Chapter 6 - Archean Gray Gneisses and the Origin of the Continental Crust: Evidence the Godthåb Region, West Greenland

Abstract

Abstract Gray gneisses having general trondhjemitic affinities are the main components of the Greenland Archean. In the Godthab region gray gneisses were formed during two periods of crustal development–the Amitsoq gneisses at 3800-3600 m.y. and the Nuk gneisses at 3000-2800 m.y. Each period of crustal development began with the outpouring of voluminous basaltic volcanics. During subsequent plutonic activity huge volumes of syn-tectonic tonalitic-granodioritic magmas were intruded and crystallized with primary gneissic textures. The chemistry of these gray gneisses is consistent with an origin by partial melting of basaltic rocks, but precludes an origin by re-melting of sialic rocks. The environments in which the Greenland gray gneisses were generated and intruded do not appear to have any close analogues in later Earth history. A model is suggested in which the continental crust of the Greenland Archean was built up mainly from magmas produced by partial melting of basaltic rocks above a zone of mantle downwelling.