Rare earth evidence for the origin of the N�k gneisses Buksefjorden region, southern West Greenland

Abstract

Rare earth element (REE) concentrations have been determined (by the INAA method) for the c. 2,800 m.y. old Nuk gneisses from the Buksefjorden region, southern West Greenland. Samples include dioritic to granodioritic gneisses and synplutonic mafic dykes; a Malene metagabbro and Qorqut granite were analysed for comparisons. The early Nuk gneisses, diorites and tonalites, have mildly fractionated REE patterns which are interpreted as resulting from partial melting of garnetbearing amphibolite or granulite. Early Nuk trondhjemitic gneisses possess downward convex patterns with prominent positive Eu anomalies; they may be related to the diorites and tonalites by the separation of hornblende in a residue of partial melting or fractional crystallization. Most of the later Nuk grey gneisses have extremely fractionated linear patterns which were derived from a source very rich in garnet, possibly eclogite. REE patterns measured in the late Nuk Ilivertalik granite complex are mildly fractionated but with a high overall abundance consistent with an origin by partial melting of mafic lower crustal material. Two sets of synplutonic mafic dykes have strongly fractionated patterns similar to those found in alkali basalts. The geochemical variations suggest that the igneous precursors of the Nuk gneisses were not cogenetic, but were derived from widely differing sources.