首页|Convergent plate boundary environments for formation of ≥ 3800 Ma mafic-ultramafic assemblages(Isua area,Greenland):Implications for early global geodynamics
Convergent plate boundary environments for formation of ≥ 3800 Ma mafic-ultramafic assemblages(Isua area,Greenland):Implications for early global geodynamics
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In the gneiss terrane on the south side of the Eoarchean Isua supracrustal belt,ultramafic rocks with relict abyssal peridotite mineralogy(Bennett et al.,2002;Friend et al.,2002;Nutman et al.,2007;Rollinson,2007;van de Löcht et al.,2020),layered gabbros with cumulate ultramafic rocks,basalts and associated siliceous sedimentary rocks were tectonically-imbricated,prior to and during intrusion of ca.3800 Ma tonalites.Together with ≥ 3800 Ma basalts in the Outer Arc Group of the nearby Isua supracrustal belt,the composition of all these mafic rocks(e.g.,Th-Hf-Nb systematics,high Th/Yb,Ba/Nb,Ba/Yb ratios and negative Nb and Ti anomalies)shows affinity with modern suprasubduction rocks whose genesis involved fluid fluxing of the upper mantle.However,the majority of these samples have Ba/Nb and Ba/Yb values less than in modern island arc magmas,but similar to many backarc basin magmas(e.g.,Pearce and Stern,2006).It is unknown whether these ca.3800 Ma mafic rocks are,(ⅰ)arc rocks where the Ba/Nb and Ba/Yb signatures reflect lower surficial Ba in Eoarchean oceanic settings,or(ⅱ)in direct comparison with Phanerozoic suites,these signatures reflect a back-arc setting with interplay between fluid fluxing and decompressional melting.The tectonic intercalation of upper mantle with lower and upper crustal rocks,combined with the fluid-fluxing influences seen in chemistry of all the mafic rocks is best accommodated in a compressional Eoarchean convergent plate boundary setting within a mobile-lid regime.Thus stagnant lid scenarios of crust formation,if operative,must have co-existed or alternated with mobile-lid regimes by 3800 Ma.
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