207Pb/206Pb whole Rock Age of the Archaean Granulite Facies Metamorphic Event in West Greenland

Abstract

IN the Archaean basement complex of West Greenland there are several widely separated granulite facies areas, up to 100 km across1. Rocks from three such areas, Sukkertoppen, Nord-land and Fiskenaesset (Fig. 1), were originally selected for this study on the assumption that the granulite facies metamorphism represented the oldest metamorphic event observable in West Greenland1. This idea has been shown to be ill founded by subsequent geological and geochronological work2,3. The granulite facies metamorphism is now recognized as a distinctive time marker which probably has the following chronological relationships with other principal events in the Archaean of West Greenland, particularly in the Godthaabsfjord area2,3: (1) Plutonic development ∼ 3,700-3,750 m.y. ago4,5 of a complex of granites, granodiorites and tonalites. Metamorphism, migmatization and deformation, with production of banded gneisses (Amitsoq gneisses). (2) Intrusion of a basic dyke swarm (Ameralik dykes) into the Amitsoq gneisses. (3) Eruption of basic volcanics and deposition of sediments (Malene supracrustals). (4) Emplacement of basic igneous complexes with prominent calcic anorthosites. Possibly broadly similar in age throughout the Archaean of West Greenland. (5) Tectonic interleaving of earlier rock units. (6) Intrusion of major suite of granites, granodiorites, tonalites and diorites at 3,040 ±50 m.y. ago6, the parent rocks of the Nuk gneisses. (7) Deformation and folding, accompanied by migmatization and metamorphism that culminated in amphibolite facies conditions in the Godthaabsfjord area, but reached granulite facies to the north and south (in the areas we discuss in this communication). Earlier felsic rocks converted to gneisses. (8) Emplacement of Qorqut granite and pegmatites, ˜ 2,600 m.y. ago (Oxford unpublished data). Intrusion of Precamb-rian dolerite dykes.