basalts in the central portion.

The tholeiitic dykes display lateral variations in geochemical enrichment (e.g. delta-Nb, La/Sm(N) and Sr isotopes) in the southern and central portions, grading to more depleted compositions in the north. Our petrological modeling suggests melting of asthenospheric mantle involving at least two source compositions at temperatures elevated about 100 °C above ambient mantle, consistent with melting of a zoned mantle plume originating from a plume generation zone at the core-mantle boundary. If the position of the present plume generation zone in the Pacific can be viewed as stationary back to 600 Ma, we entertain the idea that the Scandinavian Dyke Complex may be used to guide plate reconstructions.

                       POSTER PRESENTATIONS                    

Pyroxenes used as probes for assessing gas loads from LIP magmas: a crystal/melt partitioning study for sulfur and halogens

Sara Callegaro¹, Don R Baker², Andrea Marzoli³, Martin Whitehouse⁴, Paul R Renne⁵ and Henrik Svensen^1
1CEED – University of Oslo, ²McGill University, ³University of Padova, ⁴Nordsim – NRM Stockholm, ⁵Berkeley Geochronology Center

Magmatism from Large Igneous Provinces (LIPs) has in several cases been causally linked to mass extinctions, with at least five conspicuous examples in the Phanerozoic. LIPs likely generated changes on the global environment by degassing of volatile species such as S, C and halogen compounds, from both melts and thermal metamorphism of volatile-rich intruded sediments. However, quantitative estimates of the degassed volatiles are hard to obtain for ancient magmatic systems, particularly in the absence of melt inclusions. We propose to fill the gap of knowledge on sulfur partitioning between minerals and melts, at the aim of using phenocrysts as probes of volatile contents in the melts from which they crystallized. Measuring a volatile concentration in natural minerals (chiefly clinopyroxene) and combining it with an experimentally determined partition coefficient (KD), the volatile load of basaltic equilibrium melts can be calculated. We measured a clinopyroxene/melt sulfur KD of 0.001±0.0003 for basaltic experiments performed at conditions typical of LIP basalts (FMQ-2; 800-1000 MPa; 1000°-1350°C), through ion microprobe (Nordsim). Experiments were also simultaneously analyzed for Cl and F. For these elements the measured KDs were 0.0106±0.0089 and 0.1837±0.0795, respectively. Compatibility of sulfur, chlorine and fluorine in clinopyroxene from basaltic systems is markedly different (F>Cl>S), in agreement with what observed by previous studies. Application of the newly measured sulfur KD to samples from thoroughly-dated lava piles from the Deccan Traps and from the Siberian Traps sills reveal that most of the basalts were at or near sulfide saturation (ca. 2000 ppm for low fO₂ melts).

Glendonite in an early Eocene Konservat-Lagerstätte (Fur Formation of Northern Denmark) and Palaeocene / Eocene formations on Svalbard are good climate and biosphere interpretive indicators.

Bo Pagh Schultz¹, Johan Lindgren², Barbara Teichert³, Morgan T Jones⁴, Christian Tegner⁵, Rene Sylvestersen⁶, Jan Audun Rasmussen⁷ and Henrik Madsen^8
1Museum curator, Museum Salling – Fur Museum, Nederby 28, 7884 Fur, Dk, ²Senior lecturer, Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden, ³Senior researcher Ph.d., Institut für Geologie und Paläontologie, Universität Münster, D-48149 Münst, ⁴Ph.D. Centre for Earth Evolution and Dynamics (CEED) University of Oslo P.O. Box 1028 , Norway, ⁵Professor, Department of Geoscience, Aarhus University, DK-8000 Aarhus C, Denmark, ⁶Museumcurator, Museum Salling – Fur Museum, Nederby 28, 7884 Fur, Dk, ⁷Senior researcher Ph.d. Fossil og Molermuseet, Museum Mors, Skarrahagevej 9, 7900 Mors., ⁸Head of department, Fossil og Molermuseet, Museum Mors, Skarrahagevej 9, 7900 Mors

The early Eocene Fur Formation of Denmark comprises a 60 meters thick diatomite holding over 200 volcanic ash layers related to the North Atlantic Igneous Province (NAIP) (Larsen 2003). The Fur Formation overlays the Ølst Formation (Stolleklint Clay) which includes both the Palaeocene-Eocene boundary and Palaeocene-Eocene Thermal Maximum (Schoon 2013, 2015). Intriguing is therefor outcrops in parts of the Fur Formation of ikaite pseudomorph termed glendonite, CaCO₃ . Sediment legit shows that the cold environment mineral ikaite, CaCO3 • 6H2O, formed shortly after deposition in relation to thick volcanic ash layers. (Hugget et al. 2005; Schultz 2009, 2014). The occurrence of glendonites indicates a period of cooling after the Palaeocene-Eocene Thermal Maximum possibly due to heavy volcanism (Brooks 2006). In the NAIP Palaeocene/Eocene Glendonites are also seen on Svalbard ( Spielhagen 2009). The interpretive powers are immense as Mo clay area Fossil Lagerstätte is unparalleled worldwide in terms of representing the initial diversification of the modern fish in the marine, offshore realm (Pedersen et al. 2011). It contains the first major post-K/T boundary diversification of marine fish in association with terrestrial organisms such as birds, insects and plants, all of which are extremely well-preserved, as has been demonstrated in pioneering works in the field of molecular palaeontology (Vinter 2008; Lindgren et al. 2012, 2014, 2017). The rich and diverse fossil content comprising both marine and terrestrial organisms makes the Mo clay area an outstanding window into the early Eocene biosphere, where fossil indicate high latitude migrations of particular faunal elements.

References
Brooks, K., 2006. When the sun died over northern Europe: the unique geology of Denmark’s inland islands. Geology Today 22, 180-186.

Huggett, J.M., Schultz, B.P,. Shearman, D.J., & Smith, A.J., 2005: The Petrology of Ikaite pseudomorphs and their diagenesis. Proceeding of the Geologists’ Association, 116, p. 207-220. (Peer reviewed)

Larsen, M.L., Fitton, J.G. and Pedersen, A:K., 2003: Paleogene volcanic ash layers in the Danish Basin: compositions and source areas in the North Atlantic Igneous province. Lithos 71, p. 47-80.

Lindgren, J., Uvdal, P., Sjövall, P. Nilsson, D.E., Schultz, B.P., & Theil, V., 2012: Molecular preservation of the pigment melamin in fossil melanosomes. Nature Communications 3, Article number:824, doi:10.1038. 7pp.

Lindgren, J,, Sjövall, P., Carney, R.M., Uvdal, P., Gren J.A., Dyke, G., Schultz, B.P., Shawkey, M.D., Barnes K.R., and Polcyn, M.J., 2014: Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles. Nature. 2014 Jan 8. (Peer reviewed) doi: 10.1038/nature12899.

Lindgren et.al, 2017, Biochemistry and adaptive colouration of an exceptionally preserved juvenile fossil sea turtle, Scientific Reports 7, Article number: 13324 (2017) doi:10.1038/s 41598-017-13187-5

Pedersen, G.K., Pedersen, S.A.S., Bonde, N., Heilman-Clausen, C., Larsen, L.M., Lindow, B., Madsen, H., Pedersen, A.K., Rust, J., Schultz, B.P., Storey, M. og Wilumsen, P.S., 2011: Molerområdets geologi – sedimenter, fossiler, askelag og glacialtektonik. Geologisk Tidsskrift p. 41-135.

Schoon, P.L., Heilmann-Clausen, C., Schultz, B.P., Damsté, J.S.S., Schouten, S., 2015: Warming and environmental changes in the eastern North Sea Basin during the Palaeocene–Eocene Thermal Maximum as revealed by biomarker lipids. Organic Geochemistry 78 (2015) 79–88. (Peer reviewed)

Schoon, P.L., Heilmann-Clausen, C., Schultz, B.P., Sluijs, A., Sinninghe Damsté, J.S., and Schouten, S., 2013: Recognition of Early Eocene global carbon isotope excursions using lipids of marine Thaumarchaeota, Earth and Planetary Science Letters 373, p.160-168. (Peer reviewed)

Schultz, B.P., 2009: Pseudomoroh after ikaite – called Glendonite is it a geological thermometer in cold sediments orgeological oddity as it occurs close to PETM in the Fur formation. IOP Conf. Series: Earth and Environmental Science 6 (2009) 072059 doi:10.1088/1755-1307/6/7/072059.

Schultz, B.P., Bonde, N., Willumsen, P.S., Sylvestersen, R., Tapdrup, J., Teichert, B., Nenning, F., Lindgren, J., & Schoon, P., 2014: Glimpse from northern hemisphere: life evolving during an ancient period of global heating and volcanic activity. Poster presentation. The IARU Sustainability Science Congress Global Challenges: Achieving Sustainability will take place in Copenhagen, Denmark from 22-24 October 2014.

Spielhagen R.F. , Tripati, A. , Evidence from Svalbard for near-freezing temperatures and climate oscillations in the Arctic during the Paleocene and Eocene, Palaeogeography, Palaeoclimatology, Palaeoecology 278 (2009) 48–56

Vinther, J., Briggs, E.G., Prum, R.O., and Saranathan, V., 2008: The Colour of fossil feathers. Biological Letters, Vol. 4, p. 522-525.

Lithological and geochemical variations during the PETM and later Eocene hyperthermals: details from a new drill core from Fur, Denmark.

Ella Wulfsberg Stokke¹, Morgan T. Jones¹, Øyvind Hammer² and Henrik Svensen^1
1Centre for Earth Evolution and Dynamics (CEED), University of Oslo, ²Natural History Museum, University of Oslo, PO Box 1172, 0318 Oslo, Norway

One of the most extreme episodes of global warming in Earth’s history is the Paleocene-Eocene thermal maximum (PETM), which lasted ~170 000 years from ~55.8 Ma, and was followed by several smaller hyperthermal events during the Eocene. The period was initiated by substantial carbon release to the ocean-atmosphere system, causing a global temperature increase of ~5-8°C. This global warming event coincided with the second pulse from the North Atlantic Igneous Province (NAIP), suggesting a possibly causal relationship. One of the best exposures covering the PETM interval is the Fur Formation and Stolleklint Clay, found on the island of Fur in northwest Denmark. Over 180 interbedded tephra layers of NAIP origin are found within this diatomite and clay rich sequence. The diatomite clays were deposited during the late Paleocene and early Eocene in a restricted, shallow marine basin with interchanging well laminated and structureless (strongly bioturbated) subsections reflecting alternating redox bottom conditions. During a recent fieldtrip as part of the Ashlantic project, a ~70m long core was drilled through this sedimentary succession in order to investigate the role of volcanism in the prolonged global warming and observed climatic changes during the PETM. In this contribution, we will present a new and detailed overview of the lithological and chemical variations through the Fur Formation, including data from high-resolution XRF-core logging. Our data give an improved insight to the PETM and post-PETM climatic perturbations, and help us understand the climatic response to ash deposition.

1.4. Tephra markers in the circum North Atlantic area

                       ORAL PRESENTATIONS                    

Explosive volcanic activity in Iceland during the Weichelian – Different from the Holocene?

Esther Ruth Guðmundsdóttir¹, Guðrún Larsen², Jón Eiríksson³ and Ólafur Ingólfsson^4
1Nordic Volcanological Center, Faculty and Institute of Earth Sciences, University of Iceland, ²Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, ³Institute of Earth Sciences, University of Iceland, ⁴Faculty and Institute of Earth Sciences, University of Iceland

Eruption frequency is high in Iceland and has been estimated to exceed 20 eruptions per century on average during the Holocene. The majority of the eruptions are mafic (~ 90%) and predominantly explosive (~70%).

For the past 10.000 years the explosive activity in Iceland is relatively well known based on numerous soil profiles and lake sediment archives across the country. What about during the Weichselian? How was the explosive volcanic activity in Iceland during this last glaciation? Was it similar to the Holocene? Which volcanoes were active and what was the level of their tephra producing activity?

To answer these questions, tephra in high-resolution lake sediments spanning 10.000 years and marine sediments extending 50.000 years have been investigated. Lake sediments in East Iceland (Lake Lögurinn), records 170 explosive eruptions during the Holocene. The marine sediments (core MD99 2272) from the North Icelandic shelf records about 200 tephra layers back to the middle Weichelian.

There are strong indications that volcanism in Iceland during the Holocene has been periodic at several timescales with apparent periods of 140, 500, and 4-5000 years. Preliminary results from this study reveal that the 4-5000 year periodicity extends as far back as 50.000 years, with eruption frequency peaks between 1-2000, 5-7000, 9-10.000, 12-14.000, ~16-18.000, ~22-23.000, ~32-33.000, 36-~37.000, ~45-46.000 and ~49-50.000 years.

Proximal tephras from São Miguel, Azores Islands, and their links with distal sites in Europe

Hans Johansson¹ and Stefan Wastegård^1
1Stockholm University

The Azores archipelago is one of the most active volcanic areas in the North Atlantic region, with approximately 30 eruptions during the last 600 years. The geochemical composition of associated tephra-derived glass is, however, not well characterized. It has recently been suggested that trachytic tephras found in distal areas such as North Africa, the British Isles and Greenland may derive from eruptions on the Azores, but proximal data from the Azores are scarce and the correlations have been tentative at best. These tephras lie clearly outside the geochemical Icelandic province and have a potential to improve the European tephro-stratigraphy/tephrochronology framework, especially in sites in south and central Europe where tephras from more than one European volcanic system have been found (e.g. Lane et al., 2015). Analyses of tephra glass from five mid and late Holocene eruptions on the Azores Islands were presented by Johansson et al. (2017) and there is a striking geochemical similarity between trachytic tephras from volcanoes on the São Miguel island and cryptotephras found on Ireland (e.g. Chambers et al., 2004). The dominant wind direction over the Azores is favourable for tephra dispersal to western Europe and we suggest that at least three tephras found in Ireland were erupted from the Furnas volcano, and that trachytic tephras erupted from São Miguel have a potential to contribute to the construction of a European-wide tephrostratigraphic framework.

References

Chambers, F.M., Daniell, J.R.G., Hunt, J.B., Molloy, K., O’Connell, M. 2004: Tephrostratigraphy of An Loch Mór, Inis Oírr, western Ireland: implications for Holocene tephrochronology in the northeastern Atlantic region. The Holocene 14, 703-720.

Johansson, H., Lind, E.M., Wastegård, S. 2017: Compositions of glass in proximal tephras from eruptions in the Azores archipelago and their links with distal sites in Ireland. Quaternary Geochronology 40, 120-128.

Lane, C.S., Brauer, A., Martín-Puertas, C., Blockley, S.P.E., Smith, V.C., Tomlinson,E. 2015. The Late Quaternary tephrostratigraphy of annually laminated sediments from Meerfelder Maar, Germany. Quaternary Science Reviews 122, 192-206.

Linking late Quaternary palaeoclimate records in the North Atlantic area: cryptotephras at Körslättamossen, S Sweden

Simon Larsson¹ and Stefan Wastegård^1
1Department of Physical Geography, Stockholm University

The spatial and temporal development of rapid climate shifts during the last deglaciation in the North Atlantic area is still uncertain (e.g. Davies et al. 2012). The method of tephrochronology has the potential to precisely link different palaeoclimate archives from sites separated by vast distances (e.g. Lane et al. 2017), which is particularly useful to assess the issue of the timing and spatial development of these climate events (e.g. Lowe 2001, Lane et al. 2011, Davies et al. 2012).

The Körslättamossen fen in Scania, southern Sweden—previously studied by Hammarlund & Lemdahl (1994)—was analysed for cryptotephras. Preliminary results indicate the presence of at least five separate cryptotephra layers (concentrations >500 shards/cm³), two of which have been geochemically analysed and identified as the Hässeldalen Tephra (HDT) and the Laacher See Tephra (LST). This is the first geochemically confirmed find of the LST on Swedish mainland, extending its dispersal area further northwest than in previous reconstructions and providing new insights to the dispersals of its several eruptive stages. The Körslättamossen fen appears to be a promising site for the usage of tephrochronology to link palaeoclimate archives in the circum North Atlantic area.

References

Davies, S.M., Abbott, P.M., Pearce, N.J.G., Wastegård, S. & Blockley, S.P.E. 2012: Integrating the INTIMATE records using tephrochronology: rising to the challenge. Quaternary Science Reviews 36, 11–27.

Hammarlund, D. & Lemdahl, G. 1994: A Late Weichselian stable isotope stratigraphy compared with biostratigraphical data: a case study from southern Sweden. Journal of Quaternary Science 9, 13–31.

Lane, C.S., Blockley, S.P.E., Ramsey, C.B. & Lotter, A.F. 2011: Tephrochronology and absolute centennial scale synchronisation of European and Greenland records for the last glacial to interglacial transition: A case study of Soppensee and NGRIP. Quaternary International 246, 145–156.

Lane, C.S., Lowe, D.J., Blockley, S.P.E., Suzuki, T. & Smith, V.C. 2017: Advancing tephrochronology as a global dating tool: Applications in volcanology, archaeology, and palaeoclimatic research. Quaternary Geochronology 40, 1–7.

Lowe, J.J. 2001: Abrupt climatic changes in Europe during the last glacial–interglacial transition: the potential for testing hypotheses on the synchroneity of climatic events using tephrochronology. Global and Planetary Change 30, 73–84.

Tephra stratigraphy in Iceland during the Holocene

Thor Thordarson¹ and Áslaug Geirsdóttir^1
1Faculty of Earth Sciences, University of Iceland

Tephrochronology plays a conspicuous role in paleoclimate studies across the North Atlantic because tephra horizons are excellent stratigraphic markers and correlation tools in Holocene sedimentary archives. Comprehensive knowledge of the tephra stratigraphy in Iceland is essential for understanding of the far-field chronologies and for tephra detection and discrimination. Over the last two decades we have seen momentous improvements in the Holocene tephra record in Iceland, especially because of the expansion of the traditional soil-based tephra studies to include glacial, lacustrine and marine sediment archives along with heightened awareness of the importance of tephra studies for volcanological and petrological research. This has been achieved via collective, inter-disciplinary effort involving several dedicated research groups and the key outcomes include: (a) a near complete high-resolution Holocene tephra stratigraphy, dating back to 8-12 ka for West, Central, South and Southeast Iceland, including the circumference of the Vatnajökull glacier, (b) a near-complete Holocene record of magma compositions produced by individual volcanoes, including detailed geochemical characterization of all major silicic tephra deposits, (c) a much improved, high precision analytical procedure for major and volatile element concentrations in volcanic glass, suitable for grains down to the size range of 3-10 mm, (d) a comprehensive geochemical procedure for tephra identification, (e) that some of the key far-field tephra marker horizons previously conceived to be the product of one eruption, are in fact comprised of several tephra layers produced over periods of few hundred year. I intend to present a comprehensive review these research endeavors and their outcomes.

Tephrochronology of North Europe: A look back and prospects for the future

Stefan Wastegård^1
1Institut för naturgeografi, Stockholm
Chronological control and the resultant ability to examine the degree of synchrony among records of different origin are critical for the understanding of climate and environmental variability. There is an increasing demand for exact time markers in the palaeoclimate

community as more investigations now aim at high temporal resolutions. Tephrochronology, which is an age-equivalent dating method, exploits these time-synchronous markers and offers a unique possibility to test hypotheses regarding synchronous or non-synchronous responses to climate forcing. Few, if any geochronological methods can match the precision it offers both temporally and spatially. A new generation of tephrochronologists has been undertaking novel research into cryptotephras (layers of nonvisible volcanic ash encompassed within sediments) in ice-cores, the North Atlantic, the Mediterranean and several adjacent areas. The range of tephrochronology has radically been extended into geographic areas not previously considered suitable for this approach and many more records can now be integrated in a ‘tephra lattice’ for precise linking of sequence. In this talk, I will give a comprehensive overview of the state-of-art of tephrochronology of North Europe and also present some prospects for the future.

                       PSOTER PRESENTATIONS                    

Holocene tephra stratigraphy in the Vestfirðir peninsula, NW Iceland

Esther Ruth Guðmundsdóttir¹, Anders Schomacker², Skafti Brynjólfsson³, Ólafur Ingólfsson⁴ and Nicolaj K Larsen^5
1Nordic Volcanological Center, Faculty and Institute of Earth Sciences, University of Iceland, ²Department of Geosciences, UiT The Arctic University of Norway, Tromsø; 3Centre for GeoGenetics, Nat, ³Icelandic Institute of Natural History, Akureyri, Iceland, ⁴Faculty and Institute of Earth Sciences, University of Iceland, ⁵Department of Geoscience, Aarhus University, Denmark; 3Centre for GeoGenetics, Natural History Museu

In recent years the terrestrial tephra stratigraphy in Iceland has been vastly improved through detailed investigations of soil and lake sediment archives revealing hundreds of tephra layers. These studies have focused on areas in south, north, east and central parts of Iceland. Fewer studies have been carried out in west Iceland. The aim of this study was to significantly improve the knowledge of tephra stratigraphy and tephrochronology in western Iceland by investigating the sedimentary records from eight lakes in the northernmost part of the Vestfirðir peninsula. In these eight lake sediment records spanning the Holocene, 39 tephra layers have been identified, thought to represent 34 eruptive events originating from five volcanic systems; Hekla, Katla, Snæfellsjökull, Grímsvötn and Veiðivötn-Bárðarbunga. Of these 39 tephra layers, 34 have not been reported before in the Vestfirðir peninsula. Six have recently been reported in the area; the Hekla 1693 tephra (Brynjólfsson et al., 2015) Snæfellsjökull Sn-1 tephra, Hekla 3 tephra, Hekla 4 tephra, Brattihjalli tephra (Schomacker et al., 2016) and Saksunarvatn tephra (Schomacker et al., 2016; Harning et al., 2016). Here we propose that the Brattihjalli tephra is in fact the 6060 year old Hekla Ö tephra marker layer, demonstrating that the Hekla Ö tephra extended much further to the west than previously reported, covering about 2/3 of the country. Thus, the Hekla Ö tephra covers an as large area in Iceland as the Hekla 5, Hekla 4, Hekla 3 and Hekla 1104 tephras, emphasizing the importance of Hekla Ö as a chronological marker.

References

Brynjólfsson S., Schomacker, A., Gudmundsdóttir, E.R. & Ingólfsson, Ó. 2015: A 300-yrear surge history of the Drangajökull ice cap, northwest Iceland, and its maximum during the “Little Ice Age”. The Holocene 1-17.

Schomacker, A., Brynjólfsson, S., Andreassen, J.M., Gudmundsdóttir, E.R., Olsen J., Odgaard, B.V., Hakansson, L., Ingólfsson, O., & Larsen, N.K. 2016: The Drangajökull ice cap, northwest Iceland, persisted into the early-mid Holocene. Quaternary Science Reviews, 148, 68-84.

Harning, D.J., Geirsdóttir, Á., Miller, G.H. & Anderson, L. 2016: Episodic expansion of Drangajökull, VEsfirðir, Iceland, over the last 3 ka culminating in its maximum dimension during the Little Ice Age. Quaternary Science Reviews 152, 118-131.

Timing of early Holocene explosive eruptions in Iceland – improved tephrochronology

Maarit Kalliokoski¹, Esther Ruth Guðmundsdóttir², Bergrún Arna Óladóttir¹, Hreggviður Norðdahl³ and Ívar Örn Benediktsson^3
1Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Iceland, ²NordVulk, Faculty of Earth Sciences, Institute of Earth Sciences, University of Iceland, Iceland, ³Institute of Earth Sciences, University of Iceland, Iceland

The Icelandic tephrochronology is an important tool for dating and correlating sedimentary archives, landforms and archaeological sites both in Iceland and overseas. A robust tephrochronology is also essential for deriving information on eruption frequency patterns. However, the early Holocene part of the Icelandic tephrochronology lacks securely dated tephra marker horizons. Ages of the tephra layers between the Hekla 5 and Saksunarvatn tephras in soil profiles and peatlands have previously been dated only indirectly by interpolating soil or sediment accumulation rates between tephra layers of known age. As this period represents about one third of the Holocene, it is critically important to establish new tephra markers within this part of the tephrochronology. The detailed tephra stratigraphy from Lake Lögurinn, East Iceland (Gudmundsdóttir et al. 2016) was used as an aid in selecting potential tephra markers in soil sections, lake sediments and peatlands in East, Northeast and North Iceland. To qualify as a tephra marker, a tephra needs to have at least regionally significant dispersal area, it must be securely dated and harbour characteristics which distinguish it from other tephras close in age (Larsen and Eiríksson 2008). Thus, new marker tephras for the early Holocene are established by electron probe microanalysis of tephra geochemistry, careful comparison of tephra stratigraphy between sites and radiocarbon dating of selected tephra layers.

References

Gudmundsdóttir, E.R., Larsen, G., Björck, S., Ingólfsson, Ó. & Striberger, J. 2016: A new high-resolution Holocene tephra stratigraphy in eastern Iceland: Improving the Icelandic and North Atlantic tephrochronology. Quaternary Science Reviews 150, 234–249.

Larsen, G. & Eiríksson, J. 2008: Late Quaternary terrestrial tephrochronology of Iceland – frequency of explosive eruptions, type and volume of tephra deposits. Journal of Quaternary Science 23, 109–120.

1.5. New trends in the study of the crystallisation and evolution of layered mafic complexes

                       ORAL PRESENTATIONS                    

Strontium isotopes in plagioclase record magma chamber dynamics of the Skaergaard intrusion

Graham Hagen-Peter¹, Christian Tegner¹, Peter Thy² and Charles Lesher^1
1Centre of Earth System Petrology, Aarhus University, Denmark, ²Department of Earth and Planetary Sciences, University of California, Davis, U.S.A.

We report Sr isotope data obtained by laser-ablation multi-collector ICPMS for individual plagioclase crystals from the Skaergaard intrusion. Guided by micro-XRF petrography, we targeted cores and rims of primocrysts, as well as interstitial plagioclase from all major zones and subzones of the Layered Series (LS), Upper Border Series (UBS), and from the Sandwich Horizon (SH) and transgressive granophyre. Current analytical protocol enables ⁸⁷Sr/⁸⁶Sr determination with precision of 1 part in 10⁴ using a Nu Plasma II mass spectrometer and RESOultion 193 nm excimer laser with a ~150-um-diameter beam. Initial ⁸⁷Sr/⁸⁶Sr ratios (weighted means for individual samples) range from ~0.7041 to 0.7045 for plagioclase ranging from 25–70 An%. There are correlations between ⁸⁷Sr/⁸⁶Sr and An%, with the largest variation in ⁸⁷Sr/⁸⁶Sr among An-rich plagioclase. Stratigraphic relations show a progressive increase in ⁸⁷Sr/⁸⁶Sr from ~0.7041 at the base of Lower Zone A to ~0.7044 at the top of Lower Zone B, a value that is essentially uniform up stratigraphy through the SH. The UBS mirrors the LS, but also contains radiogenic An-rich plagioclase (⁸⁷Sr/⁸⁶Sr up to ~0.7045) near the roof of the intrusion in contact with Tertiary volcanic rocks. In the field these samples are associated with rafts of partially assimilated Precambrian gneiss.  The plagioclase Sr isotopic record suggests an important role for in situ AFC processes during early stages of Skaergaard evolution that diminished with inward solidification and insulation of the magma reservoir from country rocks and, in particular, the roof zone of the UBS containing rafts of basement material.

Magmatic fractionation by cumulate rock formation with expulsion of residual melts: a case from the Tertiary Kærven Syenite Complex, East Greenland

Paul Martin Holm¹ and Niels-Ole Prægel^2
1University of Copenhagen, ²Copenhagen University Library

The evolution of silicate melts by crystal fractionation processes in magma chambers is relatively well understood from experimental petrology, thermodynamic modelling, and suites of volcanic rock. However, from studies of plutonic complexes it is evident that our understanding of the physical processes acting in magma chambers during the crystallisation of magmas, and which are believed responsible for the evolution of silicate melts, needs further clarification, even with respect to quite fundamental issues. We present new major and trace element data from an intrusive unit of the Tertiary Kærven Syenite Complex, East Greenland. The geochemistry of the syenitic rocks of the WK-2 unit displays ambiguous indications for their formation. On one hand accumulation of a gabbroic mineral assemblage is indicated from linear correlation of several elements, and on the other hand an alkali feldspar dominated assemblage significantly affects the rock compositions as shown by high Eu/Eu*, Ba and K/Rb. A simple liquid line of descent by fractional crystallisation executed by crystal settling or open system wall rock crystallisation cannot explain the geochemical variation. Instead, we present a crystallisation model for the WK-2 unit consisting of a) partial crystallisation of an isolated mush during which the liquidus assemblage changes from gabbroic to alkali feldspar syenitic, b) expulsion of a fraction of the residual liquid, and c) final crystallisation of the remaining liquid fraction. The model is in line with the in the situ crystallisation of Langmuir (1989), and also, with variations, apply to some other KSC intrusive units.

