Crustal growth and crustal recycling in the Nagssugtoqidian orogen of West Greenland:: Constraints from radiogenic isotope systematics and U–Pb zircon geochronology

Abstract

We present Sr–Nd–Pb whole-rock isotopic data for regionally collected samples of late Archean to Paleoproterozoic gneisses and granitoids from the Nagssugtoqidian orogen of West Greenland, together with U–Pb zircon (SHRIMP) geochronological data for Paleoproterozoic granitoids. These data are used, together with additional published geochronological data, to place constraints upon late Archean (>2.7 Ga) crustal growth, and subsequent Paleoproterozoic (ca. 1.92 to 1.76 Ga) crustal growth and recycling in the Nagssugtoqidian. Nd-isotope data suggests that the oldest recognisable components dated at ca. 2.85 Ga contain varying amounts of older (>3.1 Ga?) light rare earth element enriched (continental crustal) component(s) as well as juvenile ca. 2.85 crust. The behaviour of Sr- and Pb-isotope systematics during Paleoproterozoic Nagssugtoqidian metamorphism is dependent upon both the grade of this event (granulite or amphibolite) and the original (i.e. Archean) grade of metamorphism. Juvenile Paleoproterozoic magmatic additions to the Nagssugtoqidian crust are represented by the Arfersiorfik quartz diorite and Sisimiut charnockite, which are shown to be contemporaneous by SHRIMP U–Pb dating. Variations in isotopic composition from these two units are related to moderate degrees of crustal contamination or extraction from a heterogeneous mantle source, probably in a magmatic arc which varied from oceanic (Arfersiorfik; minimal interaction with pre-existing continental crust) to continental (Sisimiut) in character respectively. Late granite sheets (Paleoproterozoic) throughout the Nagssugtoqidian orogen generally have a local late Archean protolith.