Geochronology and isotopic variation of the early Archaean Amitsoq gneisses of the Isukasia area, southern West Greenland

Abstract

Relatively unaltered samples of the Amitsoq gneiss complex in the Isukasia area were identified and analysed despite the effects of metamorphism and local Proterozoic hydrothermal alteration and contamination. The parents of the three lithologies that make up the Amitsoq gneisses of the area were emplaced over about 320 Ma. Taking a weighted average of dates from U-Pb on zircons, Rb-Sr on WR, Sm-Nd on WR and Pb-Pb on WR, the most probable ages of intrusion are: grey gneisses at 3690 ± 50 Ma, white gneisses at 3590 ± 40 Ma and pegmatitic gneisses at 3370 ± 60 Ma. The Rb-Sr isochron initial 87Sr 86Sr ratios for the grey, white and pegmatitic gneisses are 0.7006 ± 1, 0.7016 ± 1 and 0.8248 ± 3, respectively. The corresponding average ϵ Nd ( T) values are +1.0, +0.7 and −0.2. Since the REE chemistry of the rocks that were the direct parents of the grey gneisses is not known, the +1.0 average ϵ Nd ( T) of the grey gneisses need not imply generation from a depleted mantle. The Sr initial ratio and ϵ Nd ( T) values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses. The progressive evolution of ϵ Nd with T for the Amitsoq gneisses in the early Archaean shows that values of ϵ Nd ( T) near zero in granitoids do not necessarily indicate derivation from the “undepleted” mantle (CHUR).