References

Langmuir, C.M. (1989) Geochemical consequences of in situ crystallization. Nature 340, 199-205.

Differential migration of interstitial immiscible liquids in the Skaergaard Layered Series

Marian Holness¹, Gautier Nicoli¹ and Victoria Honour^1
1Dept. Earth Sciences, University of Cambridge

The liquid line of descent of the Skaergaard magma intersects a binodal creating an immiscible conjugate pair comprising a dense Fe-rich liquid and a buoyant Si-rich liquid. These two liquids have different wetting properties: the Si-rich liquid wets plagioclase, whereas the Fe-rich liquid wets oxides, pyroxene and olivine. The two liquids may therefore undergo differential migration within a gabbroic crystal mush: the Fe-rich liquid sinks and accumulates in mafic layers, while the Si-rich liquid rises and accumulates in plagioclase-rich regions. Microstructural evidence demonstrates that differential migration occurred in rapidly deposited modally graded layers from LZc upwards in the Layered Series. The melanocratic bases of modally graded layers in UZa develop finely-spaced internal layering, consistent with patterning driven by different wetting properties of the immiscible interstitial liquid.

Evidence for differential migration is also recorded by mafic and felsic rims developed on the top and bottom margins of anorthositic roof blocks in LZc. Highly tabular blocks have an upper mm-thick mafic rim and a lower leucocratic rim. As the block aspect ratio decreases, the rims disappear, with the mafic rim retained at lower aspect ratios than the leucocratic rim. These rims are formed as the Fe-rich interstitial liquid sinks through the mush until it reaches the relatively impermeable blocks, whereas the Si-rich liquid rises and ponds beneath them. Tabular blocks are most effective at trapping these liquids.

These observations demonstrate the complexity of behaviour in a crystal mush containing two-phase interstitial liquid. Simple cumulate paradigms do not apply in such a system.

Sedimentary layering in mafic intrusions: the Skaergaard trough bands

Marian Holness¹, Zoja Vukmanovic¹, Jens Andersen², Gautier Nicoli¹, Sam Weatherley³, Brendan Dyck⁴ and Victoria Honour^1
1University of Cambridge, ²Camborne School of Mines, University of Exeter, ³Geological Survey of Denmark and Greenland, ⁴Simon Fraser University, Vancouver

The trough bands of UZa of the Skaergaard Intrusion comprise individual stacks of upwardly concave, crescentic, modally graded layers, forming elongate structures pointing towards the intrusion centre from the nearby vertical wall on the western part of the chamber floor. Their origin is contentious, with some advocating a sedimentary origin while others argue for post-accumulation formation via recrystallization and metasomatism primarily triggered by localised gravitationally-driven compaction.

Crescentic modally graded layers are present throughout the Layered Series below UZa. Although they do not commonly form extensive stacks, their bases are clearly erosional surfaces, attesting to their sedimentary origin. Similar erosional surfaces are commonly preserved in the UZa trough bands, associated with changes in axial positon or width of the trough. The UZa trough bands occur within a ~300m thick stratigraphic interval containing abundant highly elongate prismatic plagioclase grains that we argue were derived from collapse of the wall mushy layer. Detailed microstructural analysis of the modally graded layers in the trough bands demonstrates a locally developed strong mineral lineation parallel to the trough axis, together with morphological evidence for deposition of individual euhedral grains. Evidence from consideration of dihedral angle variation and the morphology of late-stage gabbroic pegmatites suggests that the floor mush near the wall at UZa times was anomalously thick. We argue that trough bands are a normal component of sedimentation on the floor of the chamber, and that the unusually extensive stacks of trough bands in UZa are a consequence of temporary overthickening and steepening of the floor.

Late-stage melt migration in the Skaergaard Intrusion

Victoria Honour¹, Marian Holness¹, Samuel Weatherley², Gautier Nicoli¹, Brendan Dyck³ and Jens Andersen^4
1University of Cambridge, ²Geological Survey of Denmark and Greenland, ³Simon Fraser University, ⁴University of Exeter

The Skaergaard Intrusion preserves a variety of late-stage melt structures such as gabbroic pegmatites, paired felsic and mafic lenses (interpreted to be solidified coarsened immiscible emulsions), dendritic anorthosites, and melanogranophyres. Previous work on such structures has focused on particular localities in the intrusion, treating each structure in isolation.

We have established relationships between the different melt migration structures, linking morphological and compositional variation to the physical properties of the crystal mush and the progressive evolution of the bulk magma. Gabbroic pegmatite morphology varies through the floor cumulates due to changes in mush rheology (Larsen & Brooks, 1994), consistent with newly developed constraints on mush thickness. Pegmatites are podiform in thick mush but planar in thin mush. They are spatially associated with paired felsic and mafic lenses, which occur 5-10 metres stratigraphically above pegmatites in LZ, but at the same stratigraphic horizon in UZ. The paired lenses form irregular, approximately layer-parallel clusters in thick mush, but thin concordant dendritic structures within strongly foliated modally graded layers. The modal mineralogy of the paired lenses evolves upward consistent with evolution down the binodal: below LZc the mafic component comprises olivine pyroxenite, but is dominated by oxides above LZb. In UZb pegmatites are melanogranophyric, and paired lenses are confined to localised horizons recording significant syn-magmatic disruption, consistent with the inferred evolution of the bulk magma towards the Si-rich side of the binodal.

Our observations demonstrate that mush physical properties control the movement of interstitial liquids, with wide-ranging implications for our understanding of sub-volcanic processes.

References
Larsen, Rune B., and Brooks, Kent C., 1994, “Origin and evolution of gabbroic pegmatites in the Skaergaard intrusion, East Greenland.” Journal of Petrology 35(6), 1651-1679.

Skaergaard gabbros: Fractionation and dissolution in crystal mush

Troels Nielsen^1
1Geological Survey of Denmark and Greenland

Recent work on the macro-rhythmic (MR) layered gabbros in the Skaergaard Intrusion that host the PGE-Au mineralisation shows that their evolution involved dissolution of early-formed liquidus phases and the convective loss of interstitial liquid (mush melt). An early formed liquidus paragenesis (P1) was replaced by a gabbroic paragenesis (P2) that crystallised from mush melt and was out of equilibrium with the contemporary bulk liquid. P2 constitutes up to 50 vol. % of the sampled gabbro and hosts droplets of immiscible sulphide melt.

Melts on the liquid line of descent reach the two-liquid field and unmixed in Fe- and Si-rich silicate melts when the intrusion was 90 % solidified. The mineralization formed after 75% crystallisation and MR layers of the mineralisation would retain ~25 wt. % dense and Fe-rich melt after crystallisation of 60% of its mass and a convective loss of ~15 wt % Si-rich melt.

In bulk MR rocks, FeO and P₂O₅ are negatively correlated. Gabbros rich in P2 are poor in P₂O₅, except for the main Au mineralisation and samples with granophyric veins. Using a SOM (self-organizing map) analysis, Au Ce, Y, Nb and Pb are shown to follow P₂O₅, and Au is added along grain boundaries to layers already enriched in precious metal bearing droplets of sulphide melt.

The present-day gabbros represent the sum of processes acting between the liquidus and subsolidus in semi-closed, proto-MR layers. Mush melts fractionated until their evolved residuals were re-mixed into  and drove the evolution ofremaining bulk liquid.

Untangling the formation of adcumulate rocks in layered intrusions: a microstructural study of the Skaergaard and Bushveld intrusions

Zoja Vukmanovic¹ and Marian Holness^1
1Department of Earth Sciences, University of Cambridge

Despite decades of study, the formation of adcumulates remains a topic of discussion in the layered intrusion community. Models for the formation of adcumulates broadly fall into two groups: 1) those based on a primary origin by the growth of primocrysts in chemical communication with the overlying bulk magma (Wager et al., 1960); and 2) those based on two-phase flow, whereby the evolved melt is expelled from the crystal mush by gravitationally-driven compaction (McKenzie, 2011, Meurer & Meurer, 2006; Sparks et al., 1985; Tegner et al., 2009).

To resolve this issue, we performed a detailed microstructural examination of two different examples: the Layered Series of the Skaergaard Intrusion, and anorthosites from Upper Zone of the Bushveld intrusion. We analysed grain orientation by electron backscatter diffraction, and compared this with mineral chemistry. Plagioclase in the Skaergaard shows almost no evidence for crystal plasticity, and no evidence for dissolution-reprecipitation. We argue that Skaergaard gabbros underwent negligible compaction and therefore that the adcumulates are primary. In contrast, Bushveld anorthosites show ample evidence for dislocation creep such as low angle boundaries, and neoblasts formed during dynamic recrystallisation. However, the extent of deformation is the same for anorthosites both immediately below and above metre-scale magnetitite layers, suggesting that the deformation is likely to be due to regional slumping, rather than internally-generated buoyancy forces.

Our results suggest that compaction is not the main mechanism that drives the formation of adcumulates but, when present, can result in a minor medication of the primary igneous fabric.

References
McKenzie, D. (2011) Compaction and crystallization in magma chambers: towards a model of the Skaergaard Intrusion. Journal of Petrology 52, 905-930.

Meurer, W. & Boudreau, A. (1998) Compaction of igneous cumulates part II: compaction and the development of igneous foliations. The Journal of Geology 106, 293-304.

Sparks, R. S. J., Kerr, R. C., McKenzie, D. P. & Tait, S. R. (1985) Postcumulus processes in layered intrusions. Geological Magazine 122, 555-568.

Tegner, C., Thy, P., Holness, M. B., Jakobsen, J. K. & Lesher, C. E. (2009) Differentiation and Compaction in the Skaergaard Intrusion. Journal of Petrology 50, 813-840.

Wager, L. R., Brown, G. M. & Wadsworth, W. J. (1960) Types of igneous cumulates. Journal of Petrology 1, 73-85.

Combined ‘sedimentary’ and in-situ origin for magmatic Fe-Ti-V deposits: new insights from the Skaergaard intrusion, East Greenland

Samuel Weatherley¹, Jens C. Ø. Andersen², Marian B. Holness³, Brendan J. Dyck⁴, Gautier Nicoli³ and Victoria C. Honour^3
1GEUS, Øster Voldgade 10, 1350 Copenhagen K. Denmark, ²Camborne School of Mines, University of Exeter, Penryn Campus, Penryn, Cornwall, TR10 9FE. UK, ³Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge C2 3EQ. UK, ⁴Department of Earth Sciences, Simon Fraser University, Burnaby, V5A 1S6, Canada

New field observations from Skaergaard, East Greenland reveal five 0.5–2m thick, oxide-rich intervals within a ~30 m thick sequence near the top of LZc. Individual intervals are continuous along strike from the centre of the intrusion to the margin. This sequence exhibits density-sorted modal layering defined by basal concentrations of Fe-Mg rich phases (including magnetite and ilmenite) and top-of-interval concentrations of plagioclase. The layering style develops from being wispy and laterally discontinuous close to the margin, to strongly defined and laterally continuous at the centre. On-lapping, layer-truncation and channel-like features are commonly observed. The oxide-rich horizons are commonly associated with stoped wall and roof blocks, up to 400 m long, which are oriented with long-axes parallel to the floor. In the size range 10^-1–10¹ m, the tops of high aspect ratio blocks exhibit cm-thick oxide rims; the undersides exhibit cm-thick rims of leucocratic (felsic) material. These paired rims are absent from low-aspect ratio blocks. Oxide-rich seeps, discordant to layering, are also commonly observed.

On the basis of these observations we propose a combined ‘sedimentary’ and in-situ magmatic origin for Fe-Ti-V deposits whereby (1) newly-crystallised, gravitationally unstable material, was removed from the wall and deposited on the floor; the deposition process resulted in density-sorted modal layers, and (2) liquid immiscibility in the mush produced conjugate Fe-rich silicate and felsic liquids that enhanced the primary modal layering, and led to the oxide-rich seeps and paired rims on blocks. This model is compatible with the absence of LZc on Skaergaard’s western margin.

1.6. Alkaline magmatism, carbonatites and associated mineral resources.

                       ORAL PRESENTATIONS                    

Clinopyroxene as a petrogenetic indicator: using integrated trace element and zoning profiles to reveal magma chamber processes in the Ditrău alkaline magmatic system

Anikó Batki¹, Elemér Pál-Molnár², M. Éva Jankovics³, Andrew C. Kerr⁴, Balázs Kiss¹ and Gregor Markl^5
1MTA-ELTE Volcanology Research Group, 1/C Pázmány Péter Street, H-1117 Budapest, Hungary, ²Department of Mineralogy, Geochemistry and Petrology, University of Szeged, Hungary, ³MTA-ELTE Volcanology Research Group,1/C Pázmány Péter Street, H-1117 Budapest, Hungary, ⁴School of Earth and Ocean Sciences, Cardiff University, Main Building, Cardiff CF10 3AT, UK, ⁵Fachbereich Geowissenschaften, Universität Tübingen, Wilhelmstrasse 56, D-72074 Tübingen, Germany

Clinopyroxene is considered as a significant petrogenetic indicator that can be used to unravel the evolution of magmatic systems. EMPA and LA-ICP-MS have been used to analyse major and trace element compositions of diverse clinopyroxene crystals in seven related rock types of the alkaline igneous suite of the Ditrău Alkaline Massif, Romania.

Phenocryst and antecryst populations from high-Cr Fe-diopside to high-Zr aegirine-augite have been distinguished based on mineral zoning patterns and geochemical characteristics. Clinopyroxenes record two major magma sources as well as distinct magma evolution trends. The diopside population is derived from an early camptonitic magma related to basanitic parental melts, whilst a group of diopside-hedenbergite crystals represents a newly identified Na-, Nb- and Zr-rich magma source for the the Ditrău magmatic system. This magma fractionated towards ijolitic and later phonolitic compositions with pronounced HREE, Nb, Zr and Hf enrichment along with increasing Na/Ca ratio.

Field observations, petrography and clinopyroxene-melt equilibrium calculations reveal open- and closed-system magma chamber processes that played a role in the magmatic evolution: magma recharge and mingling, pyroxene recycling, fractional crystallisation and accumulation. Repeated recharge events of the two principal magmas resulted in multiple interactions between more primitive and more fractionated co-existing magma batches.

Investigation of complex zoning characteristics and equilibrium melt calculations show that antecryst recycling is a significant process during magma recharge and demonstrate that incorporated crystals can significantly affect the host magma composition and so whole-rock geochemical data should be interpreted with great care.

Probing the structural state of Y and Nd in eudialyte using X-ray absorption spectroscopy

Anouk M. Borst¹, Platon N. Gamaletsos², Adrian A. Finch¹, Nicola J. Horsburgh¹, Henrik Friis³, Takeshi Kasama², Joerg Goettlicher⁴, Ralph Steiniger⁴ and Kalotina Geraki^5
1School of Earth and Environmental Sciences, University of St Andrews, United Kingdom, ²Center for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark, ³Natural History Museum, University of Oslo, Oslo, Norway, ⁴ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Ge, ⁵Science Division, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, United Kingdo

Eudialyte group minerals (EGM) are important rare earth-bearing alkali-zirconosilicates in agpaitic syenites. Alongside c.12 wt.% ZrO₂ and 1-2 wt.% Nb₂O₅, EGM host c.1-10 wt.% total rare earth oxides with relatively high proportions of valuable heavy REE (c.35%). The EGM structure accommodates REE in various sites, mostly inferred to occupy the 6-fold coordinated Ca-dominated M1-site, or the low symmetry 8-9-10-fold coordinated Na-site. However, XRD refinement is relatively insensitive to REE site allocation, and HREE and LREE may occupy different sites due to their varying ionic radii. How REE are distributed on the micro-/nanoscale, particularly following fluid interaction, is poorly known. We use synchrotron radiation X-ray absorption spectroscopy to study the structural state and distribution of LREE and HREE in EGM and their alteration products. We collected Y K-edge and Nd L_III-edge µ-XANES and μ-EXAFS to quantify coordination numbers and bond distances, with Y and Nd as proxies for HREE and LREE respectively. Micro-XRF element mapping combined with SEM and TEM provide insights in the micro-/nanoscale REE distribution in EGM and its alteration products. Crystalline (XRD-confirmed) EGM from Ilímaussaq, Narssârssuk, Norra Kärr, Kipawa and Lovozero yield near-identical µ-XANES for Nd and Y, indicating similar LREE/HREE site occupations in EGM of varying compositions. Preliminary Y EXAFS processing of the Narssârssuk EGM yield optimal fits in 6-fold coordination and Y-O bond distances of c. 2.26-2.32 Å, consistent with Y substitution for Ca on the octahedral M1-site. Cerium absorption in the Nd L_III pre-edge region demonstrates minor variations in Ce-oxidation state across the localities.

The Spatio-Temporal Evolution of Hf isotopes in the Gardar: Evidence for Mid-Proterozoic Crustal Recycling?

Adrian A Finch¹, Anouk M Borst¹, William Hutchison¹, Nicola J Horsburgh¹, Tom Andersen² and Siri L Simonsen^2
1University of St Andrews, ²University of Oslo

The Gardar Province of Greenland refers to alkaline magmatism from intraplate rifting in the Mesoproterozoic. To understand further the sources of heavy REE and HFSE in the Gardar, we analysed the Lu-Hf isotopes of several Gardar centres, representing early (Motzfeldt, North Motzfeldt, Ivigtut) and late (Ilímaussaq, Østfjordsdal, Narsarsuk, Nunarsuit, Paatusoq) Gardar magmatism and the geographical extent of the Province from Ivigtut in the West to Paatusoq in the East. Gardar magmas intruded Archaean crust (Ivigtut), Ketilidian metasediments (Paatusoq) and Ketilidian granitoids (the others).

Age-corrected Hf isotopes show low Hf values inconsistent with sourcing from depleted mantle in Gardar times. Early Gardar zircons have significantly lower age-corrected Hf than those of the Late Gardar. Hf signatures in Early Gardar zircons project back to Ketilidian or older mantle extraction ages (> 1.8 Ma). One might interpret such data to indicate that Hf in the Early Gardar zircons was scavenged from Ketilidian basement. However, primary Gardar melts are unlikely to have contained negligible Hf and an unrealistic proportion of assimilated Ketilidian Hf is required. However the data are consistent with a model whereby subduction of Archaean crust (i.e. very low ¹⁷⁶Hf/¹⁷⁷Hf) during the Ketilidian enriched the subcontinental lithospheric mantle with Archaean Hf. In Gardar times, mantle melting preferentially accessed this subducted (Archaean) mantle component, providing magmas with anomalously low Hf isotopic ratios. As rifting continued, proportionately more contemporary Hf was present in the melts. The model suggests recycling of Archaean crust via the mantle in Gardar times.

Geochemical constraints on the formation of the Archean Siilinjärvi carbonatite-glimmerite complex, Fennoscandian shield

Esa Heilimo¹, Hugh O’Brien¹ and Pasi Heino^2
1Geologica Survey of Finland, ²Yara Suomi Oy, Siilinjärvi mine

With an age of 2610 ± 4 Ma the Siilinjärvi carbonatite-glimmerite complex in Finland is one of the oldest known carbonatites and the oldest being currently mined for phosphorous. It was emplaced in the Karelia Province following cratonic stabilization. The Siilinjärvi complex consists of carbonatite, calcio-carbonatite, carbonatite-glimmerite, and glimmerite surrounded by a fenite, forming a tabular body 16 km long and 1.5 km wide. The complex is affected by north-south Palaeoproterozoic steep shearing. Carbonatite occurs as thin (several cm) roughly vertical lenses in glimmerite that forms thicker (10 cms) vertical veins. Despite the large carbonate-phlogopite modal variablity, compositions of the phases calcite, dolomite, tetraferriphlogopite, apatite and richterite do not vary significantly across the complex. The distribution of apatite is rather uniform, with average glimmerite and carbonatite containing 10.4 and 9.9 modal % apatite, respectively. Compositionally the carbonatite veins are calcio-carbonatites, whereas the glimmerites are potassic ultramafic rocks with Mg# over 0.8. All are cumulates, but are geochemically linked, showing similar trace element trends. Average C-O isotopic compositions are δ¹³C=3.7‰ and δ¹³O=7.4‰. The geochemical data indicate that the carbonatite and glimmerite are cogenetic with fractionation processes possibly playing a significant role in their genesis. Isotopic compositions show relatively uniform patterns and, we propose that the carbonatite and related lithologies are evolved from primitive carbonatite liquids produced by low degree melting of a carbonated mantle source. We propose that the complex has represents a relatively deep well mixed magma chamber with consequent accumulation and crystallization, feeding magmatic activity at higher levels.

Fluid-rock reactions in the carbonatites of the 1.3 Ga Grønnedal-Íka alkaline complex, Southwest Greenland

Eemu Ranta¹, Gabrielle Stockmann², Thomas Wagner³, Tobias Fusswinkel³, Erik Sturkell⁴, Elin Tollefsen² and Alasdair Skelton^2
1Nordic Volcanological Center, University of Iceland, Reykjavik, ICELAND., ²Department of Geological Sciences, Stockholm University, Stockholm, SWEDEN, ³Institute of Applied Mineralogy and Economic Geology, RWTH Aachen University, GERMANY, ⁴Department of Earth Sciences, University of Gothenburg, SWEDEN

Petrogenetic studies of carbonatites – carbonate-dominated igneous rocks – are challenging because carbonatite mineral assemblages and mineral chemistry typically reflect variable pressure-temperature conditions during crystallization, and fluid-rock interaction caused by magmatic-hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid-rock interaction history of carbonatites.

Our study focuses on the 1.3 Ga Grønnedal-Íka carbonatite-syenite complex in Southwest Greenland. The alkaline complex marks the onset of the Gardar continental rifting event, during which it was subsequently intruded by basaltic dikes and underwent extensive faulting. We present LA-ICP-MS trace element data for magnetite, calcite, siderite and ankerite-dolomite from the iron-rich carbonatites of Grønnedal-Íka, and use this data, in combination with detailed cathodoluminescence imaging, for identifying magmatic and secondary geochemical fingerprints preserved in these minerals.

The chemical and textural gradients point toward a 55 m wide basaltic dike as the focal point of a fossil hydrothermal fluid flow system, providing heat for F and CO₂ rich fluids that mobilized LREEs, Nb, Ta, Ba, Sr, Mn and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40 m from the dike contact, where a high-grade magnetite mineralization was formed through oxidation of siderite. Our results can be used for discriminating between primary magmatic minerals and later alteration related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed.

A review of the Transitional Layered Kakortokite (TLK), IlÍmaussaq Complex, South Greenland

Hans Kristian Schønwandt¹, Gregory B. Barnes¹ and Thomas Ulrich^2
1G.B. Barnes & Associates, 47 Labouchere Road, South Perth, W.A. 6151, Australia, ²Department of Geoscience, Aarhus University, Denmark

Kakortokite represents the lowest member of the peralkaline magmatic stratigraphy of the Mesoproterozoic IlÍmaussaq Complex, South Greenland. Kakortokite is an agpaitic nepheline syenite of which the lowest part shows spectacular layering. Kakortokite is originally subdivided into three parts: (1) A lower layered kakortokite (LLK) about 200 m thick, (2) slightly layered kakortokite (SLK) about 35 thick, which is overlain by (3) transitional layered kakortokite(TLK) with a thickness of about 40 m. The LLK consists of three-layered units composed of black, red, and white layers formed by modal enrichment of arfvedsonite, eudialyte and feldspar-nepheline respectively. SLK is almost unlayered and appears as a grey kakortokite, locally with faint black layers. TLK represents a return to the black-red-white-layered units.

TLK is best exposed north-west of the proposed fault along Lakseelv where it gradually grades into the overlaying lujavrite. In this section, nine eudialyte-enriched layers occur named A to I in descending order from the contact to the lujavrite. The uppermost four layers (A-D) occur as a complex intergrowth of several rock types, which pinch and swell and include eudialyte-rich and naujaite-like rock. These layers are not a simple modal enrichment of eudialyte, but are interpreted by us as a reaction between naujaite slabs and kakortokite magma. The remaining layers (E-I) occur as faint layers in grey kakortokite and represent a local modal enrichment of eudialyte. Therefore, we suggest the kakortokite stratigraphy to be described as LLK (≥ 200 m) overlain by SLK (≈ 75 m).

Alkaline pegmatites of the Larvik Plutonic Complex – what are they?

Øyvind Sunde¹, Henrik Friis¹ and Tom Andersen^2
1Natural History Museum, University of Oslo, P.O. Box 1172, Blindern, 0318 Oslo, Norway, ²Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway

The Larvik Plutonic Complex is a differentiated alkaline complex situated in the Permian Oslo Region and consists of a series of semi concentric ring intrusions of monzonitic rocks (Petersen 1978; Neumann 1980). The monzonitic rocks are collectively known as larvikite and extensively quarried as a dimension stone. Within the host plutons numerous nepheline syenite and syenite pegmatites, as well as minor nepheline syenite dykes occur with an ambiguous relationship and contrasting mode of emplacement. Although many of these pegmatites have been extensively studied for their mineral content and provided first descriptions of 30 mineral species, the petrological evolution of the pegmatites is still poorly understood. Different classification schemes have been devised to relate and compare contrasting pegmatite features such as morphology, mineralogical zonation, and rock textures (e.g., Brogger 1890; Piilonen et al. 2012). However, such differences are not directly related to petrogenetic similarities between these pegmatites. Wöhlerite, ideally NaCa₂(Zr,Nb)(Si₂O₇)(O,OH,F)₂, occurs abundantly among the early liquidus mineral assemblages of the nepheline syenite pegmatites. The flexible crystal structure of wöhlerite is able to incorporate cations of different size and valence, and thus record contrasting pegmatite-forming magmas.

This contribution provides a regional comparison of wöhlerite major- and trace-element composition coupled with detailed field mapping of selected pegmatites to reveal a close petrogenetic relationship between pegmatites and heterogeneities of the host larvikite.

References
Brøgger, W.C. 1890: Die Mineralien der syenitpegmatitgänge der südnorwegischen augit- und nephelinsyenite. Zeitschrift für Krystallographie 16, 1-235.

Neumann, E.R. 1980: Petrogenesis of the Oslo region larvikites and associated rocks. Journal of Petrology 21, 499-531.

Petersen, J.S 1978: Structure of the larvikite-lardalite complex, Oslo-region, Norway, and its evolution. Geologische Rundschau 67, 330-342.

Piilonen, P.C., McDonald, A.M., Poirier, G., Rowe, R., & Larsen, A.O. 2012: The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik plutonic complex, Oslo rift valley, Norway. Part 1. Magmatic and secondary zircon: implications for petrogenesis from trace-element geochemistry. Mineralogical Magazine 76, 649-672.

Trace element and Nd-isotope data of eudialyte from the Kakortokite sequence of the Ilímaussaq complex suggest subsolidus remobilization of rare earth elements by magmatic fluids

Mathijs van de Ven¹, Anouk Borst², Gareth Davies¹ and Adrian Finch^2
1Vrije Universiteit Amsterdam, ²St. Andrews University

The kakortokites, agpaitic floor cumulates of the peralkaline Ilímaussaq Complex, South Greenland (1160 Ma¹), constitute a world-class deposit of rare earth elements (REE), largely hosted in the alkali-zirconosilicate eudialyte. Autometasomatic and/or hydrothermal alteration of eudialyte has caused formation of pseudomorphs exhibiting a variety of secondary REE-/Zr-phases. Previous work² identified three alteration types, characterized by the secondary zirconosilicates catapleiite, gittinsite and zircon.

Here we present ICP-MS trace element and TIMS Sm/Nd isotope data of micro-drilled eudialyte-pseudomorph pairs from four samples spanning the kakortokite stratigraphy (three catapleiite-type, one gittinsite-type) and one zircon-type pseudomorph sample from the ‘hybrid’ units that cross-cut them, to study the nature of altering fluids and their high field strength element (HFSE-)mobilizing capacity.

