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Geochemistry of Archaean Supracrustal Belts in SW Greenland

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Geochemistry of Archaean Supracrustal Belts in SW Greenland FACULTY OF SCIENCE UNIVERSITY OF COPENHAGEN PhD thesis Kristoffer Szilas1,2,3 Geochemistry of Archaean supracrustal belts in SW Greenland Academic advisor: Robert Frei1 External advisors: Minik T. Rosing2, Thomas F. Kokfelt3 & Anders Scherstén4 Submitted: April 16th 2012 Affilations: 1Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350, Copenhagen K, Denmark 2Natural History Museum of Denmark, Øster Voldgade 5-7, 1350, Copenhagen K, Denmark 3Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350, Copenhagen K, Denmark 4Department of Geology, Lund University Sölvegatan 12, 223 62 Lund, Sweden Contents Abstract I Summary II Resumé (in Danish) IV Preface and acknowledgements VI Objectives VIII 1. Introduction to the Archaean Eon 1.1. Events in the early Earth 1 1.2. The cratonic crust 3 1.3. The sub-cratonic mantle 8 1.4. Archaean supracrustal belts 11 1.5. The geodynamic settings of supracrustal rocks 14 1.6. Previous work on Archaean supracrustal belts in SW Greenland 18 2. Discussion of the thesis papers 2.1. Paper I: Remnants of arc-related Mesoarchaean oceanic crust in the Tartoq Group of SW Greenland 26 2.2. Paper II: Complex calc-alkaline volcanism recorded in Mesoarchaean supracrustal belts north of Frederikshåb Isblink, southern West Greenland: implications for subduction zone processes in the early Earth 29 2.3. Paper III: Origin of Mesoarchaean arc related rocks with boninite/komatiite affinities from southern West Greenland 31 3. Conclusions and future work 33 References 35 Curriculum Vitae 41 Annex Paper I: Remnants of arc-related Mesoarchaean oceanic crust in the Tartoq Group of SW Greenland Supplementary material for Paper I Paper II: Complex calc-alkaline volcanism recorded in Mesoarchaean supracrustal belts north of Frederikshåb Isblink, southern West Greenland: implications for subduction zone processes in the early Earth Supplementary material for Paper II Paper III: Origin of Mesoarchaean arc related rocks with boninite/komatiite affinities from southern West Greenland Supplementary material for Paper III Abstract This PhD thesis investigates the petrogenesis of Mesoarchaean (c. 3200-3000 Ma) supracrustal rocks from SW Greenland, using whole-rock geochemical, Sm-Nd and Lu-Hf isotope data. The main findings, for each of the three study areas, are the following: (1) The Tartoq Group contains a lithological assemblage of pillow lavas, shallow mafic sills/dykes, deeper level gabbros, and serpentinites of harzburgitic composition. This rock assemblage is interpreted as representing a section through oceanic crust. Whole-rock Lu-Hf isotope data yields an errorchron of 3189 ± 65 Ma. The geochemical data indicates an arc-affinity, similar to that of modern forearc- to backarc mafic magmas. (2) The Ikkattup Nunaa supracrustal association comprises basaltic and andesitic amphibolites, which display pillow lava and volcaniclastic structures, respectively. Whole-rock Lu-Hf data yield an errorchron of 2990 ± 41 Ma. The geochemical data are consistent with mixing of mafic and felsic magmas and mantle metasomatism in a process similar as that seen in modern island arcs. The graphical abstract below illustrates a model for the formation of the Ikkattup Nunaa supracrustal association, as an example of the processes that that may have operated in the Mesoarchaean Era. (3) The Nunatak 1390 area comprises mafic and ultramafic rocks. The latter have geochemical features that are practically identical to Ti-enriched komatiites described in the literature. However, they show mixing with a slab-melt component and have Nd isotope compositions similar to the mafic rocks. The petrogenesis of these ultramafic rocks is suggested to reflect flux-melting of a strongly depleted mantle source in a similar way as modern boninites. In conclusion, all of the three studied areas contain characteristics that are entirely compatible, and best explained, by formation in an arc setting. Thus, it is suggested that modern-style subduction zone processes were in operation since at least the Mesoarchaean Era. Graphical abstract. Model for the formation of Mesoarchaean calc-alkaline volcanic rocks in SW Greenland. a) Primitive island arc stage, which was dominated by tholeiitic basalts with characteristic negative Nb-anomalies. b) Evolved island arc stage where calc-alkaline andesites were formed by slab melt metasomatism and mixing of mafic melts with felsic melts produced by partial melting of the base of the arc due to mafic