Field evidence relating to the origin of ∼ 3,000 Myr gneisses in southern West Greenland

Abstract

SOME prominence has been given recently to the results and implications of isotope age studies^1–4 in the early Precambrian rocks of the Godthåb region, southern West Greenland. The studies were initiated after field investigations⁵ suggested that the Godthåbsfjord region may contain outcrops of the oldest rocks in the earth's crust. McGregor⁵ has grouped these rocks into an older and a younger set of gneisses (respectively the Amîtsoq and Nûk gneisses) separated in time by dykes (the Ameralik dykes), metavolcanic and metasedimentary rocks (the Malene supracrustals) and stratiform meta-anorthosites. Among conclusions drawn from the isotope work is that based on initial ratios of ⁸⁷Sr/⁸⁶Sr that the parents of the Nûk gneisses in the Godthåbsfjord area cannot have been derived by partial or complete melting of the older Amîtsoq gneisses³. We wish to comment on the applicability of this conclusion to the Nûk gneisses mapped in the area immediately to the south.