Trace element data show catapleiite-alteration caused a 15-25% decrease in REE, Ta, Nb, Zr, Sr and Y (relative to precursor eudialyte) and up to four-fold increase in Rb and Th ([Th]_pmo = 27-100 ppm), while gittinsite- and zircon-alteration caused stronger heavy REE (HREE) depletion (≤50%). REE fractionation is evidenced by lowered Sm/Nd ratios in all but one pseudomorph-sample, indicating that altering fluids were capable of remobilizing and fractionating HFSE and REE.

¹⁴³Nd/¹⁴⁴Nd_t=1160 ratios of fresh eudialyte (µ_eud = 0.511090, s.d. = 5) are indistinguishable from those of pseudomorphs (µ_pmo = 0.511097, s.d. = 7) at the 90% confidence level, suggesting that fluids were (late-)magmatic in origin. However, zircon-type pseudomorphs yield higher ¹⁴³Nd/¹⁴⁴Nd_t=1160 (0.511166±7), which, combined with stronger HREE depletion and enrichment in Sr (130%) and Ba (79%) hints at a different origin for the zircon-forming fluid.

References
1) Krumrei, T. V., Villa, I. M., Marks, M. A. W., & Markl, G. (2006). A 40Ar/39Ar And U/Pb Isotopic Study Of The Ilímaussaq Complex, South Greenland: Implications For The 40K Decay Constant And For The Duration Of Magmatic Activity In A Peralkaline Complex. Chemical Geology, 227(3–4), 258–273.

2) Borst, A., Friis, H., Andersen, T., Nielsen, T., Waight, T., & Smit, M. (2016). Zirconosilicates In The Kakortokites Of The Ilímaussaq Complex, South Greenland: Implications For Fluid Evolution And High-field-strength And Rare-earth Element Mineralization In Agpaitic Systems. Mineralogical Magazine, 80(October), 1129–1133.

                       POSTER PRESENTATIONS                    

Mineralogy and petrology of mafic dikes from the Siilinjärvi carbonatite complex, Finland

Matias Carlsson¹, Hannes B. Mattsson², Karin Högdahl³, Alireza Malehmir² and Pasi Heino^4
1Geology and Mineralogy, Åbo Akademi University, Akatemiankatu 1, 20500 Turku, Finland, ²Dept. of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden, ³Dept. of Earth Sciences, Uppsala University and Geology and Mineralogy, Åbo Akademi University, ⁴Yara Suomi Oy, Siilinjärvi, Finland

The 2.6 Ga Siilinjärvi carbonatite complex in central Finland is transected by a large number of mafic dykes. Here we combine field-relations, with mineralogical and petrological analysis to infer dike emplaced in three distinct stages. The older two generations (Gen I and II) are intesely deformed in constrast to the composite dike of Gen III.

Mineralogically the dykes are comparable, containing plagioclase, amphibole, quartz, calcite as major phases, with accessory chlorite, apatite, epidote, titanite and opaques. Gen I dykes have characteristically allanite and near-euhedral chamosite crystals (chlorite), surrounded by fine-grained titanomagnetite/ilmenite. These chamosites are likely pseudomorphs replacing original phlogopite crystals.

The alkaline composition of Gen I dykes (predominantly K-rich foidites, with up to 13 wt.% Na2O+K2O) suggests that they are related to different degrees of mantle melting. It is simply not possible to produce the observed compositions by fractionation of any potential phenocryst phase. The two younger dyke generations can be reproduced by a relatively “normal” basaltic fractionation series (spanning from basalts, via K-rich alkali-basalts to trachytic compositions).

The Gen I dykes seem to be directly associated with a mantle source similar to that which produced the apatite-bearing carbonatites. The later dyke generations have bulk-rock compositions that are not normally found in direct relation to carbonatites, indicating a significant time gap between the Gen I dykes and subsequent generations in which time the mantle source had changed significantly to produce these chemically contrasting magma suites.

This is an ERA-MIN StartGeoDelineation contribution sponsored by Vinnova, SGU, Tekes, NIO and Yara.

Geochronology of the Särna alkaline complex, Dalarna, Sweden

John A. Eliasson¹, Thomas Zack¹ and Axel SL Sjöqvist^1
1Department of Earth Sciences, University of Gothenburg

The Särna alkaline complex is a well-known yet poorly studied locality of highly peralkaline igneous rocks in middle Sweden. It mainly consists of two rock types: varieties of intrusive cancrinite nepheline syenite (särnaite) and crosscutting tinguaite dikes. While different whole-rock särnaite samples did not form an isochron, mineral separates derived from särnaite have yielded a Rb–Sr age of 287±14 Ma (2σ) (Bylund and Patchett, 1977), broadly coeval with the Permian Oslo Rift magmatism (ca. 308–245 Ma: Larsen et al., 2008), but not precise enough to draw confident parallels.

To further constrain the timing of magmatic activity at Särna, we are studying the Rb–Sr systematics of these alkaline igneous rocks by a newly developed technique for in situ Rb–Sr dating by LA-ICP-MS/MS (Zack and Hogmalm, 2016). Detailed characterization of mineral textures and chemistry by SEM in combination with micro-analytical dating can give reliable Rb–Sr ages of alkaline rocks, which are notorious for pervasive alteration and open-system behaviour. Preliminary results include the first radiometric age of a Särna tinguaite, which was dated at 296±7 Ma (2σ) in biotite phenocrysts and confirms that the särnaite and tinguaite are coeval. Deriving more precise ages from a variety of rock types will also allow us to make more robust connections with the phases of formation of the Oslo Rift.

References

BYLUND, G. & PATCHETT, P. J. 1977. Palaeomagnetic and RbSr isotopic evidence for the age of the Särna alkaline complex, western central Sweden. LITHOS, 10, 73-79.

ZACK, T. & HOGMALM, K. J. 2016. Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chemical Geology, 437, 120-133.

Thorium in eudialyte ore deposits from Greenland studied by synchrotron radiation (μ-XRF/-XAFS) and electron microscopic techniques (SEM, TEM)

Platon N. Gamaletsos¹, Anouk M. Borst², Takeshi Kasama¹, Adrian A. Finch², Joerg Goettlicher³, Ralph Steininger³, Berit Wenzell¹, Zoltan I. Balogh¹ and Athanasios Godelitsas^4
1Center for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark, ²School of Earth and Environmental Sciences, University of St. Andrews, St Andrews, UK, ³ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Ge, ⁴Department of Geology and Geoenvironment, National & Kapodistrian University of Athens, Athens, Gree

The Nordic region holds some of the world’s largest eudialyte deposits, which contain promising rare earths concentrations. Due to low content of actinides (including thorium / Th) in such rocks, eudialyte would avoid many of the issues of potential radioactive contamination in tailings that bedevil e.g., monazite and xenotime extraction. Here, we study the nature of Th associated with rare earth elements (REE) in eudialyte ore deposits from Greenland in micro-/nano-scale using the technical advance of combining synchrotron radiation (SR μ-XRF/-XAFS) and electron microscopic (SEM, TEM) techniques. Preliminary synchrotron radiation & electron microscopic observations of altered eudialyte reveal the presence of Th-bearing phases at the micro-/nano-scale. Similar heterogeneity is observed for the REE in eudialyte replacement products. We collected Th L_III-edge extended x-ray absorption fine structure (μ-EXAFS) spectral data of Th-enriched microareas, identified by SR μ-XRF mapping that confirmed the heterogeneous distribution of Th, which provide information on the local structural environment of Th within altered eudialyte. Th-bearing phases were detected along altered margins of eudialyte, in secondary phases, and interstitial to primary phases such as feldspar, amphibole and eudialyte. Concerning fresh eudialyte, Th is undetectable by SR μ-XRF. However, further research is needed to clarify these phenomena.

Varena suite in the crystalline crust of the Southern Lithuania: implication to the genesis and mineralization.

Vaida Kirkliauskaite¹, Gediminas Motuza² and Raminta Skipityte^1
1PhD student, ²Prof.dr.

Crystalline crust in Southern Lithuania is covered by 200-500 m thick sedimentary cover and is investigated by potential field mapping and drilling. It is composed by amphibolites (metabasalts), biotite-quartz-plagioclase gneisses (metapsammites and porphyry metadacite and metaandesite) of Orosirian period. Each lithology is predominant in alternating bands extended NNE-SSW. Supracrustals are metamorphosed on the level of amphibolite facies and migmatized. Intrusive rocks are represented by coeval rare bodies of gabbro, peridotite and widespread Calymmian granitic plutons (Motuza 2005; Linnemann et al. 2008).

Varena suite comprises olivine, enstatite, diopside, olivine-magnetite, magnetite, apatite-bearing, and presumably also dolomite rocks. They form integrated bodies few sq. km large, concentrated in the ~300 km² area extended in N-S direction(Motuza et al. 2016).

Rocks of Varena suite and the country rocks (amphibolites, metaporphyres) are affected by strong metasomatic alteration. The mineralization of REE (up to 2759-3100ppm of  La and Ce respectively), Th, U, P hosted by monazite, apatite(?), allanite is spatially related to the Varena suite and some metasomatized supracrustals.

The views on the genesis of Varena suite are contradicting. By various authors they are regarded as skarns, presuming metasomatic origin, as layered intrusions or products of alkaline and carbonatitic magmatism(Motuza et al, 2016).

In this presentation the genetic model is reviewed based on reinterpretation of structural, geochemical, petrographical data, and new isotopic studies of O, C, Sr in the rocks of Varena suite. It is also an attempt to raise methodological questions such as Lu-Hf dating of apatite and find opportunities for cooperation in further studies.

References
Please see all mentioned references in this article.

Motuza, G., Kirkliauskaitė, V., 2016. Ultramafic Varėna Suite in the crystalline basement of the Southern Lithuania – implications for the origin. Baltica, 29 (2), 93–106. Vilnius.

New Rb-Sr and Zircon U-Pb dating of the Grønnedal-Íka igenous complex, SW Greenland

Gabrielle Stockmann¹, Andreas Karlsson², Alexander Lewerentz¹, Tonny B. Thomsen³, Thomas F. Kokfelt³, Elin Tollefsen¹, Erik Sturkell⁴ and Lena Lundqvist^5
1Department of Geological Sciences, Stockholm University, SWEDEN, ²Department of Geosciences, Swedish Museum of Natural History, Stockholm, SWEDEN, ³Geological Survey of Denmark and Greenland (GEUS), Copenhagen, DENMARK, ⁴Department of Earth Sciences, University of Gothenburg, SWEDEN, ⁵Swedish Geological Survey (SGU), Gothenburg, SWEDEN

Dating of the Grønnedal-Íka igneous complex in Southwest Greenland is important to understand the continental rifting event – the Gardar episode. In this study, three different methods have been applied to rock specimens and previously published data to better constrain the age of the complex: a) U-Pb dating of zircons extracted from hand specimens of Upper Series nepheline syenites and from drillcore samples of a central carbonatite plug collected by Kryolitselskabet Øresund A/S, b) new in-situ Rb-Sr analysis on Lower Series granular syenite, and c) re-calculation of whole rock Rb/Sr data by Blaxland et al. (1978).

Prismatic zircons, 100-200 µm in size, light brown in colour, showing magmatically zoned cores and irregularly shaped alteration rims with overgrowths of U and Th oxides were analysed with ThermoFinnigan Element 2 Inductively Coupled Plasma Sector Field Mass Spectrometer instrument at GEUS. This gave an U-Pb age of 1325±6 Ma after having selected suitable data points from CL.

In-situ Rb-Sr dating using an Agilent 8800 ICP MS/MS on selected minerals from granular syenite gave an isochron age of 1314±14 Ma on the basis of 20 spots in biotite and k-feldspar using the technique outlined in Zack and Hogmalm (2016). Re-evaluation and re-calculation of the whole-rock dataset from Blaxland et al. (1978) with the newly revisioned Rb-Sr decay constant (Villa et al., 2015) yields an age of 1311±26 Ma.

This confirms the Grønnedal-ìka complex as the oldest of the Gardar intrusions and most likely marks the onset of the Gardar continental rifting of South Greenland.

References
Blaxland, A. B., van Breemen, O., Emeleus, C. H. & Anderson, J. G. (1978). Age and origin of the major syenite centers in the Gardar province of south Greenland: Rb-Sr studies. Geol. Soc. Am. Bull. 89, 231-244.

Villa, I. M., De Bièvre, P., Holden, N. E. & Renne, P. R. (2015). IUPAC-IUGS recommendation on the half life of ⁸⁷Rb. Geochim. Cosmochim. Act. 164, 382-385.

Zack, T. & Hogmalm, K. J. (2016). Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chem Geol. 437, 120-133.

Experimental simulation and predictive modelling of rare earth element enrichment in carbonatites and alkaline magmas

Johann Tuduri¹, Zineb Nabyl¹, Fabrice Gaillard¹, Malcom Massuyeau², Giada Iacono-Marziano¹, Jérémie Melleton¹ and Laurent Bailly^1
1BRGM, UMR7327, 3 avenue Claude Guillemin, 45100 Orléans, France and also University of Orléans, ISTO, ²University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa

Carbonatites and alkaline magmas are key pieces of the deep carbon cycle and constitute one of the principal resources of rare metals including REE. Several experimental studies have tackled the rare metal partitioning between immiscible carbonate and silicate liquids (Martin et al. 2013; Veskler et al. 1998, 2012). They show negative to massive enrichments in the carbonate liquid. However, no prevailing cause has been clearly isolated as such enrichments can be ruled by experimental conditions (P, T, ƒO₂), melt compositions (water and alkali contents), or technical issues such as unequilibrated experiments.

The aim of this study is to simulate, by HP-HT experiments in the nephelinite-carbonatite system, crystal fractionation and immiscibility between carbonate and silicate liquids, in order to assess the factors ruling REE enrichment during the differentiation of alkaline magmas.

Thirty experimental charges were synthetized using piston-cylinder and internally heated pressure vessel. The partitioning of REE between carbonate liquids, silicate liquids and crystals (pyroxene, calcite, nephelinite, perovskite, titanite) has been characterized. REE partition coefficients between carbonate and silicate liquid increase while Ca partition coefficient increases, suggesting that both have the same behavior. Also, the more the silicate liquid is polymerized, the more REE are concentrated into carbonate liquids. The Ca partition coefficient has been thus calibrated by an empirical model based on the silicate melt composition.

This study reveals the optimum conditions for which carbonatite melts get enriched by >10 times with respect to the residual silicate melts. This predictive approach may serve as guide for prospection of REE-enriched carbonatites.

References
Martin, L.H.J., Schmidt, M.W., Mattsson, H.B., & Guenther, D. 2013: Element partitioning between immiscible carbonatite and silicate melts for dry and H₂O-bearing systems at 1-3 GPa. Journal of Petrology 54, 2301-2338.

Veskler, I.V., Petibon, C., Jenner, G.A., Dorfman, A.M., & Dingwell, D.B. 1998: Trace element partitioning in immiscible silicate-carbonate liquid systems: an initial experimental study using centrifuge autoclave. Journal of Petrology 39, 11-12.

Veskler, I.V., Dorfman, A.M.,  Dulski, P., Kamenetsky, V.S., Danyushevsky, L.V., Jeffries, T.  & Dingwell, D.B. 2012: Partitioning of elements between silicate melt and immiscible fluoride, chloride, carbonate, phosphate, and sulfate melts, with implications to the origin of natrocarbonatite. Geochimica et Cosmochimica Acta 79, 20-40.

Polymetallic, REE and precious elements new results within Suwalki Anorthosite Massif, NE Poland

Janina Wiszniewska¹ and Michal Ruszkowski^2
1Polish Geological Institute -National Research Institute, ²Department of Geology, University of Warsaw

The Suwalki Anorthosite Massif (SAM) is located in NE part of Poland, within Mezoproterozoic, beltiform magmatic AMCG [anorthosite – mangerite – charnokite – granite rapakivi] suite known as a Mazury Complex. This complex associate with tectonic deep rift crustal structure. The SAM occupies an area of 250 km² of the oval shape diapiric structure. Its central part is made up of anorthosites surrounded by rings of norites, gabronorites and diorites. Fe-Ti-V ore minerals concentrations with subordinate Fe-Cu-Co-Ni sulphides that represent 1 to 4% of the rock volume (Wiszniewska J., 2002, Wiszniewska J. et al. 2002) locally even more to a maximum content of 8%.

Chalcopyrite, pyrrhotite, cubanite, pentlandite, cobaltite, bornite, chalcocite, millerite and sphalerite are most common magmatic sulfide minerals. These minerals coexist with Fe-Ti oxides in variable proportion and forms of monomineral aggregates of 3-100 µm, interstitial veins or schlieren of 30-250 µm dimensions. Pyrrhotite makes up nearly 75% of sulphides bulk. Pyrrhotite, chalcopyrite and pentlandite have magmatic origins confirmed by sulfur isotopes of δ34S_CDT of about 0 ‰. (Wiszniewska J. et al.) Sulfides contain often nanoforms of REE (La, Ce and Nd) and precious elements (gold, platinum) mineral admixtures of 10-100 µm or even up to 300 µm in sulfides. Untypical zircon grains of blown candle flame structure and rims around ore minerals were observed and prepared for age determinations.

New results of polymetallic and precious metal sulfides together with REE dispersed mineralization may increase an economic value of Fe-Ti-V documented deposits in SAM.

The research was funded by NCN project 2015/17/B/ST10/03540 and Warsaw University Foundation 21/April/2017.

References:

Wiszniewska, J. (2002). Age and the genesis of Fe-Ti-V ores and related rocks in the Suwalki Anorthosite Massif (Northeastern Poland). Biuletyn PIG. Nr 401.

Wiszniewska J., Duschesne J-C., Stein H., Jedrysek M.O.(2001) .Petrologic and isotope evidence for crustal source of ore-bering Suwalki Anorthsites, Poland. Mineral deposites at the beginig of 21st century. Ed. Pestrzyński, Balkama, p.635-638.

Wiszniewska J., Claesson S., Stein H., Vander Auwera J. and Duchesne J.C. (2002) The north-eastern Polish anorthosite massifs: Petrological, geochemical and isotopic evidence for a crustal derivation, Terra Nova, Vol. 14, 451–460.

Janina Wiszniewska¹ Polish Geological Institute -National Research Institute, Rakowiecka 4, 00-975 Warsaw janina [dot] wiszniewska [at] pig [dot] gov [dot] pl

Michał Ruszkowski² Department of Geology, University of Warsaw, Żwirki i Wigury 93, 02-089 Warsaw; e-mail: ruszkowskimichal [at] wp [dot] pl

1.7. Magmatic Plumbing Systems.

                       ORAL PRESENTATIONS                    

A seismic study of the 2014-15 Bárðarbunga-Holuhraun rifting event

Thorbjörg Ágústsdóttir¹, Robert S. White¹, Jennifer Woods¹, Tim Greenfield², Tom Hudson¹ and Bryndís Brandsóttir^3
1University of Cambridge, ²University of Southampton, ³Institute of Earth Sciences, Science Institute, University of Iceland

On 16 August 2014 an unusual sequence of earthquakes began near the southeastern rim of the ice-covered Bárðarbunga caldera in central Iceland. Over the course of two weeks a dyke propagated 48 km beneath the glacier northeastwards and into the Holuhraun lava field, where it erupted for six months, becoming the largest eruption in Iceland for over 200 years. During this time, a gradual, incremental caldera collapse took place in the Bárðarbunga central volcano. This study analyses the seismic response to the event, both due to the dyke propagation, and the associated caldera collapse. Approximately 35,0000 earthquakes were recorded during the pre-intrusive, intrusive, eruptive and post-eruptive periods, whereof ~4,000 earthquakes were associated with the caldera collapse, delineating faults accommodating the subsidence and showing good correlation with geodetic data. Detailed analysis of the earthquake source mechanisms shows that ~90% can be explained by a double-couple solution. The dominant failure mechanism during the collapse is steep normal faulting, with sub-vertical P-axes, striking sub-parallel to the caldera rim. The northern and southern sides of the caldera, however experienced very different seismicity rates, highlighted by the order of magnitude difference in the cumulative seismic moments. We present evidence of a complex asymmetric caldera collapse, not controlled by a single caldera ring fault. The ~31,000 earthquakes delineating the segmented, lateral dyke intrusion fractured a pathway through the crust, utilizing pre-existing weaknesses. Despite the extensional rift setting, the dyke emplacement generated exclusively double-couple earthquakes with solely strike-slip faulting mechanisms.

Dyke tip processes and large-scale deformation: implications for geodetic modelling

Håvard Svanes Bertelsen¹, Frank Guldstrand¹, Olivier Galland¹ and Karen Mair^1
1Physics of Geological Processes, University of Oslo, Norway

Geodetic modelling of dykes using the Okada dislocation model has become a standard

approach when interpreting geodetic data monitored over active volcanoes. The Okada model

assumes that (1) dykes are tensile fractures and (2) the host is purely elastic. However, recent

field observations and laboratory results suggest an alternate dyke propagation mechanism;

the viscous indenter, i.e. the dyke tip propagates by pushing its host rock ahead, failing

in shear. To which extent different dyke propagation mechanisms affect the associated surface

deformation is currently unknown.

To address this, we compare two series of laboratory models of dyke emplacement, during

which we monitored surface deformation at high resolution and precision. In the first series,

the model crust was elastic gelatine and the dyke propagated as tensile fracture; the resulting

surface deformation was comparable with the predictions of the elastic Okada model, i.e.

an elongated topographic trough above the dyke apex, surrounded by two uplifted zones.

Conversely, in the second series, the model crust was a cohesive Coulomb silica flour and the

dyke propagated as a viscous indenter; the resulting surface deformation was only uplift.

The first-order differences between the two series of models show that local tip processes

indeed control large-scale deformation induced by propagating dykes, and so the surface de-

formation patterns. Our results question the systematic use of tensile elastic fracture models

to interpret geodetic measurements associated with dyke emplacement. Finally, our results

show that relevant interpretation of geodetic measurements requires a proper physical un-

derstanding of dyke propagation mechanisms.

Seismic Interpretation of Sill-Complexes in Sedimentary Basins: The ‘Sub-Sill Imaging Problem’

Christian Haug Eide¹, Nick Schofield², Isabelle Lecomte¹, Simon J. Buckley³ and John A. Howell^2
1University of Bergen, ²University of Aberdeen, ³Uni Research CIPR

Application of 3D-seismic reflection-data to igneous systems in sedimentary basins has led to a revolution in the understanding of mafic sill-complexes. However, there is considerable uncertainty on how geometries and architecture of sill-complexes within the subsurface relates to those imaged in seismic reflection-data. To provide constraints on how sill-complexes in seismic data should be interpreted, we present synthetic seismograms generated from a seismic-scale (22×0.25 km) outcrop in East Greenland constrained by abundant field-data. This study highlights how overlying igneous rocks adversely affect imaging of underlying intrusions and rocks by decreasing seismic amplitude, frequency and making steeply dipping features near-impossible to image. Furthermore, seismic modelling in this study shows that, because of the high impedance contrast between siliciclastic host-rock and dolerites, very thin (1-5 m) intrusions should in principle be imaged in reflection-seismic data at 3 km depth. However, comparison with actual seismic data with well-data shows significant amounts of un-imaged sill intrusions, and this is likely due to limited seismic resolution, overburden complexity, inadequate velocity-models, and interference between reflections from closely spaced sills and sill-splays. Significant improvements to sill imaging and interpretation could be made by better predicting occurrence and geometry of sill intrusions and including these in velocity models.

Fracture networks in and around igneous intrusions on Svalbard: implications for fluid flow

Marte Festøy¹, Kim Senger² and Sten-Andreas Grundvåg^3
1Master graduate, ²Primary supervisor, ³Co-supervisor

Igneous intrusions exert a strong influence on petroleum systems, groundwater aquifers and CO₂ storage reservoirs. This is especially true for migration pathways, as any given intrusion may act as a barrier to, or a carrier for fluid flow, potentially compartmentalizing a reservoir or creating seal by-pass structures. The fracture network in the intrusion and its surroundings will control the permeability along and across a single intrusion. However, the various fracturing processes are relatively poorly constrained, being controlled by magma emplacement, magma cooling and tectonic events, and field data are required to better understand this interplay. In this study we investigate 15 outcrops from central Spitsbergen, Arctic Norway, in order to constrain the impact of Early Cretaceous igneous intrusions on subsurface fluid flow. We combine traditional field mapping with photogrammetry to construct high-resolution 3D virtual outcrop models, providing 330 m of scanlines, 928 fracture measurements using a geological compass and an iPhone and 7993 fracture measurements from virtual outcrop models. Fracture sets, both within the dolerites and the nearby host rocks, are predominantly oriented either parallel or perpendicular to the intrusion contact. At one of the study sites, the fracture frequency in the intrusion-proximal host rocks increases toward the intrusion contact, from 6 fractures/m (f/m) in the background to 10 f/m approximately 1 m from the intrusion contact. Calcite cemented fractures are frequently observed in within intrusions, but also within intrusion-proximal host rocks. A conceptual model for rock evolution and fracture development in and around igneous intrusions is presented.

References
Festøy, M. H. (2017). Integrated characterization of igneous intrusions in Central Spitsbergen (Master’s thesis, UiT Norges arktiske universitet).

The relevance of sills and related flat-lying intrusions in volcanic plumbing systems

Olivier Galland¹, Håvard S. Bertelsen¹, Christian H. Eide², Frank Guldstrand¹, Øystein T. Haug¹, Héctor A. Leanza³, Karen Mair¹, Octavio Palma⁴, Sverre Planke⁵, Ole Rabbel¹, Benjamin Rogers¹, Tobias Schmiedel¹, Alban Souche¹ and Juan B. Spacapan^6
1Physics of Geological Processes, Department of Geosciences, University of Oslo, Norway, ²University of Bergen, Norway, ³Museo Argentino de Ciencias Naturales-CONICET, Argentina, ⁴Y-TEC-CONICET, Argentina, ⁵VBPR, CEED, University of Oslo, Norway, ⁶YPF-CONICET, Argentina

Although dykes are considered to be the main upward magma pathways through the Earth’s crust, the last two decades of research showed that significant parts of volcano plumbing systems consist of flat-lying igneous intrusions, namely sills. The aim of this contribution is to provide a didactic overview of our current understanding of the formation of sills and of their scientific and economic relevance (Galland et al., accepted).

Sills form mainly in the layered parts of the crust, principally in volcanic deposits and sedimentary basins where lateral magma flow through sill complexes and networks can reach several hundreds kilometers. Sill exhibits various shapes, from strata-concordant, transgressive sheets to saucer shapes. Furthermore, sills represent intermediate feeder structures for volcanic eruptions and, therefore, a better understanding of sill emplacement and evolution is essential for assessing volcanic hazards. Sills emplaced in sedimentary basins also deeply affect petroleum systems and are essential components in exploring hydrocarbons. Finally, the massive and fast emplacement of sills resulting from LIPS in sedimentary basins triggered catastrophic climate changes and mass extinctions during Earth’s history.

In this presentation, I will develop the main points of our understanding of sill formation, i.e. the feeders of sills, the factors controlling their initiation, their propagation mechanisms and subsequent evolutions to saucer-shaped intrusions or laccoliths.

References
Galland O., Bertelsen H. S., Eide C. H., Guldstrand F., Haug Ø. T., Leanza H. A., Mair K., Palma O., Planke S., Rabbel O., Rogers B., Schmiedel T., Souche A., Spacapan J. B., accepted. Chapter 5 – Storage and transport of magma in the layered crust – Formation of sills and related flat-lying intrusions. In: Volcanic and Igneous Plumbing Systems – Understanding magma transport, storage and evolution in the Earth’s crust (Ed. Burchardt, S.), Elsevier.

