Convergent plate boundary environments for formation of ≥ 3800 Ma mafic–ultramafic assemblages (Isua area, Greenland): Implications for early global geodynamics

Abstract

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.