Further rubidium–strontium age and isotope evidence for the nature of the late Archaean plutonic event in West Greenland

Abstract

THE gneisses of the Godthaabsfjord region of southern West Greenland have been grouped by McGregor1 into an older and a younger set, respectively termed the Amitsoq and Nuk gneisses. They are separated by dykes (the Ameralik dykes), metavolcanic and metasedimentary rocks (the Malene supracrustals) and stratiform meta-anorthosites. The Amitsoq gneisses have been dated at ∼ 3,700–3,800 Myr (refs 2–4). Pankhurst et al.5 reported a Rb–Sr whole rock isochron age of 3,040±50 Myr and an initial 87Sr/86Sr ratio of 0.7026±0.0004 on a suite of Nuk gneisses from Bjorneoen, some 25-km northeast of Godthaab (Fig. 1). The isochron age was interpreted as the age of emplacement of the precursors of the Nuk gneisses, while the low initial ratio suggested that these precursors could not have been derived by partial or complete melting of the older Amitsoq gneisses. Several geologists have expressed disagreement with this interpretation6,7. In particular, Chadwick et al.7 disagreed with our conclusion that the Nuk gneisses represent essentially juvenile sialic crust formed in West Greenland ∼ 2,800–3,000 Myr ago. We here present further evidence to support our interpretation.