Deep and shallow magma storage on Bali

Harri Geiger¹, Valentin R. Troll¹, Ester M. Jolis¹, Frances M. Deegan¹, Chris Harris², Martin J. Whitehouse³, David R. Hilton⁴ and Carmela Freda^5
1Department of Earth Sciences, Uppsala University, Uppsala, Sweden, ²Department of Geological Sciences, University of Cape Town, South Africa, ³Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden, ⁴Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA, ⁵Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy

The architecture of sub-volcanic magma supply systems exerts a fundamental control on eruptive behaviour and thus on volcanic hazards. The island of Bali in the Sunda arc, Indonesia, is densely populated and home to two active stratovolcanoes, Agung and Batur, but relatively little is known of their underlying magma plumbing systems. Here we reconstruct magma storage and evolution under Bali by employing clinopyroxene and plagioclase mineral-melt equilibrium thermobarometry, oxygen isotope crystal stratigraphy, and He isotopes in pyroxene. Clinopyroxene records magma storage at the crust-mantle boundary, consistent with mantle-like He isotope pyroxene values and mildly submantle δ¹⁸O values. Plagioclase, in turn, reveals upper crustal magma storage and mantle-like δ¹⁸O values, indicating differentiation of ascending primary magma in upper crustal reservoirs. Indeed, recent arc-wide InSAR data for the Sunda arc revealed frequent shallow-level magma storage along the arc, which contrasts traditional petrogenetic models for arcs. Multi-level magma storage likely favours prolonged differentiation of Sunda arc magmas, which could promote future hazardous eruptions on Bali.

Dyke induced quasi-2D deformation in a Coulomb britte host — the influence of host strength on propagation and emplacement

Frank Guldstrand¹, Olivier Galland¹, Alban Souche¹, Håvard Svanes Bertelsen¹ and Alain Zanella^2
1Physics of Geological Processes, University of Oslo, ²L.P.G-Le Mans, Le Mans Université

The emplacement of magmatic dykes within the Earth’s crust represents a fundamental process of magma transport and feeding of volcanic eruptions. Understanding the dyke emplacement mechanism requires an understanding of how the host deforms to accommodate the dyke propagation and growth. Most established dyke emplacement models solely account for tensile opening of an elastic host, whereas the Earth’s crust also exhibits prominent inelastic Coulomb brittle behavior. In this study, we aim to encompass how brittle deformation affects dyke emplacement in cohesive Mohr-Coulomb material.

We present quantitative experimental results obtained from a quasi-2D experimental setup (Hele-Shaw cell type), which consists of two vertical glass plates (3 cm gap) containing fine-grained dry Coulomb mixes of silica flour and glass beads with varying cohesions. Golden syrup is slowly injected into the silica flour via a bottom inlet at a constant flow rate for ~1 hour. While the quasi-2D setup restricts the injected syrup to a “sheet-like” intrusion, it allows direct observation of the host deformation such that we can measure displacement maps, shear strains, and surface deformations accommodating dyke emplacement from strong to weak host rocks.

To further interpret our experimental dataset we attempt a numerical comparison using a static elastoplastic 2D code to investigate the expected elastic and plastic deformation using the imposed experimental intrusion geometry and host rock properties.

Ground deformation in the Bárðarbunga Volcanic System, Iceland, Following the 2014-2015 Lateral Dyke, Caldera Collapse and Major Effusive Eruption

Siqi Li¹, Freysteinn Sigmundsson¹, Vincent Drouin¹, Benedikt G. Ófeigsson² and Michelle Parks^2
1Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland,101 Reykjavik, ²Icelandic Meteorological Office, 150 Reykjavik, Iceland.

The largest instrumentally recorded ground deformation event in Iceland occurred during 2014-2015 within the Bárðarbunga volcanic system. A 48-km-long lateral dyke propagated away from the caldera, followed by six-month-long effusive eruption at the far end of the dyke, while the Bárðarbunga caldera gradually collapsed by 65 meters (Sigmundsson et al. 2015, Gudmundsson et al. 2016). We study ground deformation in the volcanic system after these events, in order to provide an improved understanding of the deformation that follows the events, and distinguish signals induced by visco-elastic responses from those related to renewed melt supply.

We observe ground deformation using Interferometric analysis of Synthetic Aperture Radar (InSAR) and GPS geodesy. Two year interferograms (2015-2017) from Sentinel-1 satellites are created to observe the overall deformation field. Interferogram stacking is implemented to reduce atmospheric noise. Observed deformation is a superpostion of several sources, including glacial isostatic adjustment and steady plate movements. These effects are subtracted from the observations (Drouin et al., 2017) to reveal excessive displacement rates we attribute to post-rifting relaxation in the dyke area. The movements are attributed to visco-elastic response of the Earth, which results in gradual recovery of the area and higher displacement rate after eruption events. The post-rifting relaxation is modelled by a two-layer model with elastic crust and visco-elastic upper mantle.

In the caldera area, a similar study will help to understand if present inflation and earthquake activity at Bárðarbunga is induced by magma inflow or is a visoelastic response to the large deflation in 2014-2015.

References
Drouin, Vincent, Freysteinn Sigmundsson, Benedikt G. Ófeigsson, Sigrún Hreinsdóttir, Erik Sturkell, and Páll Einarsson. 2017: Deformation in the Northern Volcanic Zone of Iceland 2008–2014: An interplay of tectonic, magmatic, and glacial isostatic deformation. Journal of Geophysical Research: Solid Earth 122, no. 4, 3158-3178.

Gudmundsson, Magnús T., Kristín Jónsdóttir, Andrew Hooper, Eoghan P. Holohan, Sæmundur A. Halldórsson, Benedikt G. Ófeigsson, Simone Cesca et al. 2016: Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow. Science 353, no. 6296, aaf8988.

Sigmundsson, Freysteinn, Andrew Hooper, Sigrún Hreinsdóttir, Kristín S. Vogfjörd, Benedikt G. Ófeigsson, Elías Rafn Heimisson, Stéphanie Dumont et al. 2015: Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland. Nature 517, no. 7533, 191-195.

Syn-emplacement fracturing in the Sandfell laccolith, eastern Iceland

Tobias Mattsson¹, Steffi Burchardt¹, Bjarne S. G. Almqvist¹ and Erika Ronchin^1
1Department of Earth Sciences, Uppsala University, Villavägen 16, SE 752 36, Uppsala, Sweden

Felsic magma commonly pools within mushroom-shaped magma chambers, so-called laccoliths or cryptodomes at shallow crustal levels, which can cause collapse of the volcanic edifice. While deformation of magma in volcanic conduits is an important process for regulating eruptive behaviour (Pistone et al., 2016), the bulk of the deformation associated with laccolith emplacement is considered to occur in the host-rock (Pollard & Johnson, 1973), and the effects of magma deformation on the intrusion emplacement is largely unexplored. Here we describe the deformation associated with the emplacement of the 0.5 km³ rhyolitic Sandfell laccolith in eastern Iceland, which formed in a single intrusive event. By combining field measurements, 3D modelling, anisotropy of magnetic susceptibility, and microstructural analysis, we quantify deformation that occurred in both the host-rock and the magma to investigate its effect on intrusion emplacement. Magmatic and magnetic fabric analyses reveal contact-parallel magma flow during the initial stages of intrusion emplacement. The magma flow fabric is overprinted by strain-localisation bands, which indicate that the magma subsequently became viscously stalled and was deformed by consecutively intruding magma. This change in magma rheology can be attributed to the interaction between the strain-localisation bands and the flow bands, which caused extensive fracture-rich layers in the magma and led to decompression degassing, crystallization, and rapid solidification of half of the magmatic body. Our observations indicate that syn-emplacement rheology change, and associated fracturing of intruding magma not only occur in volcanic conduits, but also play a major role in the emplacement of shallow viscous magma intrusions.

References
Pistone, M., Cordonnier, B., Ulmer, P. & Caricchi, L. 2016: Rheological flow laws for multiphase magmas: An empirical approach. Journal of Volcanology and Geothermal Research 321, 158–170.

Pollard, D.D. & Johnson, A.M. 1973: Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, II: Bending and failure of overburden layers and sill formation. Tectonophysics 18, 311–354.

How do sills become laccoliths? An answer from integrated laboratory and numerical modelling

Tobias Schmiedel¹, Olivier Galland¹, Øystein T. Haug¹ and Christoph Breitkreuz^2
1Physics of Geological Processes (PGP), Department of Geosciences, University of Oslo, Norway, ²Geological Institute, Technical University Bergakademie Freiberg, Germany

Igneous intrusions in the upper brittle crust exhibit diverse shapes ranging from thin sheets (dykes, sills, cone sheets), to thick, massive intrusions (laccoliths, plutons, plugs). Presently, none of the established models of magma emplacement have the capability to simulate this diversity because they account for end member rheology of the host rocks (elastic, viscous or plastic), whereas natural rocks are complex elasto-plastic materials. We investigated the effects of host rock rheology on magma emplacement using scaled laboratory models. The model rocks were dry Coulomb granular materials of variable strength (cohesion). We show that strong (high-cohesion) host rock, results in the emplacement of thin, sheet intrusions (sills, cone sheets). Conversely, weak (low-cohesion) host rock results in the emplacement of massive intrusions (laccoliths, plugs). We integrate our laboratory results with numerical simulations to constrain the host rock deformation mechanism that accommodates magma emplacement in the experiments. Our results show how both sills and laccoliths result from initial thin sills that spread horizontally until triggering shear failure of the overburden at a critical radius. Two scenarios can then happen: (1) the overburden is cohesive enough and allows space opening in the sub-surface to accommodate viscous magma inflow along the failure planes, so sills evolve as sheets (saucer shape or cone sheets), or (2) the overburden is not cohesive enough and does not allow sub-surface space opening to accommodate viscous magma inflow along the failure planes, so the sill inflates and lifts up the overburden along shear zones to form a massive laccolith.

Structure and dynamics of the Krafla magma domain and its associated geothermal area: Ground deformation and magma plumbing at the Krafla Magma Testbed, Iceland

Freysteinn Sigmundsson¹, Vincent Drouin¹, Pall Einarsson¹, Knutur Arnason², Gylfi Pall Hersir², Léa Levy³, Elías Rafn Heimisson⁴, Erik Sturkell⁵, Anette K. Mortensen⁶ and John Eichelberger^7
1Nordic Volcanological Center (Nordvulk), Institute of Earth Sciences, University of Iceland, Iceland, ²ÍSOR – Iceland GeoSurvey, Reykjavík, Iceland, ³Nordvulk, ÍSOR and Ecole Normale Supérieure, Paris, ⁴Stanford University, California, USA, ⁵University of Gothenburg, Gothenburg, Sweden, ⁶Landsvirkjun, Reykjavík, Iceland, ⁷University of Alaska Fairbanks, USA

We interpret extensive volcano geodesy results at Krafla volcano, Iceland, together with results from various other observations, including geophysical surveying and drilling. The combined data is interpreted in terms of a magma domain model, which we consider to represent the crustal volume hosting magma at a shallow level. It may be of variable size and shape, containing variable amounts of magma, melts and crystals and comprising magma bodies with variable connectivity. At Krafla caldera, multiple constraints can be applied, including direct constraints from accidental drilling into rhyolitic magma at 2.1 km depth. We interpret the data in terms of at least two magma bodies at shallow depth: (i) a basaltic magma body with a pressure center near the center of the Krafla caldera in the 3-5 km depth range, contributing to S-wave shadows and influencing seismic propagation, as well as being responsible for an inflation/deflation pattern during the 1975-1984 Krafla rifting episode, and (ii) a shallower body of rhyolitic magma with an upper surface at 2.1 km depth underlying a high-temperature geothermal area. This rhyolitic body was inactive during 1975-1984, but also contributes to S-wave shadows. It has been drilled close to or into multiple times and is responsible for superheated steam near the bottom of several geothermal wells. Future advance in our understanding volcano interiors is expected from the Krafla Magma Testbed initiative (http://kmt.is); an international effort to establish an in-situ magma laboratory at Krafla, including drilling again to magma and coring of the magma-geothermal interface.

Impact of host rock heterogeneity on failure mechanism around magmatic intrusions

Alban Souche¹, Olivier Galland¹ and Øystein T. Haug^1
1Physics of Geological Processes, Geosciences Department, University of Oslo, Norway

Rock failure mechanism accommodating the emplacement of magmatic intrusions is dominantly controlled by the local stress regime within the host rock. Most of the mechanical models addressing rock failure conditions (e.g., for a given tectonic regime or magma overpressure) simplify the stress calculation by assuming the homogeneity of the host rock properties.

In this study, we highlight the importance of local heterogeneity in controlling the failure mechanism ahead of a magmatic intrusion. The heterogeneity is introduced by mean of a stochastic perturbation with given wave-length and amplitude on the host rock cohesion. We numerically model and quantify the elasto-plastic deformation of the system in response to the overpressurization of an intrusion. Depending on the characteristics of the heterogeneity, we observe the development of plastic zones representative of either tensile or shear failure. We show that shear failure is the dominant mechanism as soon as heterogeneities are introduced, even for relatively large dilatancy property of the host rock (up to the limit of associative plasticity). We conclude that heterogeneities within the host rock may locally “seed” shear faults ahead of the magmatic intrusion in the propagating direction. We further illustrate the implications of these results by considering the emplacement of a dyke in an extensional setting (Icelandic case) and show that shear failure may be a dominant mechanism in pre-eruptive seismic activity consequent to dyke emplacement, in good agreement with seismic data monitored during recent magmatic events.

The effect of the host-rock rheology on dyke propagation energy balance: a numerical modelling study.

Rémi Vachon¹ and Christoph Hieronymus^1
1Uppsala University

Propagation of magmatic intrusions such as dykes and sills is an essential factor that controls volcano dynamics. It has been shown that an intrusion, or a fracture in general, will propagate when G, the energy released due to crack extension, exceeds a critical energy release rate value Gc, which is considered a material property of the host-rock. Estimates of Gc from observations of dyke length and aperture are generally around 100kJ/m^2 for competent rock, whereas lab measurements on similar rocks yield estimates Gc which are significantly lower.

In order to understand the discrepancy between in-situ and lab measurements of Gc, we test host-rock rheologies that are more realistic than the commonly used linear elastic theory. These rheologies take additional processes into account which affect the Griffith energy balance of a growing crack, such as: (1) pressure dependent elasticity with reduced moduli in regions of low-pressure, (2) plastic failure where shear stresses exceed a critical yield stress, and (3) viscous flow affecting the host rock during the cooling phase of the magmatic intrusion.To test these hypotheses, we construct three numerical models, each testing one of the host-rock rheologies described above.

Our results show that the most significant changes to the Griffith energy balance are obtained from the elasto-plastic model where G values are almost 100 time lower than results from the linear elastic solution. We thus find that large-scale fracturing, which we model as plasticity, can resolve the long-standing problem of disparate critical energy release rate Gc obtained by different means.

                       POSTER PRESENTATIONS                    

A new model for saucer-shaped intrusions: shear failure versus tensile opening at sill tips

Øystein Haug¹, Olivier Galland¹, Pauline Souloumiac², Alban Souche¹, Frank Guldstrand¹ and Tobias Schmiedel^1
1PGP, University of Oslo, ²Géosciences et Environnement Cergy (GEC), Université de Cergy-Pontoise, France

Sills with a characteristic saucer shape are common features in many sedimentary basins worldwide. Previous models of sill emplacement usually assume that pure elastic bending of the overburden of growing sills control their evolution to saucer shape. However, field observations show that significant shear damage also accommodate sill emplacement. To which extent such damage plays an active role on sill emplacement or not is not understood. To address this, we study the condition for shear failure and the distribution of damage in the overburden of sills using the limit analysis software Optum G2. Through a parameter study, we investigate the effect of the length-to-depth ratio (L/D) of the sill, the cohesion of the host rock and the emplacement depth on the over-pressure within the sill at failure. The results show that the characteristic saucer shape is ubiquitously reproduced using this approach (Haug et al, 2017). The over-pressure required for shear failure scales linearly with the host cohesion and as a power-law of L/D. From these observations, we propose a scaling law for a new shear failure criterion in the overburden of a sill. We compare our scaling law to an analytical solution of hydrofractures, and show that tensile tip propagation is favored for small sills, but overburden shear failure may become favorable for large sills. From these results, we suggest that sills initially grow laterally by tensile propagation until reaching a critical L*, when the overburden fails in shear, leading to the emplacement of saucer-shaped sill’s inclined sheets.

References

Haug, Ø.T., Galland, O., Souloumiac, P., Souche, A., Guldstrand, F. & Schmiedel, T. 2017: Inelastic damage as a mechanical precursor for the emplacement of saucer-shaped intrusions, Geology, DIO: 10.1130/G39361.1 |

THE SEILAND IGNEOUS PROVINCE: IMAGING A WELL PRESERVED EDIACARAN DEEP SEATED MAGMATIC SYSTEM BY 3D GRAVITY AND MAGNETIC MODELLING

Zeudia Pastore¹, Christine Fichler¹ and Suzanne McEnroe^1
1Department of Geoscience and Petroleum at the Norwegian University of Science and Technology (NTNU)

The Ediacaran Seiland Igneous Province (SIP) is the largest complex of mafic and ultramafic intrusions in northern Fennoscandia, and one of the few examples in the world of a well preserved deep-seated magmatic plumbing system. The intrusive rocks of the SIP cause a prominent gravity anomaly. A model for the subsurface structure of the SIP has been constructed by forward modelling of 3D gravity and magnetic data using density and magnetic attributes from the outcrops.

There are multiple roots, two of which reach a depth of 9 km. The annular distribution of the roots suggests that this is close to the original geometry, which, despite the complex geodynamic history, has been preserved. Therefore, the Kalak nappe unit hosting the SIP, may not have experienced a strong tectonic reworking.

Forward magnetic modelling of the SIP allowed for estimating the geometry and the magnetization of the magnetic sources. This indicates that most of the sources of the magnetic anomalies are related to gabbroic bodies, and to a lesser extent to the contacts of the ultramafic intrusions with the country rock, possibly indicating these contacts have some degree of serpentinization, or mineralization. Based on the combined results of 3D gravity and magnetic modelling we suggest that the SIP is weakly magnetic at depth. Furthermore, the depths of the numerous mafic and ultramafic complexes vary geographically within the SIP. A low angle detachment fault dipping towards the north truncating the deeper part of the SIP would explain these observations.

Rock magnetic properties and magnetic carriers of a deep crustal magmatic system, the Reinfjord Ultramafic Complex, Seiland Igneous Province, Northern Norway

Geertje W. ter Maat¹, Suzanne McEnroe¹, Nathan Church¹, Zeudia Pastore¹, Alexander Michels¹, Hirokuni Oda² and Rune Larsen^1
1Norwegian University of Science and Technology, ²Geol. Surv. Japan, AIST, Tsukuba, Japan

The Reinfjord Ultramafic Complex (RUC) is part of the 5000km2 Seiland Igneous Province (SIP) in northern Norway. The SIP is considered to be a deep-seated conduit system of a large igneous province. The SIP was emplaced into continental crust at 25-30 km depth at 570-560 Ma. The RUC intruded into gabbros and meta-sedimentary gneisses resulting in three formations. 1) Central Series consisting of dunites; 2) Upper Layered Series consisting of dunites and wehrlites, and 3) Lower Layered Series containing the most pyroxene-rich rocks, with an associated but mineralogically distinct marginal zone resulting from contact metamorphism of the melts and host rocks. Exposures of deep ultramafic systems are rare, and RUC gives a unique opportunity to study the magnetic properties of the roots of a deep seated magmatic system.

The rock magnetic properties of the different formations give insight in the magnetic carriers, and can be associated to primary (spinel, and exsolution in pyroxene), or secondary processes, such as serpentinization. Scanning SQUID microscopy of thin sections combined with optical and electron microscopy indicate which minerals carry the magnetization, that then can be linked to the rock magnetic properties. Characterizing the primary, and secondary, magnetic carriers, allows for assessing the contribution of bodies such as the RUC to magnetic signals from deep crustal rocks. Understanding the nature of the magnetic properties of deep ultramafic rocks now exposed at the Earth’s surface will aid in the interpretation of ground-, aeromagnetic, and satellite surveys and paleomagnetism.

1.8. Building Baltica

                       ORAL PRESENTATIONS                    

Fennoscandia, Sarmatia and Volgo-Uralia: the building stones of the East European Craton/Baltica

Svetlana Bogdanova^1
1Department of Geology, Lund University, Sweden

The three lithosphere segments Fennoscandia, Sarmatia and Volgo-Uralia (Fig.), which constitute the East European Craton (Baltica), have each specific crustal architecture and evolution (Bogdanova, 1993; Gorbatschev & Bogdanova, 1993; Bogdanova et al. 2008). While Volgo-Uralia is nearly all Archean, but complicated by thorough Paleoproterozoic reworking, Fennoscandia is Archean in the north-east but Proterozoic in the south-west, while Sarmatia features several separate Archean blocks cemented by Archean and Paleoproterozoic collisional belts. The segments are separated by the major ca. 2.1 Ga Volga-Don and ca. 1.8 Ga Central Russian Paleoproterozoic collisional belts followed by Meso- to Neoproterozoic aulacogens.

Baltica was built up during numerous collisional and accretionary orogenies, and also breakups ( in Ga):

• 8-2.7: most Archean blocks amalgamated forming the protocratons of Fennoscandia and Volgo-Uralia but still separated by oceans in Sarmatia
• 5-2.4: first records of protocraton breakup (Fennoscandia and Volgo-Uralia)
• 4-2.0: continuing breakup of Archean Fennoscandia when several small oceans were opened
• 1-2.0: the Sarmatian and Volgo-Uralian Archean blocks collided forming megacontinent Volgo-Sarmatia
• 0-1.95: the combined NW margin of Volgo-Sarmatia became the site of a wide continental magmatic arc facing an ocean; strong deformation of Volgo-Uralia ‘s crust and upper mantle
• 94-1.90: the Lapland-Kola collision encompassed Archean Fennoscandia
• 93-1.83: the Svecofennian accretionary orogeny at SSW edge of the Karelian protocraton
• 80-1.75: collision of Fennoscandia and Volgo-Sarmatia; Baltica was created 1.7-1.5: Gothian and Telemarkian accretion at the western Baltica margin
• 55-1.40: the Danopolonian (in part Hallandian) intracontinental orogeny
• 35-1.25: intracontinental mafic magmatism
• 0-0.9: Sveconorwegian collisional orogeny; Baltica participate in Rodinia supercontinent

References

Bogdanova, S.V. 1993: The three-segment hypothesis for the East European Craton. Terra Nova 5(Suppl 1), 313-314.

Gorbatschev, R. and Bogdanova, S.: 1993. Frontiers in the Baltic shield. Precambrian Research 64(1), 3-21.

Bogdanova, S.V., Bingen, B., Gorbatschev, R., Kheraskova, T.N., Kozlov, V.I., Puchkov, V.N. & Volozh, Y.A. 2008: The East European Craton (Baltica) before and during the assembly of Rodinia Precambrian Research 160(1-2), 23-45.

Trace element geochemistry and Sm-Nd isotopes of 2.1 Ga mafic magmatism in the Karelia-Kola, Wyoming and Superior cratons

Sarah Davey¹, Wouter Bleeker², Sandra Kamo³, Richard Ernst⁴ and Brian Cousens^1
1Carleton University, Canada, ²Geological Survey of Canada, ³Jack Satterly Laboratory, University of Toronto, Canada, ⁴Carleton University, Canada, Tomsk State University, Russia

The Karelia-Kola, Wyoming, and Superior cratons are proposed “nearest-neighbours” within the Superia supercraton reconstruction (Bleeker and Ernst, 2006). At 2.1 Ga, their kinship is best constrained by matching paleolatitudes in the Marathon (Superior) and Bear Mountain (Wyoming) mafic dyke swarms, and overall similar apparent polar wander paths (Halls et al., 2008; Kilian et al., 2015). Although Karelia’s position within Superia is not as well constrained by paleomagnetic data, it is hypothesized to have been positioned at the southeast margin of Superior based on the overall correlation of mafic magmatic events (“magmatic barcodes”) and the geometry of associated giant dyke swarms, including the 2.1 Ga Tohmajärvi dykes and Misi gabbro sills (Pekkarinen and Lukarinen, 1991; Niiranen et al., 2003; Bleeker and Ernst, 2006; Salminen et al., 2014, Davey et al., 2017). Here we use whole-rock and trace element geochemistry and Sm-Nd isotopes to test whether Marathon (Superior), Bear Mountain (Wyoming) and Tohmajärvi dykes and Misi sills (Karelia) were generated under similar conditions and from a common source. Halls et al. (2008) report a transition from enriched to flat trace element patterns through time between 2125 to 2100 Ma in the Marathon dykes. This geochemical progression provides an ancillary means in which to compare contemporaneous dyke swarms found in other cratons. If the Bear Mountain and Tohmajärvi dykes and Misi sills were fed by the same magmatic pulses as the Marathon dykes, then the same secular changes in geochemistry should be observed, in addition to sharing comparable paleolatitudes, geometry, and geochronology.

References
Bleeker, W. & Ernst, R.E. 2006: Short-lived mantle generated magmatic events and their dyke swarms: The key unlocking Earth’s paleogeographic record back to 2.6 Ga. In: Hanski, E. et al. (eds): Dyke Swarms—Time Markers of Crustal Evolution, A.A. Balkema Publishers, Rotterdam, 1-22.

Davey, S.C., Bleeker, W., Kamo, S., Vuollo, J., Huhma, H., & Ernst, R.E. 2016: Overview of major 2100-2125 Ma mafic dyke swarms and sill provinces across Archean cratonic fragments. GAC-MAC 2017 Conference Abstract.

Halls, H.C., Davis, D.W., Stott, G.M., Ernst, R.E. & Hamilton, M.A. 2008: The Paleoproterozoic Marathon Large Igneous Province: New evidence for a 2.1 Ga long-lived mantle plume event along the southern margin of the North American Superior Province. Precambrian Research 162, 327-353.

Killian, T.M. 2015: Precambrian Paleomagnetism of Mafic Dyke Swarms and the Wyoming Craton: Implications for an Archean Supercontinent. Yale University, Ph.D. thesis.

Pekkarinen, L.J. & Lukarinen, H. 1991: Paleoproterozoic volcanism in the Kühtelysvaara –

Tohmajärvi district, eastern Finland. Geological Survey of Finland, Bulletin 357.

Niiranen, T., Hanski, E. & Eilu, Pasi. 2003: General geology, alteration, and iron deposits in the Palaeoproterozoic Misi region, northern Finland. Geological Society of Finland, Bulletin 75 (1–2). 69–92.

Salminen, J., Halls, H.C., Mertanen, S., Pesonen, L.J., Vuollo & J., Söderlund. 2014: Paleomagnetic and geochronological studies on Paleoproterozoic diabase dykes of Karelia, East Finland—Key for testing the Superia supercraton. Precambrian Research 244, 87-99.

Polyphase, transpressive deformation of the Archean Siilinjärvi carbonatite complex, central Finland

Karin Högdahl¹, Bjarne S.G. Almqvist², Fredrik Karell³, Pietari Skyttä⁴, Aleksi Salo⁵, Alireza Malehmir² and Pasi Heino^5
1Dept. of Earth Sciences, Uppsala University and Geology and Mineralogy, Åbo Akademi University, ²Dept. of Earth Sciences, Uppsala University, ³Geological Survey of Finland, ⁴Dept. of Geography and Geology, Turku University, ⁵Yara Suomi Ltd

The 2.6 Ga Siilinjärvi carbonatite complex is one of few known Archean carbonatites and the oldest currently being mined. The 16 x 1.5 km N-S trending curved and tabular complex comprises dominatly glimmerite with subordinate carbonatite, enclosed within a fenite halo, all cut by three generations of mafic dykes. The complex has been affected by polyphase deformation, including emplacement of the dykes and their subsequent structural overprint. Strain is heterogeneously distributed with the highest strain localised into the glimmerite and the contacts of more competent rocks represented by fenite blocks, mafic dykes, and glimmerite hosting richterite and carbonates. The relationship between dyke emplacement and deformation has been obtained using a combination of structural data and magnetic fabric measurements. The older dykes are mainly steep, N-S to NW-SE trending, parallel to both the overall structural grain and the magnetic foliation, and narrow dykes are characterised by boudinage observed in both steep and sub-horizontal sections. Streching lineations within and along the margins of these dykes are sub-vertical and coincides with the magnetic lineation. Shear zones at dyke margins and elsewhere in the complex show dextral horizontal shear with a W-block-up vertical component. Later conjugate NNW-SSE dextral and NNE-SSW sinistral zones are likely related to folding of the youngest dyke generation. The consistency between structural and magnetic fabric measurements indicate two episodes of dextral transpressive shear separated by an extensional event related to emplacement of the youngest dyke generation.

This is an ERA-MIN StartGeoDelineation contribution sponsored by Vinnova, SGU, Tekes, NIO and Yara.

