TY - JOUR
T1 - Complex calc-alkaline volcanism recorded in Mesoarchaean supracrustal belts north of Frederikshåb Isblink, southern West Greenland
T2 - Implications for subduction zone processes in the early Earth
AU - Szilas, Kristoffer
AU - Hoffmann, J. Elis
AU - Scherstén, Anders
AU - Rosing, Minik T.
AU - Windley, Brian F.
AU - Kokfelt, Thomas F.
AU - Keulen, Nynke
AU - van Hinsberg, Vincent J.
AU - Næraa, Tomas
AU - Frei, Robert
AU - Münker, Carsten
N1 - Funding Information:
We acknowledge the Greenland Bureau of Minerals and Petroleum (BMP) for financial support of the field and analytical work and thank the Geological Survey of Denmark and Greenland (GEUS) for permission to publish this work. We thank Fiorella Fabra Aguilera and Mojagan Alaei for help with the zircon separation at GEUS. We are grateful for the help provided by Susanne Rømer at GEUS with preparation of the tectonic sketch figure. K. Szilas would like to thank Geocenter Danmark for funding his PhD project at GEUS with the title ‘Archaean supracrustal belts of SW Greenland’, which this work will form a part of. J.E. Hoffmann was supported by Grant Mu 1406/08 of the DFG (German Research Foundation). A. Scherstén acknowledges financial support from the Swedish Research Council through grant # 2008-3447 . V.J. van Hinsberg acknowledges financial support from the European Union Seventh Framework Program ( FP7/2007-2013 ) under grant agreement no. 254015 . This study is a contribution to IGCP project 599.
PY - 2012/7
Y1 - 2012/7
N2 - We present new geochemical data for three Mesoarchaean supracrustal belts (Ravns Storø, Bjørnesund and Perserajoorsuaq) situated north of Frederikshåb Isblink in southern West Greenland for which we propose the collective name 'The Ikkattup Nunaa Supracrustal Association'. They comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc-alkaline andesite composition. Both lithological units are cut by aplite sheets of tonalite-trondhjemite-granodiorite (TTG) composition with U-Pb zircon ages of c. 2900. Ma. Lu-Hf and Sm-Nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 ± 41. Ma and 3020 ± 78. Ma, respectively, which are within error of the age of the Fiskenæsset Complex situated 5-25. km to the north. Leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and TTG gneisses, as the end-members. By assimilation-fractional-crystallisation (AFC) modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a TTG-like end-member and another group of high P
2O
5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low-silica adakite end-member. However, the leucoamphibolites are juvenile with e{open}Nd
(2970Ma) from +2.1 to +3.5 and e{open}Hf
(2970Ma) 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 (<60Ma) as the volcanism took place. Our preferred interpretation of the 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 AFC 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 other 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 processes operating at lower crustal or source levels in arcs, such as melting-assimilation-storage-homogenisation (MASH) or slab-melt (low-silica adakite) metasomatism of the mantle source, respectively.
AB - We present new geochemical data for three Mesoarchaean supracrustal belts (Ravns Storø, Bjørnesund and Perserajoorsuaq) situated north of Frederikshåb Isblink in southern West Greenland for which we propose the collective name 'The Ikkattup Nunaa Supracrustal Association'. They comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc-alkaline andesite composition. Both lithological units are cut by aplite sheets of tonalite-trondhjemite-granodiorite (TTG) composition with U-Pb zircon ages of c. 2900. Ma. Lu-Hf and Sm-Nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 ± 41. Ma and 3020 ± 78. Ma, respectively, which are within error of the age of the Fiskenæsset Complex situated 5-25. km to the north. Leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and TTG gneisses, as the end-members. By assimilation-fractional-crystallisation (AFC) modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a TTG-like end-member and another group of high P
2O
5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low-silica adakite end-member. However, the leucoamphibolites are juvenile with e{open}Nd
(2970Ma) from +2.1 to +3.5 and e{open}Hf
(2970Ma) 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 (<60Ma) as the volcanism took place. Our preferred interpretation of the 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 AFC 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 other 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 processes operating at lower crustal or source levels in arcs, such as melting-assimilation-storage-homogenisation (MASH) or slab-melt (low-silica adakite) metasomatism of the mantle source, respectively.
KW - Archaean
KW - Calc-alkaline volcanism
KW - Southern West Greenland
KW - Supracrustal belt
KW - The Ikkattup Nunaa Supracrustal Association
UR - http://www.scopus.com/inward/record.url?scp=84860316460&partnerID=8YFLogxK
U2 - 10.1016/j.precamres.2012.03.013
DO - 10.1016/j.precamres.2012.03.013
M3 - Article
VL - 208-211
SP - 90
EP - 123
JO - Precambrian Research
JF - Precambrian Research
SN - 0301-9268
ER -