A Discussion on the evolution of the Precambrian crust - The early Precambrian gneisses of the Godthåb district, West Greenland

Abstract

Intensely metamorphosed and deformed basic dykes, the Ameralik dykes, have been used to divide the amphibolite-facies gneisses of the Godthåb district into the Amitsoq gneisses (older) and the Nuk gneisses. Metavolcanic and metasedimentary rocks (the Malene supracrustals), and stratiform meta-anorthosites are also present and are probably younger than the Amitsoq gneisses, but are older than the Nuk gneisses. The Amitsoq gneisses contain abundant fragments of Ameralik dykes. They appear to have been derived from homogeneous granitic (s.l.) parents, but most of them have been intensely reworked before and/or after the intrusion of the Ameralik dykes and are now banded gneisses. They range from dioritic to granitic in composition and potassic varieties are common. Isotopic data indicate that the parent rocks of the gneisses were emplaced or metamorphosed about 3750 Ma ago. Ameralik dykes are absent from the Nuk gneisses, which are the most abundant rocks in the area. These gneisses are derived from intrusive calc-alkaline parents, mainly tonalites and granodiorites, and represent a massive addition of granitic material to this level of the crust about 3080 Ma ago, after the first supracrustal rocks had been laid down.