Building or breaking Baltica – the Svecofennian ’intraorogenic’ Herräng dyke swarm, east-central Sweden

Åke Johansson^1
1Department of Geosciences, Swedish Museum of Natural History, Box 50 007, 104 05 Stockholm, SWEDEN

The mafic Herräng dykes are found in the Roslagen coastal area of east-central Sweden, where they intrude early Svecofennian metavolcanics and granitoids (ca. 1.87-1.91 Ga), but are themselves cut by late- to post-orogenic pegmatites and metamorphosed to greenschist- or amphibolite-grade. This dominantly E-W-trending dyke swarm has thus traditionally been referred to as ‘intraorogenic’ with respect to the Svecofennian orogeny, e.g. by Stålhös (1991).

In the present study, twentyone samples of mafic dykes have been obtained from the Herräng type area (Magnusson 1940), from the area around lake Limnaren south of Norrtälje (Lundegårdh 1946), and the islets Fogdö and Riddarskäret between Väddö and Singö (Lundqvist 1959). The sampled rocks are dominantly composed of plagioclase and amphibole (presumably hornblende), with variable amounts of quartz and biotite, and accessory titanite, apatite, zircon and opaque minerals. A couple of samples, however, have deviating mineralogy and geochemistry.

Geochemically, the sampled dykes are subalkaline basalts to andesites, with 48-60 wt% SiO₂. They have variable character on different discrimination diagrams: calc-alkaline or tholeiitic; MORB, volcanic arc or within-plate, possibly reflecting their origin during extension (perhaps incipient back-arc spreading) of juvenile subduction-related continental crust.

Initial Epsilon-Nd (at 1.87 Ga) falls between -0.1 and +1.6, and initial Epsilon-Sr between +5 and +24, suggesting a similar ‘mildly depleted’ mantle source as for other Svecofennian mafic rocks. Spot analysis of eleven titanite grains from one sample yields a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 1848 ± 13 Ma, interpreted to date amphibolite-grade metamorphism, and yielding a minimum age of crystallization.

References
Lundegårdh, P.H. 1946: Rock composition and development in central Roslagen. Kungliga Vetenskapsakademien, Arkiv för Kemi, Mineralogi och Geologi vol. 23A, no. 9, 160 pp.

Lundqvist, Th. 1959: Berggrunden på Riddarskäret i nordöstra Uppland. GFF 81, 99-126.

Magnusson, N.H. 1940: Herrängsfältet och dess järnmalmer. SGU serie C 431, 78 pp. (with English summary).

Stålhös, G. 1991: Beskrivning till berggrundskartorna Östhammar NV, NO, SV, SO, med sammanfattande översikt av basiska gångar, metamorfos och tektonik i östra Mellansverige. SGU serie Af 161, 166, 169, 172, 249 pp. (with English summary).

High-Nb and adakite-like 1.86 Ga magmatism in southern Finland

Jaakko Kara¹, Markku Väisänen¹, Hugh O’Brien² and Yann Lahaye^2
1Department of Geography and Geology, University of Turku, ²Geological Survey of Finland

Mafic to intermediate c. 1.86 Ga magmatism has been discovered within the boundary area between central and southern Svecofennia in southern Finland (Kara et al. 2016). The magmatism occurs as NW-SE trending dykes and small intrusions which crosscut the surrounding migmatitic country rocks. The magmatism can be divided into three different varieties based on their geochemical and isotopic characteristics: i) high niobium basaltic dykes (HNB), ii) high magnesium basaltic dykes (HMB) and iii) high silica adakite-like rocks (HSA).

The HNB are alkalic and enriched in HFSE, particularly Nb (>20 ppm) and show high LREE, P₂O₅, TiO₂ and F contents. In-situ zircon Hf-analyses show very high average initial ε_Hf value of c. +10. The HMB exhibit high MgO (up to 12 wt%) and Cr concentrations and show high average initial ε_Hf values of c. +3 and +5 (two dykes). The HSA group is enriched in Sr and LREE and depleted in Y (<20 ppm) and Yb (mostly <1.9 ppm) among the other high silica adakite characteristic signatures (Martin et al. 2005). Two HSA-intrusions exhibit average initial ε_Hf value of zero and c. -3.

The ages as well as geochemical and isotope data are clearly different from the adjacent, c. 1.90-1.88 Ga, arc-related igneous rocks suggesting a different kind of petrogenesis for these two age groups. The similar ages and close spatial occurrences support a genetic link between the 1.86 Ga groups. However, separate sources are likely due to differences in geochemistry and isotope signatures.

References
Kara, J., Väisänen, M., Lahaye, Y. and O’Brien, H., 2016. Post-kinematic mafic dykes in southern part of Central Svecofennia, Finland. In: Lithosphere 2016 Symposium, November 9-11, 2016, Espoo, Finland. Institute of Seismology, University of Helsinki, Report S-65, 47-50.

Martin, H., Smithies, R. H., Rapp, R., Moyen, J. F. and Champion, D., 2005. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos, 79, 1-24.

Paleoproterozoic Inari orocline of northern Fennoscandia: progressive or secondary orocline

Raimo Lahtinen¹, Mohammad Sayab¹ and Stephen Johnston^2
1Geological Survey of Finland, ²University of Alberta, Canada

Oroclines are curvatures of previously linear arcs (or belts), and are divided into two main types: progressive and secondary (Johnston et al., 2013). Progressive oroclines are formed by strain heterogeneity experienced during progressive thin-skinned thrusting: a common feature of modern orogens. Secondary oroclines are formed in response to an orogen-parallel shortening, as witnessed by Paleoproterozoic coupled Bothnian oroclines of Fennoscandia (Lahtinen et al., 2014). The main component of the Inari orocline is the Lapland granulite belt (LGB) having an arcuate shape geometry in the northern Fennoscandia. Recent studies of Tuisku and co-workers (e.g., Tuisku et al., 2006; 2012; cf. Cagnard et al., 2011) provide a solid base for the age and tectono-metamorphic history of the belt. Based on the existing studies, new field and age data, and re-interpretations of geophysical data, we propose following stages for the evolution of the LGB: 1) extension and intrusion of enderbites at ca. 1.92 Ga in a linear rift basin; 2) thrusting and recumbent folding during basin inversion at 1.92-1.91 Ga leading to peak metamorphic conditions; 3) intrusion of post-collisional appinites at ca. 1905 Ma; 4) extension and decompression melting at 1.90-1.88 Ga; 5) renewed shortening of the linear orogen at 1.88-1.87 Ga that lead to upright folding and SW vergent thrusting; 5) orogen-parallel shortening at ≤1.87 Ga. Our preliminary interpretation is that the Inari orocline, including the LGB, is a secondary orocline formed during buckling about a vertical axis of rotation.

References
Cagnard, F., Barbey, P., Gapais, D., 2011. Transition between “Archaean-type” and “modern-type” tectonics: Insights from the Finnish Lapland Garnulite Belt. Precambrian Research 187, 127-142.

Johnston, S.T., Weil, A.B., Gutiérrez-Alonso, G., 2013. Oroclines: Thick and thin. Geological Society of America Bulletin 125, 643–663.

Lahtinen R., Johnston S.T., Nironen M., 2014. The Bothnian coupled oroclines of the Svecofennian Orogen: a Palaeoproterozoic terrane wreck. Terra Nova 26, 330-335.

Tuisku, P., Mikkola, P., Huhma, H., 2006. Evolution of migmatitic granulite complexes: implications from Lapland Granulite Belt, part I: metamorphic petrology. Bulletin Geological Society Finland 78, 71–105.

Tuisku, P., Huhma, H., Whitehouse, M.J., 2010. Geochronology and geochemistry of the enderbite series in the Lapland Granulite Belt: generation, tectonic setting, and correlation of the belt. Canadian Journal of Earth Sciences 49, 1297-1315.

New paleomagnetic and isotopic data for the Late Paleoproterozoic mafic intrusions in the Blekinge Province (southeastern Sweden)

Natalia Lubnina¹, Svetlana Bogdanova² and Ulf Söderlund^2
1Faculty of Geology, M.V. Lomonosov Moscow State University, Russia, ²Department of Geology, Lund University, Sweden

To the east of the Sveconorwegian orogen, the Precambrian bedrock in Blekinge contains granitoids of the Transscandinavian Igneous Belt to the north of the Småland-Blekinge Deformation Zone (SBDZ), and the c. 1.76 Ga Tving granitoids in the south (Johansson et al. 2006). In this study, we have investigated the mafic intrusions cutting the Tving granitoids.

The new paleomagnetic and AMS examinations of the Tving granitoids in eastern Blekinge show NW-striking foliation parallel to the SBDZ. The studied mafic intrusions carry a stable component with shallow downward NNW direction. The primary remanence is supported by positive contact tests. New isotopic data for olivine gabbro and metabasic rocks using U-Pb baddeleyite and Ar-Ar amphibole geochronology suggested a protolith age of ca 1760 Ma for these rocks. The mean paleopole for these mafic intrusions from Blekinge thus corresponds to the c. 1.76 Ga pole for Fennoscandia.

In the entire Blekinge, AMS lineations mainly dip NW. The NNE upward overprint component is found in the Tving granitoids near the Karlskrona Deformation zone, that is close to that in the 1.45 Ga Karlshamn granites, probably due to regional heating and deformation of the Blekinge block (Čečys & Benn 2007). AMS and structural data indicate that the magnetic fabrics of the mafic intrusions are continuous and the metamorphic fabrics in the country rocks were formed during ENE–WSW compression, and can be referred to the Danopolonian orogeny. The new paleomagnetic pole is close to 1.45 Ga pole for Baltica (Lubnina et al. 2010).

References

1.     Čečys, A. & Benn, K. 2007: Emplacement and deformation of the ca. 1.45 Ga Karlshamn granitoid pluton, southeastern Sweden, during ENE-WSW Danopolonian shortening. International Journal of Earth Sciences (Geologische Rundschau) 96(3), 397-414.

2. Johansson, Å., Bogdanova, S. & Čečys, A. 2006: A revised geochronology for the Blekinge Province, southern Sweden. GFF 128(4), 287-302.
3. Lubnina, N.V., Mertanen, S., Söderlund, U., Bogdanova, S., Vasilieva, T.I. & Frank-Kamenetsky, D. 2010: A new key pole for the East European Craton at 1452Ma: Palaeomagnetic and geochronological constraints from mafic rocks in the Lake Ladoga region (Russian Karelia). Precambrian Research 183(3), 442-462.

Early Paleoproterozoic paleogeography of Karelian and Superior Cratons: new paleomagnetic and AMS data from 2.45-2.1 Ga mafic intrusions of Central Karelian and Kianta terranes

Natalia Lubnina¹, Alexandra Stepanova², Nikolay Tarasov¹ and Grigory Nazarov^1
1Department of Dynamic Geology, M.V. Lomonosov Moscow State University, Russia, ²Institute of Geology Kar RC RAS, Petrozavodsk, Russia

We present a new paleomagnetic and Anisotropy of magnetic susceptibility (AMS) data from the Early Paleoproterozoic mafic dykes and Archean host rocks within two terranes of the Karelian Craton, eastern Fennoscandian Shield. Three groups of dykes have been collected within Pyaozero area of Central Karelian terrane: NE-trending ca. 2450 Ma gabbronorite and diorite dykes, NW-trending ca. 2310 Ma dolerite dykes, and NNW-trending ca. 2130 Ma continental MORB-type tholeiitic dykes (Stepanova et al. 2014). All these dykes were typified based on the AMS data. Samples from 2130 Ma dolerite dykes within Tulos area of Kianta terrane were also collected.

The paleomagnetic results show that a strong Svecofennian overprinting is pervasive in the area.

All studied mafic dykes carried two stable components. Most typical is component of intermediate down to the NNW, corresponds to the Svecofennian remagnetisation (Mertanen 1995). Component of SE intermediate down direction yielding a paleomagnetic pole 2450 Ma based on a positive baked contact test is interpreted to represent the primary magnetization.

The paleomagnetic results for dolerite dyke within Tulos area show that a strong Svecofennian overprinting is pervasive in the area, based on a negative baked contact test.

The new paleomagnetic data from the Karelia Craton compared to similar-aged paleomagnetic data from the Superior Craton does not support the recently proposed Superia configuration (Bleeker & Ernst 2006), based upon dyke swarm trajectories.

We propose a new Early Paleoproterozoic paleogeography at 2.45-2.1 Ga for the Karelia and Superior craton.

This is a contribution to projects RFBR 17-05-01270.

References
Bleeker, W. & Ernst, R. 2006: Short-lived mantle generated magmatic events and theirdyke swarms: The key unlocking Earth’s paleogeographic record back to 2. 6 Ga. In: Hanski, E. et al. (eds.): Dyke Swarms – Time Markers of Crustal Evolution. Taylor and Francis/Balkema, London, 3–26.

Mertanen, S. 1995: Multicomponent remanent magnetizations reflecting the geo-logical evolution of the Fennoscandian Shield – a palaeomagentic study withemphasis on the Svecofennian orogeny Ph.D. thesis with original articles (I–IV). Geological Survey of Finland, Espoo, 46 pp.

Stepanova, A.V., Samsonov, A.V., Salnikova, E.B., Puchtel, I.S., Larionova, Y.O., Larionov, A.N., Stepanov, V.S., Shapovalov, Y.B., Egorova, S.V. 2014: Palaeoproterozoic continental MORB-type tholeiites in the Karelian Craton: petrology,geochronology, and tectonic setting. Journal of Petrology 55, 1719–1751.

Development of the Paleoproterozoic Svecofennian orogeny, new constraints from Central Finland

Perttu Mikkola¹, Esa Heilimo¹, Jouni Luukas¹ and Jukka Kousa^1
1Geological Survey of Finland

Significant amounts of new field and analytical data (whole-rock geochemistry, age determinations) were gathered by Geological Survey of Finland from a little studied area along the southeast boundary of the Central Finland Granitoid Complex (CFGC). As the area is the culmination point of several major geological units this new data allows revaluation of previous interpretations and correlations. The new data mostly affects the following aspects:

Units belonging to the older Svecofennian magmatic phase (1.93–1.91 Ga) extend 100 km further southeast as tentatively suggested earlier. All of the paragneiss units in the area display similar detrital zircon patterns, defining ~1.92 Ga as the maximum depositional age for most of the samples. The eruptive ultramafic units occurring as interlayers in different paragneiss units display differences in trace element patterns. But otherwise the earlier division of paragneisses mainly reflects differences in deformation and metamorphic history. The arc-type calc-alkaline magmatism in the area is similar in age (1895–1875 Ma) and composition to that of the classical Tampere group 200 km further west. The voluminous granitoid magmatism took place in two stages, at ca. 1895 and 1885–1875 Ma. The latter phase can be divided into separate units on compositional bases, which display distinct field relationships, although the obtained ages overlap within errors. Thus the earlier division into syn- and post-kinematic units should be abandoned. Instead the units represent coeval magmas originating from different levels of the crust. Lower crust melts rose only locally to the present erosion level via favorable structures.

Kinematics and deformation regime of the Kynsikangas ductile shear zone, SW-Finland.

Sebastian Reimers¹, Jon Engström², Johanna Berckhan¹ and Ulrich Riller^1
1University of Hamburg, ²Geological Survey of Finland

The NW-striking Kynsikangas Shear Zone (KSZ) is one of many prominent Paleoproterozoic ductile shear zones in SW Finland that formed during the Svecofennian orogeny. The Kokemäki segment of the KSZ consists of an approximately 16 km long and 2 km wide core of highly strained metagranitoid and migmatite rocks displaying variable mineral foliation (S) – lineation (L) geometry. Moreover, the core hosts abundant small-scale kinematic indicators, such as C-S fabrics, rotated rigid objects and folded metamorphic layering. Besides unravelling the significance of major shear zones in the Svecofennian orogen, the KSZ allows us also to elucidate to what extent small-scale kinematic indicators are useful for identifying the overall sense-of-shear of ductile deformation zones in general. The central portion of the shear zone is characterized by pronounced sub-horizontal L fabrics, which merge along-strike of the shear zone in both directions into S > L fabric geometry. East of the fault core, the curvature in the strike of foliations, sub-horizontal mineral stretching lineation and C-S fabric geometry point to a strong component of left-lateral displacement. By contrast, foliation pattern and moderately to steeply plunging mineral lineations west of the core indicate dip-slip. Despite complex internal ductile flow, evident by the along-strike variation in S-L fabric geometry, the KSZ seems to have accommodated a strong component of left-lateral transpression.

Long-lived Late Paleoproterozoic to Mesoproterozoic connection of Baltica and Laurentia – a paleomagnetic view

Johanna Salminen^1
1Physics Department, P.O. Box 64, 00014 University of Helsinki, Finland

To understand processes occurring from the planetary interior to the surface environment, a robust paleogeography of tectonic plates is important. Paleomagnetism coupled with geochronology is powerful quantitative method for providing ancient latitudes and azimuthal orientations of continents. Mafic dykes are amenable for precise U-Pb dating and often preserve a stable record of ancient magnetic fields. Recently new high quality paleomagnetic and geochronological data from Meoproterozoic mafic dykes in Southern Finland have been produced allowing reconstructions of position of Baltica (Salminen et al., 2014; 2016; 2017).

There is a general agreement that Baltica and Laurentia form a tectonic “core” of the Mesoproterozoic Nuna supercontinent in a geologically and paleomagnetically viable connection between Northern Europe and North America (NENA), where Baltica is in “upside-down” position relative to Laurentia (Gower et al., 1990; Salminen and Pesonen, 2004). However, contradicting reconstructions have been proposed (Halls et al., 2011). We show that new high quality Mesoproterozoic paleomagnetic data with new geochronology results for Baltica support NENA connection. Those include data from 1.64 Ga Häme, 1.64 Ga Suomenniemi, 1.58 Ga Åland, and 1.58 Ga Satakunta dyke swarms in Southern Finland.

The rotational transition to right-way-up Baltica within Rodinia (e.g. Patchett et al., 1978; Piper, 1980) is proposed to occurred between 1.12 Ga and 1.05 Ga (Salminen et al., 2009; Evans, 2009). It agrees with distinctive patterns of orogenesis and sedimentation around the common pivot point shared between Scoresby Sund, East Greenland, and Finnmark, Norway (Cawood et al., 2010; Evans, 2013).

References

Cawood, P.A. et al., 2010. Geology, 8, 99–102

Evans, D.A.D., 2009. in Murphy et al. Ancient Orogens and Modern Analogues. Geological Society of London Special Publication 327, 371–404.

Evans, D.A.D., 2103. GSA Bull., 11-12, 1735 – 1751.

Gower, C.F. et al.  1990. In: Gower, C.F., Rivers, T., Ryan, B. (Eds.), Mid Proterozoic Laurentia-Baltica. Geological Association of Canada Special Paper, 38, 1–20.

Halls, H.C. et al., 2011. In: Srivastava, R.K. (Ed.), Dyke Swarms: Keys for Geodynamic Interpretation. Springer-Verlag, Berlin, 509–535.

Patchett, P.J. et al., 1978. Earth Plan. Sci. Lett., 40, 349–364.

Piper, J.D.A., 1980. Nature, 283, 845–847.

Salminen et al., 2009. in Reddy et al. (eds.), Palaeoproterozoic Supercontinents and Global Evolution: Geological Society of London Special Publication 323, 99–217.

Salminen, J. et al. 2014. Precambr. Res. 244, 170–191.

Salminen, J.M. et al. 2016. In: Li, Z.X., Evans, D.A.D., Murphy, J.B. (Eds.), Supercontinent Cycles Through Earth History. vol. 424. Geological Society, London, 95–118.

Salminen, J.M et al.  2017. Precambr. Res. 288, 1-22.

 Paleoproterozoic Osnitsk–Mikashevichi-Moscow Igneous Belt (OMMB): new geochronology and petrology data for the Russian segment and tectonic implication

Alexander Samsonov¹, Svetlana Bogdanova², Alexander Postnikov³, Viktor Spiridonov⁴, Yulia Larionova¹ and Alexander Larionov^5
1Institute of geology of ore deposits, petrography, mineralogy and geochemistry of RAS, Moscow Russia, ²Department of Geology, Lund University, Lund, Sweden, ³Department of Lithology, Gubkin State University of Oil and Gas, Moscow, Russia, ⁴VNIIgeosystem, Moscow, Russia, ⁵Isotope Research Centre, VSEGEI, St Petersburg, Russia

The OMMB is a large Paleoproterozoic north-east-trending belt in the central part of the East European Craton (EEC). It is traced from Ukraine through Belarus to Central Russia (Bogdanova et al., 2006, 2016). The main part of the OMMB is covered by platform sediments, thus sampling of the belt is mostly provided by deep drill-hole cores. Petrology, U-Pb zircon geochronology and Sm-Nd isotope data for the Russian segment of the OMMB will be presented.

Major results are listed as below:

1) In the Russian segment, the OMMB consists of tonalite, trondhjemite, granodiorites (TTG) and granites with subordinate gabbros, diorites and mafic metavolcanic rocks.

2) Granites from north-west flank of the OMMB (Vorob’evo and Roslavl’ wells) yield U-Pb zircon ages of 1963 – 1970 Ma that is somewhat younger than the 2005 – 2028 Ma TTG and granites from south-east flank of the belt (Yasnogorsk, Kaluga and Vorotilovo holes) . The latter may have caused high-T low-P metamorphic event at 2020 – 2036 Ma, which has affected metapelites from Ingul-Sevsk and Volgo-Don orogens of the Volgo-Sarmatia block near the OMMB rocks (Bogdanova et al., 2004; Savko et al., 2017 and our data).

3) The OMMB TTG, granites and related mafic igneous rocks have calc-alkaline affinities, subduction-related geochemical features and show variation of eNd(T) values from +2,5 to -1,4 where juvenile rocks with eNd(T)>0 predominated.

4) All the data suggest that the OMMB was formed in two stages, at 2.03 and 1.97 Ma along the Andean-type active margin of Volgo-Sarmatia.

References
Bogdanova, S.V., Bibikova, E.V., Postnikov, A.V., Taran, L.N., 2004. A Palaeoproterozoic magmatic belt near Moscow. Doklady (Thansactions) of the Russian Academy of Sciences 395, 376-380.

Bogdanova, S., Gorbatschev, R., Grad, M., Guterch, A., Janik, T., Kozlovskaya, E., Motuza, G., Skridlaite, G., Starostenko, V., Taran, L., 2006. EUROBRIDGE: New insight into the geodynamic evolution of the East European Craton in: Gee, D.G., Stephenson, R.A. (Eds.), European Lithosphere Dynamics, Geological Society, London, Memoirs, 32. Geological Society London, pp. 599-628.

Bogdanova, S.V., Gorbatschev, R., Garetsky, R.G., 2016. EUROPE – East European Craton, in: Reference Module in Earth Systems and Environmental Sciences. Elsevier, Amsterdam.

Savko K.A., Samsonov A.V., Kotov A.B., Salnikova E.B., Korish E.H., Larionov A.N., Bazikov N.S., 2017. Early Precambiran Metamorphic Events in Eastern Sarmatia. Precambrian Research, 2017, in press

1.86-1.79 Ga magmatic events in the western East European Craton: from subduction to back-arc settings

Grazina Skridlaite¹, Laurynas Siliauskas¹, Andrius Rimsa², Martin Whitehouse³, Ake Johansson³ and Svetlana Bogdanova^4
1Institute of Geology and Geography, Nature Research Centre, Vilnius, Lithuania, ²Institute of Geosciences, Vilnius University, Vilnius, Lithuania, ³Swedish Museum of Natural History, Stockholm, Sweden, ⁴Department of Geology, Lund University, Lund, Sweden

Remnants of a ca. 1.86-1.84 Ga continental margin were found in southern part of the Mid-Lithuanian domain (MLD), the western East European Craton (Bogdanova et al., 2015), while some new data supports its continuation to the NW, towards central Sweden.

The Randamonys gabbro-diorite-granodiorite-granite in southern Lithuania of calc-alkaline affinity, metamorphosed in amphibolite facies, was emplaced between 1.86 and 1.84 Ga. The suite has volcanic-arc, and hence, subduction-related characteristics. Ca. 100 km to northwest, the ca. 1.85 Ga diorites metamorphosed in granulite facies at 1.80 Ga (Skridlaite et al., 2014) were interpreted as evidencing the MLD margin continuation in central Lithuania. A new ca. 1.85 Ga SIMS age obtained from a felsic granulite (metatonalite) further west (in western Lithuania) confirms the extension of margin northwestwards.

Gabbro-norites which intruded into the Randamonys complex at 1.79 Ga are tholeiitic, have positive initial ε_Nd, hence juvenile, mantled-derived characteristics. Some slightly older (ca. 1.82-1.81 Ga) fine-grained gabbro-norite with a very primitive REE distribution and positive initial ε_Nd resembles oceanic crust.

The 1.86-1.84 Ga continental margin appears to continue through the Baltic Sea and merge with the 1.87–1.84 Ga Askersund-Loftahammar magmatic belt along southern Bergslagen in Sweden (cf. Stephens et al., 2009). Later, an Andean-type subduction in the present west was responsible for the formation of back-arc basins and 1.79 Ga mafic magmatism in the MLD, coeval with the intrusions of the 1.81–1.76 Ga TIB-1 suite. Some remnants of ca. 1.82-1.81 Ga oceanic crust completely reworked elsewhere may be preserved in these back-arc settings.

References
Bogdanova, S., Gorbatschev, R., Skridlaite G., Soesoo A., Taran L., D., K., 2015. Trans-Baltic Palaeoproterozoic correlations towards the reconstruction of supercontinent Columbia/Nuna. Precambrian Research 259, 5-33.

Skridlaite, G., Bogdanova, S., Taran, L., Baginski, B., 2014. Recurrent high grade metamorphism recording a 300 Ma long Proterozoic crustal evolution in the western part of the East European Craton. Gondwana Research 25, 649–667.

Stephens, M.B., Ripa, M., Lundström, L., Persson, L., Bergman, T., Ahl, M., Wahlgren,C.-H., Persson, P.-O., Wickström, L., 2009. Synthesis of the bedrock geology in the Bergslagen region, Fennoscandian Shield, south-central Sweden. SGU Ba 58. Geological Survey of Sweden, Uppsala, Sweden, 259 pp.

Paleoproterozoic mafic dyke swarms in Archean Kola-Murmansk and Karelia provinces, eastern Fennoscandia: barcode comparison and implications for paleocontinental reconstructions

Alexandra Stepanova¹, Alexander Samsonov², Ekaterina Salnikova³, Andrey Arzamastsev³, Svetlana Egorova¹, Yulia Larionova², Alexander Larionov⁴, Ksenia Erofeeva² and Maria Stifeeva^3
1Institure of geology Karelian Research Center RAS, Petrozavodsk, Russia, ²Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia, ³Institute of Precambrian Geology and Geochronology, Saint-Petersburg, Russia, ⁴Centre for Isotopic Research, VSEGEI, Saint-Petersburg, Russia

Eastern Fennoscandia consists of Archean Kola-Murmansk and Karelian provinces, separated by ca. 1.98-1.85 Ga Lapland-Kola orogen (Daly et al., 2006). New baddeleyite U-Pb (ID TIMS) and zircon (SIMS) ages, geochemical and isotopic data for 2.65-1.86 Ga mafic dyke swarms in Kola-Murmansk province give insights into Lapland-Kola Ocean lifespan and clarify a position of Kola-Murmansk province within Superia supercraton (Bleeker and Ernst, 2010).

Wide-spread mafic magmatism of age 2650 Ma is recognized in Murmansk domain. It is well correlated with 2640-2660 Ma magmatic event in Keivy terrane, Kola domain (Bayanova, 2004). Mafic magmatism of this age in Karelia is unknown. It suggests strong differences in the Neoarchean history of Karelian and Kola-Murmansk provinces.

Nevertheless both Karelian and Kola-Murmansk provinces most likely belong to the Superia supercraton as suggested by Bleeker and Ernst (2010). It supported by similarity in the events succession since 2500 to 2400 Ma.

The noteworthy differences occur within the period 2400-2060 Ma. Mafic dykes of such age do not recognize in the Kola-Murmansk province yet. In contrast, several voluminous events of age ca. 2310, 2220 and 2140 Ma occur in the Karelian province (Stepanova et al., 2014, 2015, Vuollo, Huhma, 2005).