underplating. c) Convergent stage where the island arc crust was stacked and thickened, so that the base reached granulite facies metamorphic conditions and underwent partial melting to produce intrusive aplites. d) Large scale melting of the base of the island arc crust produced TTG batholiths. I Summary This PhD thesis investigates the petrogenesis of Mesoarchaean (c. 3200-3000 Ma) supracrustal rocks from SW Greenland by whole-rock geochemical, Sm-Nd and Lu-Hf isotope, and U-Pb zircon data. The foundation of this PhD thesis is the three papers found in the Annex. Additionally, three preceding chapters provide a general introduction to the Archaean Eon, previous work on supracrustal belts in SW Greenland, a discussion of the three papers, and finally conclusions and suggestions for future work in this region. The three papers present field- and geochemical data from the Tartoq Group, the Ikkattup Nunaa supracrustal association and the Nunatak 1390 area. The main findings are the following: The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean tonalite-trondhjemite-granodiorite (TTG) orthogneisses along the southern margin of the North Atlantic craton. It comprises an assemblage of supracrustal rocks, of mainly tholeiitic basalt composition, which includes pillow lavas, sills/dykes, gabbros, and serpentinites. This rock assemblage resembles that of Phanerozoic ophiolites (ocean floor). The metamorphic grade ranges from greenschist- to upper amphibolites-facies. LA-ICP-MS U-Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 ± 4 Ma for the Tartoq Group. This age is consistent with a MC-ICP-MS Lu-Hf whole-rock errorchron of 3189 ± 65 Ma and a Sm-Nd errorchron of 3068 ± 220 Ma for the supracrustal rocks. Overall the geochemical features of the Tartoq Group rocks are characterised by chondrite- normalized REE patterns with LaCN/SmCN of 0.67-1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26-1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4, indicative of an arc affinity when compared to rocks from modern settings. The geochemical signatures of the Tartoq Group rocks thus resemble rocks from modern forearc ophiolites, as well as Archaean supracrustal rocks found elsewhere in SW Greenland, such as the Isua and Ivisaartoq supracrustal belts in the Nuuk region. Therefore the Tartoq Group likely represents remnants of dismembered Mesoarchaean suprasubduction zone oceanic crust. The Ikkattup Nunaa Supracrustal Association is a collective name that is proposed for the three Mesoarchaean supracrustal belts of Ravns Storø, Bjørnesund and Perserajoorsuaq. It is situated north of Frederikshåb Isblink and south of the Fiskenæsset complex in SW Greenland. The Ikkattup Nunaa Supracrustal Association mainly comprises tholeiitic amphibolites and calc-alkaline leucoamphibolites of basaltic and andesitic composition, respectively. Primary structures such as pillow lavas and tuffs are well preserved in the Ravns Storø supracrustal belt. The supracrustal lithologies are cut by aplite sheets of TTG composition with LA-ICP-MS U-Pb zircon ages of c. 2900 Ma. Whole-rock MC-ICP-MS Lu-Hf and Sm-Nd errorchrons based on amphibolites and leucoamphibolites from the Ravns Storø and Bjørnesund supracrustal belts combined yield ages of 2990 ± 41 Ma for the Lu-Hf system and 3020 ± 78 Ma for the Sm-Nd system. The leucoamphibolites form apparent geochemical mixing trends between the tholeiitic amphibolites and TTG gneisses as end-members. By assimilation-fractional crystallisation (AFC) modelling it can be shown that one group of leucoamphibolites can be explained by contamination of the parental melts by a TTG-like end-member and another group of high P2O5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low- silica adakite end-member. However, the leucoamphibolites are juvenile with εNd(2970 Ma) from +2.1 to +3.5 and εHf(2970 Ma) of +3.5 to +4.3. Thus, the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time (<60 Ma) as the volcanism took place. Our preferred interpretation of the II whole-rock geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting, where early tholeiitic volcanism gave way to calc-alkaline volcanism in a maturing arc. The apparent contamination trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile TTG- and adakite-melts that mixed with mafic magmas or contaminated their mantle source to produce the calc-alkaline leucoamphibolite protolith. This model has important implications for the general interpretation of Archaean supracrustal belts, because AFC and geochemical mixing trends towards a TTG end-member are not uniquely diagnostic of crustal contamination, but may rather reflect
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