Extent and age of Mesoarchean components in the Nagssugtoqidian orogen, West Greenland: Implications for tectonic environments and crust building in cratonic orogenic belts

Abstract

The Nagssugtoqidian orogen (NO), which forms the northern margin of the North Atlantic craton in West Greenland, is largely comprised of ~2870 to 2720 Ma juvenile continental crust reworked during Paleoproterozoic orogenic events. Sparse evidence of components older than 3100 Ma in the area (U–Pb zircon ages and bulk-rock Sm–Nd isotopes) have hinted at a unit called the Qorlortoq gneiss but were not substantiated by regional studies. Here we report the “rediscovery” of the Qorlortoq gneiss, documented through a new geochemical and geochronological dataset of this orthogneiss and related components, which include ultramafic cumulate enclaves and late basaltic dykes. Laser Ablation Split Stream (LASS) U-Pb/Lu-Hf isotopic analyses of zircon from the gneiss give a U–Pb concordia upper-intercept age of 3177 ± 12 Ma and weighted mean εHf(t) of 1.7 ± 0.5, indicating a juvenile protolith extracted from a depleted mantle source.Ultramafic enclaves of cumulate origin make up a key component of the Qorlortoq gneiss. Their trace element systematics indicate a parental melt with tholeiitic affinities, sourced from depleted mantle beneath relatively thin lithosphere (<80 km), that was subsequently contaminated by crustal assimilation. Re–Os isotope systematics of the cumulates and their relationship with the younger Qorlortoq dykes place the age of these cumulates close to that of the Qorlortoq gneiss.The age of the Qorlortoq gneiss makes it the oldest known component in the NO by ~300 Ma. The extended timescale between episodes of crustal production in this area, could indicate a geodynamic environment characterized by stagnant-lid or episodic mobile-lid tectonics, at least on a regional scale. These constraints provide important information about the diversity and nature of Archean geodynamic environments.