Since 2060 Ma Karelian and Kola-Murmansk provinces are similar in events succession and composition of mafic rocks. So huge ca. 1980 Ma event widespread both in Kola-Murmansk and Karelian provinces indicates Lapland-Kola Ocean closure.

The study is supported by RSF, grant 16-17-10260.

References
Bayanova, T.B., 2004. Age of reference geological complexes in the Kola region and duration of magmatic processes.

Daly, J.S., Balagansky, V. V., Timmerman, M.J., Whitehouse, M.J., 2006. The Lapland-Kola orogen: Palaeoproterozoic collision and accretion of the northern Fennoscandian lithosphere. Geological Society, London, Memoirs 32, 579–598.

Ernst, R., Bleeker, W., 2010. Large igneous provinces (LIPs), giant dyke swarms, and mantle plumes: significance for breakup events within Canada and adjacent regions from 2.5 Ga to the PresentThis article is one of a selection of papers published in this Special Issue on the the them. Canadian Journal of Earth Sciences 47, 695–739.

Stepanova, A.V., Salnikova, E.B., Samsonov, A.V., Egorova, S.V., Larionova, Y.O., Stepanov, V.S., 2015. The 2.31Ga mafic dykes in the karelian craton, eastern fennoscandian shield: U-Pb age, source characteristics and implications for continental break-up processes. Precambrian Research 259, 43–57.

Stepanova, A. V., Samsonov, A. V., Salnikova, E.B., Puchtel, I.S., Larionova, Y.O., Larionov, A.N., Stepanov, V.S., Shapovalov, Y.B., Egorova, S. V., 2014. Palaeoproterozoic continental MORB-type tholeiites in the Karelian Craton: Petrology, geochronology, and tectonic setting. Journal of Petrology 55, 1719–1751.

Vuollo, J., Huhma, H. 2005. Paleoproterozoic Mafic Dikes in NE Finland. In Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield, 195–236.

                       POSTER PRESENTATIONS                    

Mineral shape fabric analysis of the Kynsikangas Shear Zone: Evidence for a transpressive origin?

Johanna Berckhan¹, Jon Engström², Sebastian Reimers¹ and Ulrich Riller^1
1University of Hamburg, Bundesstraße 55, 20146 Hamburg, ²Geological Survey of Finland, Betonimiehenkuja 4, FI-02151 Espoo

The NW-SE trending Palaeoproterozoic Kynsikangas Shear Zone (KSZ) is a prominent ductile shear zone in the Svecofennian Orogen of SW-Finland. The shear zone is located at the boundary between the Western Finland and Southern Finland Subprovinces. In the Kokemäki area, the KSZ formed under high-grade metamorphic conditions in migmatite and gneissic rocks. The shear zone is believed to have formed under left-lateral shear. A more complex kinematic, possibly transpressive, origin of the shear zone, is evident from our analysis of kilometer-scale pattern of metamorphic foliations, maximum principal stretching orientations and small-scale kinematic indicators. In order to elucidate the kinematic regime of shear zone formation, we quantify the geometry and intensity of shape-preferred orientation (SPO) of metamorphic minerals. Therefore, oriented block samples from 50 stations are cut according to the principal planes of finite strain and the SPO of elongate minerals and other strain markers are quantified by image analysis. The results of this classical analysis of SPO will be used to calibrate visual estimates of fabric intensity across the Kokemäki segment of the KSZ and will be compared with more sophisticated 3D fabric analysis techniques.

Provenance of Paleoproterozoic clastic metasedimentary rocks in Norrbotten, northern Sweden

Anna Ladenberger¹, Stefan Bergman¹, Risto Kumpulainen², George Morris¹, Fredrik Hellström¹, Tonny B. Thomsen³, Edward P. Lynch¹, Stefan Luth¹, Susanne Grigull¹, Martiya Sadeghi¹ and Høgni Vesturklett^3
1Geological Survey of Sweden, Uppsala, Sweden, ²Stockholm University, Stockholm, Sweden, ³Geological Survey of Denmark and Greenland, Copenhagen, Denmark

Provenance zircon ages (U-Pb geochronology by LA ICPMS) were obtained from twenty samples of clastic metasedimentary rocks from several supracrustal units in Northern Norrbotten. The majority belong to the Svecofennian sequences, with a few samples representing stratigraphically lower Karelian units.

The Karelian rocks generally have unimodal age distributions with the main peaks at c. 2.7 Ma and 2.55-2.60 Ga. The maximum depositional age vary from c. 2.6-2.5 Ga for Tjärro quartzite and a similar quartzite collected in the Vakko Zone. Early Paleoproterozoic depositional ages (c.2.3Ga) occur in quartzite at Tärendo and in arkosic metasandstone from the Sockberget group.

In the Svecofennian metasedimentary rocks, bimodal age distributions dominate with major peaks at Middle Paleoproterozoic and subordinate ones at Archean. The oldest single ages are found in the Hauki quartzite (3742±19Ma), the Kiilavaara quartzite (3649±44Ma), the Stora Sjöfallet quartzite (3732±10 Ma) and in metasandstone from the Råneå group (3628±31 Ma). Maximum depositional ages vary between 1.9 and 1.8 Ga. Ages as young as 1.75-1.8 Ga are reported, however, they may have been affected by metamorphism, hydrothermal alteration or inaccurate common lead corrections.

The presumed source of the clastic sediments is generally thought to be local.

The obtained geochronology data from northern Sweden, together with available detrital ages from northern Norway and Finland constitute a major database for building models of tectonic and lithostratigraphic evolution of the Fennoscandian Shield.

Petrogenesis of c. 1.9 Ga meta-volcanosedimentary rocks in the Nautanen-Aitik area, northern Sweden: Geological, lithogeochemical and Sm-Nd isotopic constraints.

Edward P. Lynch¹, Tobias E. Bauer², Hannu Huhma³ and David Drejing-Carroll^4
1Department of Mineral Resources, Geological Survey of Sweden, Uppsala, Sweden, ²Division of Geosciences and Environmental Engineering, Luleå Technical University, Luleå, Sweden, ³Geological Survey of Finland, Espoo, Finland, ⁴Exploration Division, New Boliden, Boliden, Sweden

In northern Sweden, Orosirian metasupracrustal successions provide critical insights into Svecofennian-cycle tectonothermal processes. Additionally, they commonly host iron oxide-apatite and/or Cu ± Au mineralization, and thus represent key exploration targets. Characterising such rocks from a primary perspective is an important prerequisite in understanding subsequent deformation, metamorphic and/or hydrothermal events. We present new petrogenetic constraints for a polydeformed, c. 1.9 Ga metasupracrustal package in the Nautanen-Aitik area (Norrbotten) hosting several economically significant Cu-Au deposits.

The investigated sequence mainly comprises tuffaceous metavolcaniclastic rocks and intercalated metasedimentary (epiclastic) horizons. Primary volcanic features include remnant feldspar phenocrysts, accretionary lapilli-like features/clasts and inferred agglomeritic horizons. Epiclastic layers locally display trough- and herring bone-type cross-stratification, suggesting relatively shallow, subaqueous depositional conditions. Compositionally, the rocks are mainly andesitic, with lesser basaltic andesitic and dacitic varieties, and have calc-alkaline series affinities. Chondrite-normalized REE patterns are LREE-enriched (La_N/Yb_N = 4.2 – 18.6), with flat to weakly negative Eu anomalies ([Eu/Eu*]_N = 0.59 – 1.16; avg = 0.88). Whole-rock ɛNd_{(1.88 Ga)} values range from -4.5 to -0.3 (n = 10), while corresponding T_DM model ages range from c. 2.2 to 2.5 Ga.

The sequence attributes are broadly consistent with formation in a continental arc-type setting undergoing extension, where precursor tephra accumulated within a syn-volcanic depocenter (cf. Wanhainen et al. 2012). Geochemical signatures also suggest parental magmas derived from a mixed continental-juvenile source. Given the spatial overlap of Orosirian and older (Rhyacian) successions across the region, however, a contribution from intracratonic tholeiitic greenstones, remobilised by Svecofennian-related magmatism, may also be valid.

References
Wanhainen, C., Broman, C., Martinsson, O., Magnor, B. 2012. Modification of a Palaeoproterozoic porphyry-like system: Integration of structural, geochemical, petrographic, and fluid inclusion data from the Aitik Cu–Au–Ag deposit. Ore Geology Reviews 48, 306–331.

Deformation history of the Archipelago of Southern Finland

Kaisa Nikkilä¹, Anna Saukko¹ and Olav Eklund^1
1Åbo Akademi University, Geology and Mineralogy, Finland

The Paleoproterozoic Svecofennian orogenic domain of Southern Finland consists principally of belts of strongly migmatized infra- and supracrustal rocks and granitoids in upper amphibolite to granulite facies, with areas of less migmatized rocks in between. The granite-migmatite belts are presumably related to each other, but their structural setting, deformation stages and thus tectonic setting are vaguely known.

Our focus is the onshore area and the archipelago around the Hanko peninsula. As the site was last researched before the plate tectonic theory was established, the goal of this study is to update the geological interpretations of the area. The project is ongoing and the results presented here are preliminary.

In the study area, at least three different ductile deformation stages are recognized (D1, D2, D3). During D1, the metasedimentary rocks were folded into tight to isoclinal folds with E-W striking nearly vertical axial plane. The E-W trending horizontal to moderately plunging mineral lineation is in places stronger than the foliation. The D2 developed open asymmetric folding with SE plunging fold axis. At this stage, mineral lineation is common, but the foliation is difficult to observe. The D3 is represented by a mineral lineation plunging towards NE.

The leucosomes in the migmatites are folded twice. This suggests that melts were generated before or during D1. The E-W trending structures indicate N-S (at present) compression at stage D1. During D2, the main shortening direction was likely in the NE-SW direction, whereas the D3 structures seem to represent a NW-SE shortening.

A ca.1.89 Ga magmatic complex in eastern Lithuania: a link connecting with the domains in Estonia and the Bergslagen terrane in Sweden

Laurynas Šiliauskas¹, Gražina Skridlaitė¹, Martin Whitehouse² and Alvar Soesoo^3
1Institute of Geology and Geography, Nature Research Centre, Akademijos 2, LT-03223 Vilnius, Lithuani, ²Swedish Museum of Natural History,Box 50 007, SE-104 05 Stockholm, Sweden, ³Tallinn University of Technology, alvar [dot] soesoo [at] ttu [dot] ee

Concealed crystalline basement of the Latvia-East Lithuania (LEL) domain is covered by 200-500 m thick sediments. It is mostly composed of basic to acidic intrusive rocks and their volcanic counterparts that host ore deposits. Bogdanova et al (2015) suggested correlations between the 1.89-1.87 Ga Bergslagen microcontinent and Livonia megadomain (including Latvia-East Lithuania, South Estonia Granulite and West Estonia domains). However, apart from a ca. 1.887±7 Ma metadiorite in the south-western LEL (Bogdanova et al., 2015) and a much younger ca. 1.5 Ga AMCG suite in the south (Sundblad et al, 1994), almost none of the rocks from the LEL have been properly dated.

Four samples with calc-alkaline affinity from the LEL were selected for zircon U-Pb SIMS dating: two samples of granodiorite from the southern part of the LEL that might be a part of a TTG suite, a sheared diorite and a meta-rhyolite from the western border of the LEL. The two granodiorites yielded concordia ages of 1892.3±5.7 Ma (MSWD 1.01) and 1893.7±7.4 Ma (MSWD 1.9), whereas diorite is somewhat younger yielding 1876.1±4.8 Ma (MSWD 1.6). The meta-rhyolite recorded a concordant age of 1898±8.3 Ma (MSWD 1.4).

Our newly obtained U-Pb zircon ages support the model proposed by Bogdanova et al. (2015) in which the Livonia and Bergslagen terranes formed simultaneously and can be correlated across central part of the Baltic Sea. It is also consistent with the general southwest younging of terranes in the western East European Craton (and Baltica).

References
Bogdanova, S., Gorbatschev, R., Skridlaite G., Soesoo A., Taran L., D., K., 2015. Trans-Baltic Palaeoproterozoic correlations towards the reconstruction of supercontinent Columbia/Nuna. Precambrian Research 259, 5-33.

Sundblad, K., Mansfeld, J., Motuza, G., Ahl, M., Claesson, S., 1994. Geology, geochemistry and age of a Cu-Mo-bearing granite at Kabeliai, Southern Lithuania. Mineralogy and Petrology 50, 43-57.

The Nisser Shear Zone – Discovery of a Sveconorwegian crustal-scale detachment zone in southern Norway

Espen Torgersen¹, Iain Henderson², Bernard Bingen², Katia Svendby² and Aziz Nasuti^2
1Geological Survey of Norway-NGU and Dept. of Geoscience and Petroleum-IGP, NTNU, ²Geological Survey of Norway-NGU

In the Telemarkia Lithotectonic Unit, S Norway, thick successions of metasedimentary and metavolcanics rocks, known as the Telemark- and Nissedal supracrustals, are structurally overlying gneiss complexes. The contact relationships between low- and high-grade rocks, whether depositional or tectonic, are contentious, although notably poorly documented.

As part of an ongoing multi-disciplinary project in the Nissedal-Drangedal area (Bedrock Infrastructure in Telemark–BITE), field mapping reveals that the c.1200 Ma basalt-dominated Nissedal supracrustal rocks are separated from the underlying c.1200 Ma granitic gneisses of the Vråvatn Complex by a ductile to brittle shear zone, here called the Nisser Shear Zone (NSZ). The shear zone corresponds to a pronounced asymmetric negative magnetic anomaly, at least 50 km long, on the high-resolution aeromagnetic map. The NSZ is up to 200 m thick and comprises a variety of high-strain rocks ranging from ultramylonite to cataclasite and fault gouge. Brittle structures appear to overprint ductile structures. It dips shallowly to the SE (c. 150/25) and carries strongly developed SE-plunging mineral lineations and slickenlines. These are associated with kinematic indicators (e.g. sigmoidal porphyroclasts and s-c shear bands) that consistently show a top-to-the ESE normal shear sense. Mylonites developed in the footwall granitic gneisses show dynamic recrystallization of K-feldspar, testifying to deformation above 500°C.

Provisionally, we interpret the NSZ as a crustal-scale extensional detachment linked to the late-Sveconorwegian orogenic decay. It exhumed lower-crustal gneisses and juxtaposed them against upper-crustal sequences of a similar age. Future studies will constrain the age, tectonic setting and environmental conditions of the NSZ.

AMCG Mazury Complex in NE Poland – a time frame of the suite formation

Janina Wiszniewska¹ and Ewa Krzeminska^1
1Polish Geological Institute -National Research Institute

A Mesoproterozoic AMCG suite known from covered part of the Eeast European Craton on the border of Poland with Lithuania and Russia, was emplaced along E–W trending linear thrusting zone. An ~1.50 Ga crystallization age of the rapakivi-like granitoids of Mazury Complex, obtained on zircon grains, was one of the first geochronological contribution, initiating more detailed investigations in the frame of EUROBRIDGE programme. Further age studies have been focused mostly on the spatial chronology of the suite, controlled by A-type granitoids as a dominant phase across the region.

A principal goal of recent studies was to constrain the time of emplacement of all AMCG components, using U- Pb zircon ages determined by SHRIMP method. The metaluminous, within-plate, A-type Mazury granites were intruded between 1.53 and 1.50 Ga. The last pulse at 1498±10 Ma was deciphered from westernmost part (Filipów hole). The geochemical and isotopic investigations pointed out that AMCG suite was formed by multiple magma batches, assembled by pulses, often in the same locality. AMCG merged pluton was finally intruded by a net system of peraluminous granite veins, that were crystallized between 1495 ±11 Ma -1488.7 ±4 Ma. This event recorded in Mazury AMCG suite, predates a formation of AMCG massifs in western Lithuania (1447 -1445±8Ma Nemunas- Geluva), and regional magmatism (1.45 Ga) in Bornholm -Blekinge area.  It also predates a dynamic high-grade metamorphism in SW Sweden related to convergent active-margin processes during the Danopolonian orogeny.

The research was financed from NCN grant no. 2015/17/B/ST10/03540

A link between Mesoproterozoic basement of Lithuania and NE Poland by inherited metamorphic zircon data

Janina Wiszniewska¹, Ewa Krzeminska¹, Grazina Skridlaite² and Svetlana, V. Bogdanova^3
1Polish Geological Institute-National Research Institute, Warsaw, Poland, ²Nature Research Centre, Vilnius, Lithuania, ³Lund University, Sveden

A number of metamorphic events between 1.7-1.6 Ga were recognized within the hidden basement of Lithuania Skridlaite et al., 2014) . In Baltica, , this period marks a tremendous change in tectonic evolution (cf Bogdanova et al., 2008, 2015), when an active accretionary margin shifted to its  western edge (cf Bingen et al., 2008).

At present, 1.7-1.6 Ga old zircons were found in the Mazury area (NE Poland) by U-Pb age determinations carried out at SHRIMP Lab, PGI, Warsaw; cf Krzeminska & Wiszniewska, 2017). They represent an inherited component from 1.49-1.48 Ga peraluminous granite veins cross-cutting the Suwalki Anorthosite Massif (SAM). The most of zircon grains have cores, containing material derived from various Paleoproterozoic sources. Important coherent group of inherited grains (about 20%) has cores in range of 1710±12 Ma – 1617±9 Ma, with signature typical for metamorphic crystallization e.g. Th/U<0.1, but the host rocks with these characteristics have not been identified so far in proximity of SAM .

This inherited population corresponds however with the records of magmatic and metamorphic activity common in Lithuania and Belarus (Skridlaite et al., 2014; Vejelyte et al., 2015, Bogdanova et al., 2001), where zircons and monazites from Randamonys (Lithuania) and Grodno (Belarus) have documented reworking at 1668±9 Ma (and 1569±12 Ma) and granulite facies metamorphism at 1727±30 Ma. This may  indicate genetic link between peraluminous the Mesoproterozoic vein granites and their basement,  that could be similar in NE Poland and Lithuania at the level of magma generation.

References

The research was financed from NCN grant no. 2015/17/B/ST10/03540

Bingen, B., Nordgulen, Ø., Viola, G., 2008. A four-phase model for the Sveconorwegian orogeny, SW Scandinavia. Norwegian Journal of Geology 88, 43-72.

Bogdanova, S., Gorbatschev, R., Skridlaite G., Soesoo A., Taran L., D., K., 2015. Trans-Baltic Palaeoproterozoic correlations towards the reconstruction of supercontinent Columbia/Nuna. Precambrian Research 259, 5-33.

Krzeminska E. & Wiszniewska J., 2017 – Magma generation processes within Mazury AMCG suite – evidences from inherited zircons. Goldschmidt 2017, Paris, 12-18.08.2017. Conf.abstracts.

Skridlaite, G., Bogdanova S., Taran L., Baginski, B., 2014. Recurrent high grade metamorphism recording a 300 Ma long Proterozoic crustal evolution in the western part of the East European Craton. Gondwana Research 25, 649-667.

Vejelyte, I., Bogdanova, S., Skridlaite, G., 2015. Early Mesoproterozoic magmatism in northwestern Lithuania: a new U–Pb zircon dating. Estonian Journal of Earth Sciences 64, 189-198.

1.9. Micro-beam frontiers in magmatic, metamorphic, and hydrothermal systems

                       ORAL PRESENTATIONS                    

U-Pb calcite and zircon dating by LA-ICP-MS image mapping

David Chew¹, Kerstin Drost¹ and Joseph Petrus^2
1Department of Geology, Trinity College Dublin, Dublin 2, Ireland, ²School of Earth Sciences, The University of Melbourne, Parkville, Australia

Traditionally, LA-ICP-MS geochronology employs spot ablations, with the limitation that the laser excavates material several tens-of-µm below the sample surface. LA-ICP-MS mapping (ablating a series of parallel, adjacent lines in a single experiment) yields much shallower ablation depths (0.3-3 µm). The data contained in the parallel line scans are reduced with software (e.g. Iolite) to produce x-y-element concentration and/or isotopic maps. We employ here a new flexible interrogation tool for LA-ICP-MS maps called Monocle (Petrus et al., 2017), and add-in for Iolite. In addition to inspectors tools such as size-and-shape adjustable loupes, user-defined regions-of-interest (ROIs) and live graphs (e.g. Wetherill or Tera-Wasserburg concordiae), Monocle allows the selection and pooling of pixels that meet user-defined criteria. Potential applications of zircon U-Pb imaging include characterisation of complex polyphase zircons and characterising the U-Pb systematics of key samples for TIMS dating. U-Pb calcite dating has important applications including dating unfossiliferous sedimentary rocks or ore-bearing mineralization events. The functionality of Monocle can circumvent limitations of the U-Pb calcite system, including low U concentrations and/or high amounts of initial Pb. Pixels on a map can be pooled into “analyses” based on ECDFs of elements or isotopic ratios (e.g. ²³⁸U/²⁰⁴Pb) to create a spread of U-Pb data on concordia. Portions of the sample with elevated detrital components or chemically-different generations of carbonate can be rejected by defining exclusion criteria (e.g. Rb<1ppm; Mg/Ca<0.004). Stratigraphically well-constrained Paleozoic limestones and shell fragments yield accurate ages with internal age uncertainties as low as ±1%, confirming the feasibility of the technique.

References
Petrus, J.A., Chew, D.M., Leybourne, M.I and Kamber, B.S. (2017) A new approach to laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) using the flexible map interrogation tool ‘Monocle’. Chemical Geology, 463, 76-93.

Promises and pitfalls of in situ Sr isotope ratio measurements by LA-MC-ICPMS

Graham Hagen-Peter¹, Rasmus Andreasen¹, Ole Skursch¹, Mark Edwards², Christian Tegner¹, Gry Barfod¹ and Charles Lesher^1
1Centre of Earth System Petrology and Aarhus Geochemistry and Isotope Research Platform, Denmark, ²Department of Earth Science, University of California, Santa Barbara, U.S.A.

The Rb-Sr isotopic system has been used for decades for geochronology and radiogenic isotope geochemistry. The advent of laser-ablation plasma-source mass spectrometry (LA-MC-ICPMS) enhanced the efficiency of Sr isotope analysis and—more importantly—enabled spatially resolved measurements. However, over two decades since the first measurements of Sr by LA-MC-ICPMS, there is still no standard routine for accurate and precise determination of Sr isotope ratios in Ca-bearing silicates with variable Rb/Sr ratios. This is largely because of the analytical challenges inherent to this method, including numerous elemental (Kr, Rb, doubly charged REE) and molecular (CaAr, Ca₂, among others) interferences on the Sr isotopes of interest. Previous studies have successfully measured Sr isotopes in situ in plagioclase, carbonates, and phosphates, but interferences have thwarted accurate measurements of materials with Rb/Sr > ~0.05. We test the effectiveness of several commonly used data-reduction approaches on a large set of LA-MC-ICPMS measurements of two in-house plagioclase standards and widely used rock glass standards with Rb/Sr up to ~0.7. Most approaches effectively reproduce the plagioclase standards but yield highly inaccurate results for higher Rb/Sr standards. We have developed custom data-reduction approaches that now accurately reproduce rock standards with a range of Rb/Sr and compositional matrices, expanding the utility of this method. We highlight several applications of the method, including measurements of plagioclase, volcanic groundmass, and archeological glass samples, and also simultaneous measurements of Sr isotopes and trace elements by laser-ablation split-stream analysis.

In tandem K–Ca and Rb–Sr dating of potassic minerals by LA-ICP-MS/MS: Method and prospective applications

Axel S.L. Sjöqvist¹, Thomas Zack¹, K. Johan Hogmalm¹ and Erik Kangas^1
1Department of Earth Sciences, University of Gothenburg, PO Box 460, SE-405 30 Göteborg, Sweden

Recent technical development in ICP-MS instruments facilitates on-line chemical separation of isobaric ions (e.g. ⁸⁷Rb⁺ and ⁸⁷Sr⁺) by gas reactions, which enables dating of beta-decay systems such as Rb–Sr by laser ablation of minerals without wet chemistry (Zack and Hogmalm 2016; Hogmalm et al. 2017). We have analysed mica-group minerals of various ages for K–Ca and Rb–Sr isotopic ratios in tandem by LA-ICP-MS/MS. Both Ca⁺ and Sr⁺ ions are highly reactive with SF₆ gas, whereas K⁺ and Rb⁺ are not. Ca and Sr isotopes were measured as interference-free fluoride reaction products CaF⁺ and SrF⁺, whereas ⁴⁰K and ⁸⁷Rb isotopes were inferred from ⁸⁵Rb⁺ and ³⁹K⁺. Overlap from isotopes of Ar, present in the plasma, was suppressed by adding H₂ gas.

By analysing K–Ca and Rb–Sr isotopes in tandem, these dual systems with different decay rates are analogous to the U–Pb decay series. Rb–K isotope ratios can be assessed on various concordia diagrams and reveal single-spot ages or open-system disturbances. Since the non-radiogenic Ca composition for the purpose of mica dating can be regarded as practically constant (cf. Valdes et al. 2014), the isochron method is not required and single-spot ages can in principle be derived, which would be useful inter alia in provenance studies of sedimentary micas.

References

Fletcher, I.R., McNaughton, N.J., Pidgeon, R.T. & Rosman, K.J.R. 1997: Sequential closure of K–Ca and Rb–Sr isotopic systems in Archaean micas. Chemical Geology 138, 289–301.

Hogmalm, K.J., Zack, T., Karlsson, A.K.O., Sjöqvist, A.S.L. & Garbe-Schönberg, D. 2017: In situ Rb–Sr and K–Ca dating by LA-ICP-MS/MS: an evaluation of N2O and SF6 as reaction gases. Journal of Analytical Atomic Spectrometry 32, 305–313.

Valdes, M.C., Moreira, M., Foriel, J. & Moynier, F. 2014: The nature of Earth’s building blocks as revealed by calcium isotopes. Earth and Planetary Science Letters 394, 135–145.

Zack, T. & Hogmalm, K.J. 2016: Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chemical Geology 437, 120–133.

Behaviour of boron isotopes during magmatic degassing: a case study from Lesbos Island, Greece.

Konstantinos Thomaidis¹, Frances M. Deegan¹, Martin J. Whitehouse², Valentin R. Troll¹, Franz A. Weis¹, Angela H. Helbling³, Horst R. Marschall³ and Harri Geiger^1
1Department of Earth Sciences, Uppsala University, Uppsala, Sweden, ²Swedish Museum of Natural History, Stockholm, Sweden, ³Goethe University Frankfurt, Frankfurt, Germany

Boron is a fluid mobile trace element routinely employed as a tracer of slab-derived fluids in subduction zones. The fidelity of boron in volcanic glass as an accurate tracer of slab degassing is called into doubt, however, by experiments that show that boron isotopes (expressed as δ¹¹B) can fractionate by up to several permil (‰) during magmatic evolution and degassing at high temperature. In order to test for isotopic fractionation during magmatic degassing in a natural case, we carried out SIMS analyses of boron-rich rhyolitic glass from Lesbos Island, Greece. Analytical points were set as traverses from bubbles into the surrounding glass in order to test for variations in δ¹¹B values due to volatile exsolution. The glass records a post-eruptive average water content similar to the calculated water solubility in the final pre-eruptive melt, suggesting minor pre-eruptive degassing took place. The average boron concentration and δ¹¹B value of the glass is 102 μg/g (n=61) and -4.9‰ (2SD=1.6‰, n=60), respectively, making it one of the most boron-rich subduction zone glasses yet recorded. Moreover, the glass is statistically homogeneous at the 2σ level, with mild heterogeneity at the 1σ level. This observation implies that during degassing at magmatic temperatures in rhyolitic systems, boron is relatively immobile and its isotopes do not significantly fractionate. Our findings allows us to reconstruct the Lesbos magma’s δ¹¹B values at its final holding stage prior to eruption, and suggest that volcanic glass may be a reliable tool for investigation of subduction fluids in arcs.

