The Archean gneiss complex of northern Labrador. 1. Preliminary U–Th–Pb geochronology

Abstract

Lithological and earlier isotopic studies suggest that several of the older rock units in the Archean gneiss complex of north Labrador can be correlated with those of the Godthaabsfjord area of west Greenland (West Greenland Correlation = WGC). However, comparison of zircon data from the two areas suggests major differences in the younger Archean geological development of the two areas. The Uivak I and II gneisses (WGC = the 3600 Ma Amitsoq gneisses) yield zircon 207Pb/206Pb ages of up to 3485 Ma but with an apparent high Pb-loss episodic disturbance pattern. Zircons from the Nulliak assemblage (WGC = Isua supracrustals and Akilia association, > 3800 Ma) give dates of ~2500 Ma and have been completely reset by thermal metamorphism accompanying the intrusion of post-tectonic granites at ~2520 Ma (WGC = Qôrqut granite 2520 Ma). A group of high Fe2O3 iron-rich minor intrusives of uncertain stratigraphic age yield zircon ages of 2500–2800 Ma. Sphene separated from all the above rock types gives essentially concordant U–Th–Pb ages at ~2530 Ma, while Uivak gneiss apatite is somewhat discordant at 2385 Ma. The whole rock, feldspar, and apatite Pb isotopic composition of the Uivak gneisses is quite different from that for the Amitsoq gneiss. The Uivak gneiss lead is much more radiogenic and probably acquired the radiogenic lead during later metamorphism between 2800–2500 Ma ago.