U-Pb isotope systematics on minerals from the gneiss complex at Isukasia, West Greenland

Abstract

207Pb/204Pb vs. 206Pb/204Pb results from sphenes from Isua gneisses show a scatter of data points between two isochrons, one at 3560 Ma and the other at 2520 Ma. Similar Pb-Pb plots on apatites from Isua show many data points which are doser to the 3560 Ma reference isochron than Pb-Pb apatite data points from Amîtsoq gneisses and Nûk gneisses in the Godthåbsfjord area. Pb from the Isua gneiss apatite and sphene is more radiogenic than that from Amîtsoq gneisses. In concordia plots of zircons all but one of the Isua data points plot to the right of the Amîtsoq zircon discordia line. Assuming a diffusion Pb loss, the 'oldest' Isua gneiss zircon is about 3700 Ma. U-Pb values for sphenes from the Isua gneisses show discordancy which could be the result of partial or local metamorphic effects between 1500 and 2800 Ma. Comparable values from sphenes from Amîtsoq and Nuk gneisses duster fairly dosely to concordant results about 2500 Ma.