Melt Inclusion studies on basaltic magmas in Iceland

Thor Thordarson^1
1Faculty of Earth Sciences, University of Iceland, 101 Reykjavík

Investigations of melt inclusions and their host macrocryst in erupted basalts from Iceland have provided important insights into subsurface magmatic processes. This presentation is intended as an overview of studies on melt inclusion in mafic magmas in Iceland. High-Mg# (˃65) melt inclusions are among the most primitive liquids known from Iceland, yet record modification of mantle-derived melts by differentiation and mixing. Furthermore, major and trace element compositions of inclusions spanning the basaltic spectrum demonstrate a trace element disequilibrium between primitive and evolved mafic melts, indicating that a range of mantle melt compositions must have contributed to the formation of the relatively evolved and compositionally constrained mafic magmas typifying the erupted products within the volcanic zones of Iceland. The erupted magmas appear to have evolved with time via polybaric storage at 1.5-8 kbars (= depths of ~5 to 30 km), with further modification up on ascent to eruption as evident from a common 0-1.5 kbar signal overprinted onto macrocrysts. Oxygen isotope measurements on host macrocrysts as well as melt inclusions reveal highly variable d¹⁸O (3.3–5.4 per mil) implying some oxygen isotopic variation within the mantle source. Volatile measurements in melt inclusions indicate a typical pre-eruption concentration in the range of 0.2-1.0 wt% H₂O, 250-˃3000 ppm CO₂ and 1000-2500 ppm sulphur. The sulphur data were used to propose a two-stage degassing model for basaltic fissure eruptions in Iceland, which now has been confirmed by recent study on the sulphur degassing during the 2014-15 eruption at Dyngjusandur, North Iceland.

Explosive ocean island volcanism caused by high water contents in ocean island basalts

Franz A. Weis¹, Valentin R. Troll¹, Frances M. Deegan¹, Henrik Skogby² and Juan Carlos Carracedo^3
1Uppsala University, Dept. of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, SE-752, ²Naturhistoriska Riksmuseet, Dept. of Geosciences SE-114 18 Stockholm, Sweden, ³University of Las Palmas de Gran Canaria, Dept. of Physics (GEOVOL), Las Palmas de Gran Canaria, Spa

Water fundamentally influences the physical properties of the mantle and mantle-derived magmas, including rheology, viscosity, buoyancy, and explosive potential at the surface. Ocean island basalts (OIBs) usually erupt effusively and generally contain ≤1 wt. % H₂O. However, explosive eruptions of OIBs occur on occasion and are thought to relate either to a volatile-enriched mantle source or to shallow gas segregation processes. Here we report on explosively erupted, upper-mantle derived crystal-rich ankaramite lavas from Tanganasoga volcano on El Hierro (Canary Islands) that record magmatic water contents up to 3.2 ±0.64 wt. %. These H₂O values are among the highest known for OIBs to date and were determined through the analysis of hydrogen defects in clinopyroxene crystals. Notably, the Tanganasoga volcano formed within the recent El Golfo giant landslide embayment (87 – 39 ky), and its formation likely reflects rapid magma ascent due to unloading following the El Golfo landslide. Our data also imply that the Tanganasoga magmas are not primary melts but record a usually hidden snapshot of magma evolution in the sub-Canary mantle. At this point strong volatile enrichment in the magma is caused by fractionation processes within the underplating zone beneath the active ocean island, yet the vertical load of the island impedes the magma’s ascent. However, in exceptional cases, sudden external pressure changes, such as from large crustal unloading caused by large landslides, may, similar to the “un-corking” of a champagne bottle, trigger fast ascent and explosive eruptions of water-rich magma and associated explosive ocean island eruptions.

Improved control on interferences in laser ablation studies utilizing N2O as a universal reaction gas: Applications to ore mineral related studies

Thomas Zack^1
1Department of Earth Sciences, University of Gothenburg

Control on reaction chemistry of ions with a range of gases are instrumental in the development of minimizing interferences in ICP-MS applications. A recent breakthrough was achieved in this regard with the introduction of a new generation of ICP-MS that utilizes a configuration called QQQ. It turns out that this is of particular importance for laser ablation-based studies as this technology precludes other chemical treatments to remove interferences.

In the Microgeochemistry Laboratory at the University of Gothenburg we are in the process of evaluating different gases for their use of interference removal/minimization. It turns out that nitrous oxide (N₂O) is particular potent reaction gas. Rigorous testing has revealed that this gas is not only able to remove ⁸⁷Rb from ⁸⁷Sr (therefore allowing in-situ Rb-Sr dating; Hogmalm et al. 2017, JAAS 32, 305), but also serves as an almost universal reaction gas for routine in-situ trace element analysis. Significant improvement in detection limits is achieved for a wide suite of elements: Si, P, Ca, Ti, V, Cr, Mn, Fe, Ge and Se. A major reason for this is the minimization or removal of MAr⁺ and M²⁺ interferences, respectively.

I will demonstrate that N2O as a universal reaction gas leads to a more reliable analytical protocol in laser ablation studies, in particular in ore related studies where a wide variety of matrixes (sulphides, oxides, etc) leads to multitude of severe interferences that can easily lead to false results (e.g., the well-known false Rh and Pd values in the presence of Cu).

                       POSTER PRESENTATIONS                    

New Scanning Electron Microscope for automated mineralogical, crystallographic and chemical analyses

Nynke Keulen¹ and Rikke Weibel^2
1Petrology & Economic Geology, GEUS, Denmark, ²Reservoir Geology, GEUS, Denmark

The Geological Survey of Denmark and Greenland (GEUS) recently acquired a ZEISS Sigma 300VP Field Emission Scanning Electron Microscope (SEM), and can now make automated mineralogic, crystallographic and chemical analyses on entire thin sections or polished mounts relevant for all areas of geoscience research, including mining and exploration, as well as investigations of reservoirs (oil & gas and geothermal energy). The instrument is equipped with 2 Bruker Xflash 6|30 129 eV EDS detectors, an Bruker e^–FlashFS EBSD detector, a 185-850 nm Light-Guide Cathodoluminescence detector, a back-scattered electron detector and three secondary electron detectors, including an in-lense detector. The SEM is able to work under variable pressure conditions, with high vacuum condition of 3.25 x 10^-4Pa and low vacuum conditions between 2-133 Pa. Also it is equipped with the Zeiss Mineralogic software platform for mining and reservoir rocks. With this software, we will be able to perform automated mineralogy on entire thin sections, polished mounts (e.g. drill cores, heavy mineral separates, cuttings samples, ores) or parts thereof. The strength of the new SEM instrument lies in the combination possibilities for the different detectors and imaging techniques, which will shed a completely new light on geoscience microanalysis. In the presentation, we will give an overview over the range of new possibilities, with a focus on magmatic, metamorphic, and hydrothermal processes. We are open to discuss collaboration opportunities.

In situ Rb-Sr dating of snowball Earth tillite clasts for provenance study on the Chuos formation, Namibia.

Martin Thor¹ and Thomas Zack^1
1Department of Earth Sciences, University of Gothenburg

Provenance studies can assist with valuable insights into transportation distance and mode of transportation when producing reconstructions on ice sheet movement and mechanics. Recent advancements in in situ Rb-Sr dating makes dating felsic clasts with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) possible. This study applies these techniques to produce accurate and precise Rb-Sr dates on clasts from the Chous formation.

The Otavi group in Namibia, which contains both the Ghaub and Chous formations, has been the subject of intense study focused on the snowball Earth hypothesis. The Ghaub and Chous formations have been hypothesized to represent snowball Earth deposits because of their cap carbonates (Hoffman et al., 1998). Some of the clasts in the tillite are of felsic composition, with abundant K-bearing mineral phases, making them suitable for dating using Rb-Sr.

Using in situ Rb-Sr dating with LA-ICP-MS paired with an reaction chamber with N₂O (Zack and Hogmalm, 2016, Hogmalm et al., 2017), clasts from the Chuos tillite will be dated to examine whether their provenance can be determined. The aim of this study is to investigate if a continental-scale transportation distance during the Sturtian glaciation can be inferred. The results could potentially have an impact on how we view and interpret snowball Earth ice-sheet dynamics. Initial tests have yielded ages from pegmatite clasts, 925 Ma and 975 Ma respectively, which does not appear to have been reset or altered during the Damaran orogeny (550-495 Ma).

Keywords: Snowball earth, In situ Rb-Sr, LA-ICP-MS, Chous formation, Provenance study

References

HOFFMAN, P. F., KAUFMAN, A. J., HALVERSON, G. P. & SCHRAG, D. P. 1998. A Neoproterozoic snowball earth. Science, 281, 1342-1346.

HOGMALM, K. J., ZACK, T., KARLSSON, A. K.-O., SJÖQVIST, A. S. & GARBE-SCHÖNBERG, D. 2017. In situ Rb–Sr and K–Ca dating by LA-ICP-MS/MS: an evaluation of N 2 O and SF 6 as reaction gases. Journal of Analytical Atomic Spectrometry, 32, 305-313.

ZACK, T. & HOGMALM, K. J. 2016. Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chemical Geology, 437, 120-133.

1.10. Geochemistry and modeling of Igneous Systems.

                       ORAL PRESENTATIONS                    

Markov Chain Monte Carlo Inversion of Mantle Temperature and Composition Beneath Iceland

Eric Brown¹, Kenni Petersen¹ and Charles Lesher^1
1Department of Geoscience, Aarhus University

Basalts are formed by adiabatic decompression melting of asthenosphere, and thus provide records of the thermal, chemical and dynamical state of the upper mantle. However, uniquely constraining mantle temperature, composition and flow through the lens of melting is challenging given the inevitability that primary basalts are aggregates of partial melts derived from heterogeneous mantle over a range of P-T conditions. We aim to place more rigorous constraints on the uniqueness and uncertainties of mantle source properties by coupling a Markov Chain Monte Carlo (MCMC) sampling technique with forward modeling of polybaric near-fractional fusion of heterogeneous mantle using REEBOX PRO [1]. The MCMC method systematically samples the distribution of mantle potential temperature (T_P), and initial abundances and compositions of the source lithologies that produce models that “fit” geochemical and geophysical observations within their associated uncertainties. We have applied the model to magmatism along Reykjanes Peninsula in Iceland, exploring melting of depleted peridotite ± G2 pyroxenite [2] ± enriched KG1 peridotite [3] ± harzburgite. Best-fit models have depleted peridotite + enriched peridotite/pyroxenite sources having T_P ~122 ± 15 °C above ambient mantle with ~95% depleted peridotite and ~5% pyroxenite spanning likely compositions for G2 and KG1. The enriched lithologies have EMORB-like [4] trace element compositions and Paleozoic mean ages, whereas the depleted peridotite exhibits very-incompatible element enrichments relative to DMM [5]. These results advance our understanding of the petrogenesis of the Iceland source that differs significantly from depleted mantle sampled by the global spreading ridge system.

References
1 Brown and Lesher (2016), G-cubed, 17, 3929-3968

2 Pertermann and Hirschmann (2003), Journal of Petrology, 44, 2173-2201

3 Kogiso et al. (1998), Earth and Planetary Science Letters, 162, 45-61

4 Gale et al. (2013), G-cubed, 14

5 Workman and Hart (2005), Earth and Planetary Science Letters, 231, 53-72

Enriched lithosphere contribution to the genesis of the Freetown Layered Complex (Sierra Leone): Isotope systematics of a high-Ti CAMP intrusion.

Sara Callegaro¹, Andrea Marzoli², Hervé Bertrand³, Janne Blichert-Toft³, Laurie Reisberg⁴, Giancarlo Cavazzini⁵, Fred Jourdan⁶, Joshua Davies⁷, Urs Schaltegger⁷ and Massimo Chiaradia^7
1CEED – University of Oslo, ²University of Padova, ³Université Lyon 1, ENS Lyon, ⁴CRPG – CNRS Nancy, ⁵IGG-CNR Padova, ⁶Department of Applied Geology, Curtin University, ⁷Département des Sciences de la Terre, Université de Genève

We present a geochemical and geochronological study of a mafic layered intrusion cropping out along the Atlantic coast of Sierra Leone – the Freetown Layered Complex (FLC). Geochronology (⁴⁰Ar/³⁹Ar on plagioclase: 201.7±0.7 and 202.3±2.3 Ma; U-Pb on baddeleyite: 198.794 ± 0.048/0.071/0.22 Ma) and crystal chemistry (high-TiO₂ pyroxene) demonstrate the connection between the FLC and the high-Ti magmatism of the Central Atlantic Magmatic Province (CAMP). Yet, Sr, Nd, Hf, Pb, and Os isotopes reveal an unusual signature for the FLC compared to most other CAMP occurrences previously studied. Particularly distinctive of the FLC rocks are their low ²⁰⁶Pb/²⁰⁴Pb and high ²⁰⁷Pb/²⁰⁴Pb, suggesting involvement of an ancient component in the genesis of these magmas. Although some lower crustal assimilation is isotopically confirmed and also suggested by the presence of a granulite xenolith in one of the analysed rocks, this process alone cannot be responsible for the observed isotopic fingerprint of the FLC. We rather propose that the most straightforward way to confer the distinctive isotopic signature to the FLC is by hybridization of an upper asthenospheric melt with small volumes (1-3%) of highly enriched alkaline melts derived from the sub-continental lithospheric mantle, possibly lamproites. This scenario is also supported by the geodynamic setting of the FLC, emplaced within a Proterozoic mobile belt (Rokelide) and bordering an Archean craton (Man), as well as by the reported presence of lamproites and kimberlites in the area. Origin of high-Ti magmatism of other Gondwana Large Ignoeus Provinces (Karoo, Paraná-Etendeka) can likewise be explained by similar processes.

Evidence of liquid immiscibility in the 1.8 Ga Raftsund monzonite, Lofoten, Northern Norway

Nolwenn Coint¹, Jakob K. Keiding¹, Peter Ihlen¹ and Suzanne McEnroe^2
1Geological Survey of Norway, ²Department of Geoscience and Petroleum, Norwegian University of Science and Technology

The significance of silicate liquid immiscibility and its petrogenetic role has been debated for many years. Although well documented in mafic layered intrusions, it is not clear if liquid immiscibility can occur in more evolved alkaline rocks. Here, we present evidence for this process in the 1.8 Ga Raftsund batholith; a monzonitic to locally granitic ferroan intrusion. Fe-Ti-P-rich mineralizations occurs as cm-scale up to 200 m x 50 m large scattered bodies, displaying both sharp and gradational contacts with the surrounding host pigeonite-clinopyroxene monzonite to syenite. The mineralizations are composed of anhedral rare pigeonite, Fe-rich olivine (Fo₂₂–₂₉), subhedral clinopyroxene (Wo_34-45 En_20-46 Fs_23-44), anhedral magnetite and ilmenite, scattered late hornblende and variable proportions of subhedral ternary feldspars. Inclusions of apatite, which can reach up to 12 modal percent, are present in all minerals but ternary feldspars. The scarcity of pigeonite in the Fe-Ti-P-rich rocks and the presence of Fe-rich olivine indicate that these rocks cannot represent direct cumulate from the host monzonite and syenite. Bulk-rock compositional variations are well explained by recent experimental work on liquid immiscibility (Charlier & Grove, 2012) with enrichment in Fe, Ti, P, Mg, Ca, Sc, Co, Zn, Cu, V and REE in the mineralizations. Clinopyroxene from the mineralizations is enriched in Sc and to a lesser extent in Ti, Zn and depleted in V, Co, Sr, Y, Nb, LREE and Al, which is consistent with co-precipitation of apatite and magnetite and the formation of such a melt by liquid immiscibility.

References

Charlier, B. & Grove, T. L. 2012: Experiments on liquid immiscibility along tholeiitic liquid lines of descent. Contributions to Mineralogy and Petrology 164, 27–44.

Evolution of a deep crustal magma conduit system

Thomas B. Grant¹, Rune B. Larsen¹, Axel Müller² and Suzanne McEnroe^1
1NTNU, ²NHM Oslo

Seiland Igneous Province (SIP), Northern Norway represents the lower crustal plumbing system of a LIP where large volumes of ultramafic, mafic and alkaline-carbonatite melts were transported from the mantle to shallower levels of the crust. The Reinfjord mafic-ultramafic intrusion is a magma conduit system and offers an excellent location in which to study differentiation processes as magmas enter the crust (Larsen et al 2016).

Field relationships clearly show the Reinfjord intrusion formed by progressive influxes of new melts into earlier cumulates, resulting in a sequence of gabbro – olivine clinnopyroxeneite – wehrlite – dunite from the margins to the core (Grant et al 2016). Here, we determine the relative contributions of fractional crystallization, assimilation and recharge on the compositions of ultramafic cumulate layers.

Trace elements in clinopyroxenes were determined using LA-ICP-MS. Melts in equilibrium with the clinopyroxenes from Reinfjord have OIB-like characteristics, similar to picrite dykes in the SIP. For dunites and wehrlites, fractional crystallization was the dominant processes. The magmas did not fractionate by more than 30%, implying large volumes of magma were transported vertically to shallower levels of the crust.  For olivine clinopyroxenites, LREE/HREE ratios record the importance of assimilation as well as fractional crystallization.

With time, the intrusion evolved to more olivine-rich and ultramafic cumulates, fractional crystallization become more dominant over assimilation, and the magmas leaving the conduit system became increasingly MgO-rich. These results may explain geochemical variations observed in flood basalt sequences (Yu et al 2015).

References

Grant, T.B., Larsen, R.B., Anker-Rasch, L., Grannes, K.R., Iljina, M., McEnroe, S., Nikolaisen, E., Schanche, M. and Øen, E., 2016: Anatomy of a deep crustal volcanic conduit system; The Reinfjord Ultramafic Complex, Seiland Igneous Province, Northern Norway. Lithos, 252, 200-215.

Larsen, R.B., Grant, T., Sorensen, B.E. and McEnroe, S.A., 2016: February. Mixing-Mingling and Modification of Magmas in a Giant Deep Crustal Magmatic Reservoir: The Seiland Igneous Province, Northern Norway. In AGU Fall Meeting Abstracts.

Yu, X., Lee, C.T.A., Chen, L.H. and Zeng, G., 2015: Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia. Geochemistry, Geophysics, Geosystems, 16(7), 2082-2096.

ORAL

Unraveling the differentiation of a flood basalt sequence using Magma Chamber Simulator

Jussi S. Heinonen¹, Arto V. Luttinen², Wendy A. Bohrson³ and Frank J. Spera^4
1Department of Geosciences and Geography, University of Helsinki, ²Finnish Museum of Natural History, University of Helsinki, ³Department of Geological Sciences, Central Washington University, ⁴Department of Earth Science and Earth Research Institute, University of California Santa Barbara

The Magma Chamber Simulator (MCS; Bohrson et al., 2014) models the effects of recharge, assimilation, and crystallization on phase equilibria, mineral chemistry, major elements, trace elements, and radiogenic isotopes through mass and enthalpy balance in a multicomponent-multiphase magma + wallrock system. Here we provide an example of utilizing MCS in deciphering the differentiation history of a compositionally uniform continental flood basalt sequence characterized by strong crustal geochemical signature from the Antarctic portion of the Karoo large igneous province. Our modeling shows that only a few percent of assimilation of sialic crust may have profound impact on the composition of the parental magma (major elements, 16 trace elements, and Nd isotopes). Isobaric and continuously assimilating models fail to produce the observed eruptive compositions. Instead, we suggest that contamination of the picritic parental magmas by assimilation of mid-crustal Archean TTGs (by stoping + hybridization or partial melting or both) must have first taken place at depths of ~10‒20 km, where orthopyroxene and olivine are stable phases in the magma. Most of the subsequent differentiation happened in a complex network of dikes and sills in the shallow upper crust without additional assimilation. There, the large temperature gradient between magma in narrow feeding channels and country rock precluded anatectic melt formation and effectively shielded the magma from incorporation of wallrock partial melts. This example illustrates the capabilities of MCS in testing possible petrogenetic scenarios and encourages application of the MCS tool to different igneous environments and tectonic settings where assimilation takes place.

References

Bohrson, W.A., Spera, F.J., Ghiorso, M.S., Brown, G.A., Creamer, J.B. & Mayfield, A. 2014. Thermodynamic Model for Energy-Constrained Open-System Evolution of Crustal Magma Bodies Undergoing Simultaneous Recharge, Assimilation and Crystallization: the Magma Chamber Simulator. Journal of Petrology 55, 1685–1717.

Deep crustal homogenization of diverse mafic and ultramafic LIP-forming mantle melts: Lessons from the Ediacaran Seiland Igneous Province (SIP), Norway

Rune B. Larsen¹, Thomas S. Grant¹, Bjørn E. Sørensen¹, Suzanne McEnroe¹ and Christian Tegner^2
1Dep. of Geosciences and Petroleum, NTNU, Trondheim, ²Earth System Petrology, D. of Geosciences, Århus University

SIP contains >5000 km2 of mafic/ultramafic intrusions with minor alkaline, carbonatite and felsic rocks emplaced at depth of 25 – 35 km during the formation of the Central Iapetus Magmatic Province at 610-550 Ma (CIMP) and exposes 85-90 % layered gabbros, 8-10 % peridotitic complexes, 2-4 % alkaline intrusion emplaced within <10 Ma. Gravimetric data suggests that SIP features six deep lithospheric roots of ultramafic rocks extending in the depths of > 9 km that resembles the conduit systems. Four of the ultramafic complexes are exposed at SIP in a right-way-up setting. They feature a marginal hybrid zone at the country rock contacts, then an olivine-mela-gabbro grading to pyroxenite, then an olivine-clinopyroxenite zone which is followed by a wehrlite zone and, finally, pure dunite in the centre. From pyroxenite to dunite, olivine composition changes from Fo₇₂ to Fo₈₅ and clinopyroxene from Di₈₀ to Di₉₂ i.e. forming a reverse fractional crystallization inwards. Parental melts were komatiitic to picritic with 16-21 wt% MgO, 1594 ppm Cr and 611 ppm Ni and were emplaced at 1450-1500 ^oC. Melts are OIB-like with LREE enriched over HREE. The high abundance of carbonated and hydrated mineral assemblages imply a volatile-rich nature of the mantle source region also corroborated by unusually high abundance of magmatic sulphides.

Essentially, the ultramafic complexes in SIP comprise deep-seated transient magma chambers that facilitated mixing and homogenization of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust.

Geochemistry of the Feragen ultramafic body, Central Norway.

Anna Pryadunenko¹, Lars P. Nilsson² and Rune B. Larsen^1
1Department of Geoscience and Petroleum, NTNU – Norwegian University of Science and Technology, ²Geological Survey of Norway

Tectonically dismembered ophiolite fragments stretch for several hundred kilometres along the Swedish-Norwegian border [1]. One of the fragments, the Feragen ultramafic body, located 30 km east of the town of Røros represents an excellent natural laboratory for studying upper mantle melting and melt extraction processes. Upper mantle rocks in Feragen are represented by harzburgite, that occasionally changes composition towards lherzolite and a subordinate amount of dunite. In the north-western part of the massif, close to a presumed mantle-crust boundary, the amount of dunite exceeds that of harzburgite. Dunitic rocks were observed both as replacive dunites with gradual transitions to harzburgite up to several meters wide, and as dunite channels with sharp borders to host harzburgite and a maximum width of 50 cm. Harzburgite relicts of various shape are often present within the dunite channels and the replacive dunites.

Olivine present in the rocks is higly magnesian with Mg numbers ranging from 90 to 91 in harzburgite and from 92 to 94 in dunite. NiO content of olivine in all the rock types does not exceed 0.4 wt %. Some of the dunites are rich in chromian spinel. Cr numbers for spinel are between 0.4 and 0.6. TiO₂ content of spinel minerals is as low as 0.15 wt % [2]. Chromitites show extreme depletion in Pt and the overall PGE content implies depleted mantle composition for the rocks analysed. Low TiO₂ content of spinel and high Fo content of olivine suggest a high degree of melting, that is greater than 25%

References
1.       Nilsson, L.-P. & Roberts, D. (2014) A trail of ophiolitic debris and its detritus along the Trøndelag-Jämtland border: correlations and palaeogeographical implications. Norges geologiske undersøkelse Bulletin, 453, 29–41.

2. Cotkin, S.J. (1983) The petrogenesis and structural geology of the Feragen peridotite and associated rocks, Sør-Trøndelag, East-central Norway. Unpubl. M.Sc. thesis, University of Wisconsin, Madison. 180 pp + map

Mica chemistry – a genesis indicator of the Sveconorwegian pegmatites, southern Norway

Nanna Rosing-Schow¹, Axel Müller² and Henrik Friis^1
1Natural History Museum, University of Oslo, P. O. Box 1172, Blindern, 0318 Oslo, Norway, ²Natural History Museum, University of Oslo; Natural History Museum, Cromwell Road, London, UK

Mica minerals are the third most common component of granitic pegmatites. They contain important major and trace elements including Li, F, and Rb, which can be used for pegmatite classification and to better understand the origin and genesis of pegmatites (Marchal et al. 2003; Breiter et al. 2017). Southern Norway is known for its large pegmatite province with more than 5000 pegmatite bodies, occurring in the Sveconorwegian orogeny (1.1-0.9) and forming one of the world’s largest pegmatite clusters. We present the first comprehensive comparison of mica from the Tørdal, Evje-Iveland and Froland pegmatite fields. Mica from Tørdal is significantly enriched in both Li₂O wt% and F % compared to mica from the Evje-Iveland and Froland. The Tørdal mica contains up to 5.4 wt% F and 6.0 wt% Li₂O, while only up to 2.6 wt% F and 0.6 wt% Li₂O are measured from the Evje-Iveland field and 1.2 wt% F, 0.3 wt% Li₂O in the Froland mica. A similar pattern is seen in the K/Rb ratios with the lowest values found in the Tørdal mica ranging from 3.63 to 27.5 contrary to 16.5-365 and 29.9-62.5 from Evje-Iveland and Froland, respectively. The Tørdal pegmatite field contains the most evolved mica and thereby the most evolved pegmatites. The strong enrichment of Li, F and Rb suggests that the source of the Tørdal pegmatite melts might be different than for the Evje-Iveland and Froland fields. The latter are thought to be of anatectic origin.

References
Breiter, K., Vaňkova, M., Galiová, M.V., Korbelová, Z. & Kanický, V. 2017: Lithium and trace-element concentrations in trioctahedral micas from granites of different geochemical types measured via laser ablation ICP-MS. Mineralogical Magazine 81, 15-33.

Marchal, K.L., Simmons, W.B., Falster, A.U., Webber, K.L. & Roda-Robles, E. 2014: Geochemistry, mineralogy, and evolution of Li-Al micas and feldspars from the mount mica pegmatite, Maine, USA. The Canadian Mineralogist 52, 221-233.

Chronology of the Nebo Granite, Bushveld Complex

Ole Skursch¹, Christian Tegner¹, Fernando Corfu² and Charles E. Lesher^1
1Earth System Petrology Group, Dept. of Geoscience, Aarhus University, Aarhus, Denmark, ²Dept. of Geosciences, University of Oslo, Oslo, Norway

The world’s largest A-type granite unit (the Nebo Granite)overlies the layered mafic and ultramafic cumulate rocks of the Rustenburg Layered Suite (RLS) Together with minor granophyre and other granites these units make up the Bushveld Complex, South Africa. The Nebo Granite often cuts the RLS and existing zircon U-Pb chronology supports the view that the granite is slightly younger and essentially unrelated to the RLS.The age of the RLS is very well constrained at 2055.91±0.26 to 2054.89±0.37 Ma (Zeh et al., 2015)We present new ID-TIMS zircon U-Pb chronology for 9 samples of the Nebo Granite and a related granophyric variety. We show that the ages obtained from the Nebo Granite are indistinguishable from each other and from the age of the RLS. Moreover, the granite also contains minor amounts of zircon that are slightly (<9 m.y.)older than the crystallization age. The older zircons are morphologically similar to the young zircons and are found in all samples we have investigated. Their ages also overlap the age of the earliest phase of magmatism in the Bushveld LIP.These observations suggest that the older zircons are inherited from magmatic rocks that formed during the earliest phase of Bushveld magmatism and are not xenocrysts from older basement rocks. Even though the Nebo Granite is probably an assembled body, it crystallized too fast for its temporal evolution to be resolved. Contemporaneity between the RLS and the Nebo Granite challenges the view that they are essentially unrelated and introduces the possibility of consanguinity between the two units.

References
Zeh, A. Ovtcharova, M., Wilson, A.H., Schaltegger, U. 2015. The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications. Earth and Planetary Science Letters 418, 103–114.

