Lu-Hf total-rock age for the Amîtsoq gneisses, West Greenland

Abstract

Lu-Hf total-rock data for the Amîtsoq gneisses of West Greenland yield an age of 3.55±0.22Gy(2σ), based on the decay constant λ 176Lu=1.96×10 −11y −1, and an initial 176Hf/ 177Hf ratio of 0.280482±33. The result is in good agreement with Rb-Sr total-rock and U-Pb zircon ages. In spite of severe metamorphism of the area at 2.9 Gy, zircons from two of the samples have remained on the total-rock line, and define points close to the initial Hf ratio. The initial 176Hf/ 177Hf lies close to a chondritic Hf isotopic evolution curve from 4.55 Gy to present. This is consistent with the igneous precursors to the Amîtsoq gneisses having been derived from the mantle at or shortly before 3.6 Gy. Anomalous relationships between Hf concentration and the 176Lu/ 177Hf ratio may suggest that trace element abundances in the Amîtsoq gneisses are partly controlled by processes related to metamorphism.