Fe and Si isotope systematics of the Thingmuli volcano, Iceland: the roles of crystal – melt fractionation, oxygen fugacity and melt structure

Niklas Stausberg¹, Christian Tegner², Gry Barfod² and Charles Lesher^2
1Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus-C, ²Earth System Petrology, Department of Geoscience, Aarhus University

The Tertiary Thingmuli volcano, Eastern Iceland, is a classic example of tholeiitic differentiation spanning the compositional range of basalt to rhyolite. Whole rock and mineral compositions coupled with forward modeling indicate fractionation of olivine, pyroxene, plagioclase and FeTi-oxides buffered at an oxygen fugacity slightly below the FMQ buffer. The well-constrained liquid line of descent for Thingmuli provides an opportunity to investigate stable isotope crystal-liquid fractionation for the two most abundant major elements, Fe and Si. Iron isotope fractionation is expected to be sensitive to redox condition, while Si is not. Fe and Si were chemically purified and analyzed for δ⁵⁶Fe and δ³⁰Si with a Nu Plasma II MC-ICPMS at Aarhus Geochemistry and Isotope Research Platform (AGiR), Aarhus University, using pseudo-high resolution and sample-standard bracketing. Fe isotope compositions of Thingmuli lavas range from δ⁵⁶Fe = 0.042 ‰ to 0.118 ‰ for mafic and intermediate compositions, and 0.124 ‰ to 0.650 ‰ for rhyolites. The heaviest values correspond to the most evolved, Fe-poor lavas. Preliminary Si isotope data ranges from δ³⁰Si = -0.36 ‰ to -0.06 ‰ and shows a positive linear correlation with whole rock SiO₂ (wt. %) content. Forward modeling of the stable isotope compositions shows that Fe³⁺/Fe²⁺ partitioning between melt and crystallizing solids is the principal contributor to iron isotope fractionation, except during the final stages of crystallization where high melt polymerization affects the coordination environment and fractionation of iron. In contrast, Si isotope fractionation reflects the degree of Si-polymerization of melt and solid phases throughout the crystallization sequence.

Geochemical discrimination criteria for indium-potential granites in the Fennoscandian Shield

Krister Sundblad¹, Ivan Alexeev², Matti Vuorisalo³, Bence Balogh³ and Susanna Metso^3
1University of Turku, Finland and State University of St Petersburg, Russia, ²State University of St Petersburg, Russia, ³University of Turku, Finland

Spectacular discoveries of indium-rich polymetallic sulphide/oxide deposits have recently been reported from the Fennoscandian Shield; Pitkäranta, Russian Karelia (Valkama et al., 2016a) and Sarvlaxviken, southern Finland (Cook et al., 2011; Valkama et al., 2016b; Broman et al., 2017). These sulphide/oxide deposits are closely associated with A-type granites, hosted by marbles (Pitkäranta) at km-distance from the 1.54 Ga Salmi Batholith or various granite types (Sarvlaxviken) within the 1.64 Ga Wiborg Batholith.

The batholiths display complex magmatic evolution trends from gabbro-anorthosites, via wiborgites, pyterlites and transitional granite phases to evolved late-stage plutons. The wiborgites and pyterlites have typical WPG geochemical patterns with shallow REE patterns and significant negative Eu anomalies but did not create any hydrothermal systems or metal accumulations.

In contrast, some (but not all) late phase granite plutons (< 1 km diameters), provided metals and heat for the metal-rich hydrothermal fluids and thus indium-rich polymetallic accumulations. These plutons have also WPG characteristics (although fake “syn-collisional granite” signals are seen in some discrimination diagrams due to hydrothermal influence).

Zr/Hf    Rb/Ba

Wiborgites, Sarvlaxviken                                                 43-50    < 0.2

Pyterlites, Pitkäranta                                                       28-40    < 0.3

Transitional phases (Repomäki and Nietjärvi), Pitkäranta  15-24    2- 18

Late granite phase (Marviken), Sarvlaxviken                      17-22   11- 23

Late granite phases, Pitkäranta                                         7-14   10-150

The late igneous phases can be evaluated by several geochemical criteria but the low Zr/Hf ratios, high Rb/Ba ratios and enrichments of heavy REE are the most characteristic features for the ore fertile granites (Torppusuo, Ristinoja-Mosautodor and Uuksu in the Pitkäranta area and Marviken in the Sarvlaxviken area).

References
Broman, C., Sundblad, K., Valkama, M. & Villar, A., 2017: Deposition conditions for the indium-bearing polymetallic quartz veins at Sarvlaxviken, south-eastern Finland. Mineralogical Magazine, in press.

Cook, N.J., Sundblad, K., Valkama, M., Nygård, R., Ciobanu, C.L. & Danyushevsky, L., 2011: Indium mineralization in A-type granites in southeastern Finland: insights into mineralogy and partitioning between coexisting minerals. Chemical Geology 284, 62-73.

Valkama, M., Sundblad, K., Cook, N.J. & Ivashchenko, V.I., 2016a: Geochemistry and petrology of the polymetallic skarn ores at Pitkäranta, Ladoga Karelia, Russia. Mineralium Deposita 51, 823-839.

Valkama, M., Sundblad, K., Nygård, R. & Cook, N.J., 2016b: Geochemistry and petrology of the polymetallic veins at Sarvlaxviken, Finland. Ore Geology Reviews 75, 206-219.

                       POSTER PRESENTATIONS                    

Chronology and geochemical and structural evolution of dykes intruding the Reinfjord ultramafic to mafic intrusion in the Seiland Igneous Province

Alf Andre Orvik¹, Bjørn Eske Sørensen¹, Rune Larsen¹ and Thomas Grant^1
1Department of Geoscience and Petroleum, NTNU, Norway

This study investigates the chronology, geochemical and structural evolution of dykes intruding the Reinfjord Ultramafic Complex (RUC) consting of large volumes of ultramafic, mafic, silicic and alkaline melts intruded the lower crust (~30 km) from 570-560 Ma in an extensional regime. The RUC was nested to baltica the Caledonian orogeny. Earlier studies have indicated horizons with anomalous concentrations of Pt+Pd+Au+Os and enriched horizons of Cu-Ni.

The ultramafic rocks are cut by several generations of dykes ranging from replacive dunite dykes intruded into the semi-consolidate host-rock with gradual contacts, to late gabbroic dykes that has a chilled margin. The dykes have complex crosscutting relations and in areas it constitutes up to 50 % of the outcrop volume. Large variability in chemistry and volatile content ranging from wherlites, olivine clinopyroxenites, hornblende bearing olivine clinopyroxenites, hornblende gabbro, alkaline and lamphrophyric dykes.

A relative chronology of the dykes was established by the cross cutting relations of the dykes during field work and post processing of 3D photogrammetry from drone images will be used to estimate the melt volumes within the different dyke types and their orientation evolution over time within selected locations of the central series. Petrographic descriptions of attained samples together with chemical analysis, and hopefully radiometric dating, will in combination with the photogrammetric data give insight in the evolution of the melt related to the dykes. Our ambition is that the data will provide valuable information and constrain the chronological progression of the geochemistry and tectonomagmatic association of the dykes.

Platinum Group Minerals in the Reinfjord Ultramafic Complex

Even Sunnanå Nikolaisen¹, Rune Berg-Edland Larsen¹, Thomas Grant, Bjørn Eske Sørensen¹ and Suzanne McEnroe^1
1Norwegian University of Science and Technology (NTNU), Department of Geoscience and Petroleum. Sem S

An upwelling mantle plume 560 – 570 Ma formed the Seiland Intrusive Province (SIP), a 5400 km² area of felsic to ultramafic rocks in Northern Norway. The Reinfjord Intrusive Complex (RUC) is the southernmost part, exposing 25km² of ultramafic rocks that are generated from three intrusive magmatic events and crystal fractionation. These events show evidence that RUC is representing an open magma chamber, connected to a conduit system at 25-30km depth and subjected to extensive magma replenishment.

Exploration drill cores in RUC in 2011 (RF-1 and RF-2) show two spikes for economic elements, one Cu-Ni reef low in PGE at depth of 86-93m and a PGE reef low in Cu-Ni at 107-113m

A detailed analysis of a 6 m drill core section, suggest PGE being segregated in different lithologies, which combine to a total of 0.79 ppm PGE. When comparing the δ³⁴S signature of the PGE reef with the Cu-Ni reef shows a distinct difference -0.40 (PGE) and -4.56 (Cu-Ni), indicating different sources.

This study looks into the details of the PGE’s, with the aim of identifying the ore forming processes. SEM imaging and mapping, EPMA, magnetic analysis and whole rock geochemistry are used to find cryptic variations in the PGE rich lithologies. BSE and EDS detects and identify the PGE’s, with EPMA data to calculate empirical formulas for PGM identification. Based on the results, it is clear that despite a sub-economical grade, the RUC provide a valuable insight in to the ore forming potential for deep-seated layered intrusions.

1.11. Open session on petrology and geochemistry

                       ORAL PRESENTATIONS                    

Additional constraints to the evolution of the early solar system? A U-Pb systematics study of ungrouped differentiated achondrite NWA4587.

Yuri Amelin¹ and Elin Rydeblad^2
1Australian National University, ²Uppsala University

The study of meteorites can help us answer questions still under debate regarding the early solar system and the processes therein. It could help us unify a timescale for the development of the solar system, help us fully understand how our planets formed, and tell us about the nature of elemental dispersal throughout the early solar system (Amelin and Ireland, 2011). It will also allow us to investigate how our solar system compares to other solar systems – giving us an invaluable tool in the search for other habitable planets and possible life, and future celestial exploration (White, 2011).

This study aims to add additional constraints to the evolution of the early solar system by the isotopic analysis of ungrouped differentiated achondrite North West Africa (NWA)4587.

All sample processing was performed at the SPIDE²R laboratory facilities, located at the Research School of Earth Sciences (RSES) at the Australian National University (ANU). For the U-Pb systematics, four separate hand-picked pyroxene fractions were analysed. The fractions were chosen based on mineral grain texture, colour, and size. This study employed an acid leaching and dissolution protocol, combined with single- and double pass μ-column protocols. The Pb ratios and isotopic composition of the samples were analysed using a thermal ionisation mass spectrometer.

The age of NWA4587 is 4563.63±0.55 Ma, suggesting that its parent body was formed during early planetary accretion. Further analyses of Rb-Sr isotope systematics are currently being conducted, and the preliminary results are concordant with the age determined by the U-Pb systematics.

References
Amelin, Y. and Ireland, T. (2013). Dating the Oldest Rocks and Minerals in the Solar System. Elements, 9(1), pp.39-44

White, W. M. (2011). Isotope geochemistry. John Wiley & Sons. (pp. 85-144).

The EARTHTIME Initiative: A community working to accelerate the development and application of integrated methodologies for the quantification of geological time.

Daniel Condon^1
1British Geological Survey, Nottinghamshire, NG12 5GG, UK

The EARTHTIME Initiative is an international community effort to ‘To accelerate the development and application of integrated methodologies for the quantification of geological time for the underpinning of Earth sciences’. The first phase (from 2004 and ongoing) was focused on addressing urgent shortcomings in the ‘high-precision’ geochronometers (e.g., U-Pb and ⁴⁰Ar-³⁹Ar), issues that limit widespread application at the quoted levels of precision/accuracy. Many of these (e.g., inter-laboratory bias) have been addressed through experiments conducted over the past 13 years and the accuracy and precision of both of these widely used dating systems has been significantly improved. The EARTHTIME Initiative has brought the ⁴⁰Ar-³⁹Ar and U-Pb isotope dilution sides together (for both experiments and application) but also expanded to include a range of decay schemes (e.g., Re-Os and U-Th) and analytical approaches (e.g., microbeam U-Pb). Perhaps the most significant outcome has been the development of a strong and lasting sense of a ‘geochronology community’.

Whilst these Phase 1 activities continue, the original motivation for accelerating progress in the various dating methods has been that they underpin a wide range of Earth science research areas. Starting in 2016 a second phase of EARTHTIME has been initiated, following two community workshops, with a renewed emphasis on engagement of the geochronology community with the ‘end-users’ of geochronology. This presentation will provide a summary of EARTHTIME efforts since 2004 to present to calibrate and inter-calibrate the geochronometers, and will provide a vision and strategy for the second phase of the EARTHTIME Initiative.

Pangea assembly and its implications to the mid Permian–Triassic magmatism on the southwestern margin of South America

Álvaro del Rey¹, Katja Deckart², César Arriagada² and Fernando Martínez^2
1Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark, ²Departamento de Geología, Universidad de Chile, Santiago, Chile.

The Chilean Frontal Andes batholiths (28°–31°S) represent part of the extensive late Paleozoic–Triassic magmatism along the southwestern margin of South America. Despite its importance, a consensus regarding the tectonic model controlling the magmatism has not been reached. Several models^1,2,3 have proposed cessation of subduction as the reason behind the apparent lack of typical arc magmas and abundance of within plate signatures. Yet the causes for this cessation differ and do not provide a comprehensive model for coeval units along the margin. Here, new LA-ICPMS U–Pb in zircon ages, and geochemical and isotope analyses (Rb-Sr, Sm-Nd, Lu-Hf, Re-Os and d¹⁸O) indicate that mid Permian–Triassic granitic magmatism originated in a slab rollback extensional setting with increasing thinning towards Triassic times. Subduction and anataxis of lower crust were the main magma-generation mechanisms, the latter caused by decompression and subsequent accumulation of underplating basalts. A comparison with other igneous units along the Chilean–Argentinian border allows extension of this model from at least 21° to 40°S. The key element triggering intense extension in a subduction setting (slab rollback) is low subduction velocities -as numerical models have predicted⁴, which can be attributed to the transitory stage of Pangea (mid Permian–Triassic). Hence, subduction of the paleo-Pacific oceanic plate beneath paleo-South America has been a continuous process from early Paleozoic times onwards; and Pangea assembly, the setting behind the observed changes in the magmatism and not the cessation of subduction as previous models have invoked.

References

¹ Mpodozis, C., Kay, S.M., 1992. Late Paleozoic to Triassic evolution of the Gondwana margin: evidence from Chilean Frontal cordilleran batholiths (28°S to 31°S). Geol. Soc. Am. Bull. 104, 999–1014. doi:10.1130/0016-7606(1992)104<0999:LPTTEO>2.3.CO;2

² Franzese, J.R., Spalletti, L. a., 2001. Late triassic- Early jurassic continental extension in SouthWestern Gondwana: Tectonic segmentation and pre-break-up rifting. J. South Am. Earth Sci. 14, 257–270. doi:10.1016/S0895-9811(01)00029-3

³ Kleiman, L.E., Japas, M.S., 2009. The Choiyoi volcanic province at 34°S-36°S (San Rafael, Mendoza, Argentina): Implications for the Late Palaeozoic evolution of the southwestern margin of Gondwana. Tectonophysics 473, 283–299. doi:10.1016/j.tecto.2009.02.046

⁴ Schellart, W.P., 2005. Influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge. Earth Planet. Sci. Lett. 231, 197–219. doi:10.1016/j.epsl.2004.12.019

Tourmaline textures and structures as a tracer for the magmatic-hydrothermal transition in SW England

Kristian Drivenes¹, Rune Larsen¹, Bjørn Eske Sørensen¹ and Sam Hughes^2
1Norwegian University of Science and Technology, ²Camborne School of Mines, University of Exeter

Tourmaline is a common mineral throughout the granites of SW England. It occurs in multiple textural and structural varieties, including veins, quartz-tourmaline orbicules, massive quartz-tourmaline rocks (MQT), pegmatitic pockets, and disseminated in the granite. Field relations indicate both magmatic tourmaline and post-magmatic, hydrothermal tourmaline, occuring as disseminated grains and veins, respectively. In addition, more genetically complex textures are observed as gradual exsolutions of quartz-tourmaline rocks within the granite, with or without associated veining. These appear on the cm scale and up to several meter large bodies. Such structures are observed in localities several km apart, and are a regional feature. Occasionally, the gradual transition from granite to MQT to miarolitic tourmaline or a pure tourmaline zone, can be observed over tens of cm. The microtextures are strongly linked to the macroscopic structures, reflecting the diverse processes that formed the different tourmaline occurrences. Strongly birefringent, green, acicular tourmaline dominates in vein-style tourmaline; disseminated tourmaline in the granite is weakly zoned and brown; tourmaline from MQT is brown with blue rims. Later tourmaline generations overgrow earlier generations, adding to the complexity. Cassiterite is only associated with the hydrothermal green acicular tourmaline. Tin concentrations in other tourmaline-rich rocks are less than 100 ppm, and these rocks are not a part of the mineralizing events of the Land’s End granite.

Dendritic Core Crystallization in Minor Planets – An empirically constrained Numerical Simulation Study

Kim Esbensen¹, Henning Haack², Kenny Erleben³ and Marek Misztal^4
1GEUS, AAU, KHE Consulting, ²Maine Mineral and Gem Museum, Maine, USA, ³Department of Computer Science, Copenhagen University, ⁴Niels Bohr Institute

The Cape York meteorite shower typifies the magmatic iron meteorites. The Agpalilik mass displays characteristic oriented, elongated troilite nodules, interpreted to testify to a dendritic crystallisation mode of surrounding (Fe,Ni) metal phase (Esbensen (1981, 2017); Esbensen & Buchwald (1982); Esbensen et al. (1982; Haack & Scott (1992). The troilite accessory mineralogy testifies that the fractional crystallisation / late magmatic differentiation history of asteroid cores can be modelled in the Fe –Ni –S- P system. Accessory schreibersite and high-Ni taenite are found lining the metal/ troilite interface, representing very late crystallisation from an evolved Fe-Ni-S-P liquid. A troilite/taenite eutectic represents the ultimate liquid-solid transformation product (the paragenesis shows that schreibersite precipitated prior to taenite crystallization, effectively purging the system for P), allowing the ultimate crystallisation to be modeled in the residual Fe-Ni-S system, which thus defines an iron meteorite analogue to the petrogenetic residual system for silicate magmatic rocks. The inter-dendrite residual melt pocket physiography (3^rd or 5^th order arms) allows to attempt an ambitious fractal numerical simulation modelling of the crystallization of the early solar system asteroid/planetoid metallic cores. Agpalilik will serve as an empirical constraint for numerical dendritic crystallization modelling, based on a simplified Fe-S system, believed to approximate (Fe,Ni)-S well. It is first in the latest years that it has been possible to tackle the numerical simulation challenges involved, Misztal MK, Bærentzen JA (2012); Misztal MK, Erleben K, Bargteil A, (2014). We present a first foray overview of the scope of this challenging multi-disciplinary study.

Age and geochemistry of granitoids in the Precambrian basement of Öland, SE Sweden – implications for the extension of the Transscandinavian Igneous Belt in the Baltic Sea region

Evgenia Salin¹, Krister Sundblad¹, Hugh O’Brien², Yann Lahaye² and Jeremy Woodard^3
1University of Turku, 20014 Turku, FINLAND, ²Geological Survey of Finland, 02151 Espoo, FINLAND, ³University of KwaZulu Natal, Westville (Durban), X5 4001, SOUTH AFRICA

The Precambrian crust in the Baltic Sea region is mostly covered by Phanerozoic sedimentary rocks but can be studied in drill cores. Two granitoids from the crystalline basement below Öland were collected from the archives of the Geological Survey of Sweden and analyzed for geochemistry and dated with the U-Pb method on zircons. The Böda Hamn granitoid (northern Öland) has a monzodioritic composition and an age of 1799.8±3 Ma while the Valsnäs granitoid (central Öland) has a quartz monzonitic composition and an age of 1784.9±5.7 Ma. These geochemical-isotopic characteristics are compatible with those of generation 1 of the Transscandinavian Igneous Belt (TIB) in the Fennoscandian Shield, ≥30 km west of Öland.

A more detailed review of the TIB-1 generation shows that two sub-generations (1a and 1b) can be distinguished on each respective side of the Oskarshamn-Jönköping Belt (OJB). Sub-generation 1a (north of OJB) has an age span of 1794-1808 Ma while sub-generation 1b (south of OJB) has an age span of 1769-1793 Ma. According to this subdivision, the Böda Hamn monzodiorite belongs to sub-generation 1a, which also can be followed to southernmost Gotland (Sundblad et al. 2003) and the Valsnäs quartz monzonite belongs to sub-generation 1b, which can be followed to the Latvian/Lithuanian border, where a marginally younger granitoid was reported from off shore drill core E-7 (Salin et al. 2016). Taken together, these data suggest that the Transscandinavian Igneous Belt can be traced across the Baltic Sea from the exposed parts within the Fennoscandian Shield to the Latvian/Lithuanian border.

References
Salin, E., Sundblad, K., Woodard, J. & Lahaye, Y. 2016: The Precambrian crust in the Baltic Sea region. Bulletin of the Geological Society of Finland, 32nd Nordic Geological Winter Meeting, Helsinki. Abstract volume, p. 162.

Sundblad, K., Claesson, S. & Gyllencreutz, R. 2003: The Precambrian of Gotland – a key to the understanding of the geologic environment for granitoids in the Baltic Sea region. Granitic systems – State of the art and future avenues. An international symposium in honor of professor Ilmari Haapala. Abstract volume, Helsinki, 102­–106.

Sr isotope zoning in plagioclase from Cenozoic andesites from Cabo De Gata, Spain: evidence for shallow and deep contamination

Tod Waight¹ and Jakob Tørnqvist^1
1Department of Geosciences and Natural Resource Management (Geology Section), Copenhagen University

Subduction-related andesites from the Cabo De Gata region in Southern Spain are part of the larger Alboran Magmatic Province which stretches from Northern Africa, through the Alboran Sea into Southern Spain. The province is of interest due to the penecontemporaneous occurrence of a wide variety of magma compositions (incl. tholeiitic basalts, calc-alkaline andesites, explosive rhyolites, peraluminous crustal melts and lamproites) and temporal overlap with the Messinian Salinity Crisis. Many plagioclase phenocrysts within andesite domes at Cabo De Gata are characterized by resorbed high-An% cores (An_73-85) surrounded by oscillatory zoned plagioclase (An_40-60). Sr isotope analyses of the high An% cores of these plagioclases show consistently radiogenic ⁸⁷Sr/⁸⁶Sr compositions (0.7127-0.7129). These compositions are inconsistent with an origin as xenocrysts from early formed mafic volcanics or from metamorphosed carbonates in the basement. We interpret these high-An% cores to be early formed phenocrysts crystallizing from more primitive and relatively water-rich magmas in the deep crust. Their radiogenic Sr isotope compositions suggest that these early magmas already had a significant crustal component, consistent with other evidence for important involvement of a subducted sediment component during magmatism. The resorbed nature of the high-An% cores is interpreted to reflect resorption during rapid magma migration into the upper crust. Lower An% rims of these same plagioclases show increases in ⁸⁷Sr/⁸⁶Sr (up to 0.7132) that are consistent with interactions with crust in upper crustal magma chambers. The plagioclase Sr isotope data therefore provide evidence for both source and crustal contamination during petrogenesis of the Cabo De Gata andesites.

                       POSTER PRESENTATIONS                    

Recycling of continental material through the suprasubduction zone Mawat ophiolite, NE Iraq

Heider Al Humadi¹, Markku Väisänen², Sabah Ismail³, Yann Lahaye⁴, Hugh O’Brien⁴, Jaakko Kara² and Marja Lehtonen^4
1Dpet. of Geogr. and Geo. Uni. of Turku, Finland/Dept. of Appl. Geo. Uni. of Babylon, Iraq, ²Department of Geography and Geology, FI-20014 University of Turku, Finland, ³Department of Applied Geology, College of Sciences, University of Kirkuk, Iraq, ⁴Geological Survey of Finland, FI-02151 Espoo, Finland

The fate of subducted continental material is not fully understood. It is estimated that up to 20% of it is recycled back to the crust (Clift et al. 2009), mainly in volcanic arcs. Recently, suprasubduction zone ophiolites formed above subduction zones have also been shown to contain recycled continental minerals (Robinson et al. 2015).

We have analysed zircons and monazites from three felsic dykes and zircons from two mafic rocks from the mantle section of the suprasubduction zone Mawat ophiolite in NE Iraq. Laser ablation single-grain U-Pb results show a wide range of ages from the Late-Cretaceous to Paleo-Archean (c. 95-3500 Ma). The youngest 95 Ma age represents the formation of the Mawat ophiolite whereas the bulk of the zircons are inherited from older unknown mixed continental material. Lu-Hf isotopes on zircons from a felsic sample show average initial ε_Hf = – 2.4. The results indicate that the magma originated from an older source possibly contaminated in the mantle wedge. In contrast, the initial ε_Hf values of the mafic sample display an average of + 9.4, indicating a juvenile magma source.

The results show that the felsic rocks of the Mawat ophiolite formed in a forearc position partly from recycled older continental material which was most likely subducted as the upper layer of the Neo-Tethyan oceanic crust. The effect of the older crustal material on the newly-formed mafic crust is significantly less based on the Hf data, but nevertheless is in clear evidence by the occurrence of old xenocrystic zircon.

References
Clift, P. D., Vannucchi, P., & Morgan, J. P. 2009. Crustal redistribution, crust–mantle recycling and Phanerozoic evolution of the continental crust. Earth-Science Reviews, 97, 80-104.

Robinson, P. T., Trumbull, R. B., Schmitt, A., Yang, J. S., Li, J. W., Zhou, M. F., … & Xiong, F. 2015. The origin and significance of crustal minerals in ophiolitic chromitites and peridotites. Gondwana Research, 27, 486-506

Oxidative weathering in the Late Neoproterozoic: Preliminary Cr Isotope results from the Urucum District, Brazil

Trygvi Bech Árting¹ and Robert Frei^2
1University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark, ²University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark

The Fe and Mn deposits in the Urucum district (Banda Alta Fm.) in Mato Grosso do Sul, Brazil, which are associated with glaciogenic deposits, are among the youngest and largest Cryogenian sedimentary Fe and Mn formations (IF, MnF). The Urucum IF comprise plane-parallel, stratified hematite-chert iron with interspersed manganese micro- and mesobands. A lower age limit of 695 ± 17 Ma is defined by detrital zircon U-Pb from shaley beds in the Banda Alta Fm. (Frei et al. 2017). Preliminary results from borehole samples (Vetria borehole 28-32) support previous findings with a positively fractionated authigenic Cr isotope signature throughout the stratigraphic section (d⁵³Cr_SRM979 = 0.80 ± 0.25 ‰, 2s, n=26/28). This indicates a continental source of oxidized Cr, implying high atmospheric O₂ levels in the late Neoproterozoic.

References
Frei, R., Døssing, L.N., Gaucher, C., Boggiani, P.C., Frei, K.M., Árting, T.B., Crowe, S.A., Freitas, B.T. 2017: Extensive oxidative weathering in the aftermath of a late Neoproterozoic glaciation – Evidence from trace element and chromium isotope records in the Urucum district (Jacadigo Group) and Puga iron formations (Mato Grosso do Sul, Brazil). Gondwana Research 49, 1-20.

Quadlab – A ⁴⁰Ar/³⁹Ar and noble gas isotope laboratory at the Natural History Museum of Denmark

Michael Storey¹ and Daniel Wielandt^1
1Quadlab, Natural History Museum of Denmark

In this poster presentation we provide an overview of Quadlab, a Villum Foundation funded ⁴⁰Ar/³⁹Ar age-dating and noble gas isotope laboratory that has recently been established at the Natural History Museum of Denmark. The laboratory has two noble gas mass-spectrometers: (1) A Nu Instruments Noblesse from 2005 – equipped with 3 ion counters (high, axial and low mass positions) and a high mass Faraday detector; (2) A Thermo-Fischer Helix MC high-resolution noble gas mass-spectrometer (commissioned spring 2016), equipped with 5 dual faraday/ion counting detectors with 10¹² and 10¹³ ohm amplifiers for improved (signal to noise ratio) of argon isotopes. The Helix has a 120 degrees magnet with a standard resolution of 900, allowing full resolution of hydrocarbon isobars in the Ar mass range 36 to 40. The high-resolution mode on channel L2 has a resolution of 1700 allowing psudeoresolution of the HCl isobar from ³⁶Ar. Three independent laser-sample gas preparation systems supply the 2 mass-spectrometers, with one preparation system designed and dedicated specifically for cosmogenic nuclide exposure dating (³He, ²¹Ne) applications. This system is equipped with a Janis closed cycle cold trap system (CCSTRAP-HT/204N) to 7K for sequential release and measurement of the different noble gases. We are mainly applying these tools to Quaternary problems, such as hominin evolution and the relationship between volcanism and climate. We are also currently working on projects related to meteorites and impact structures.