UPb single-zircon age for the Tinissaq gneiss of southern West Greenland: A controversy resolved

Abstract

Ion probe UPb dating of single zircons from a layered gneiss from Tinissaq, southern West Greenland, resolves a long standing controversy about the age of the oldest recognizable orthogneiss unit within the partly retrogressed granulite-facies area south of Ameralik. In spite of whole-rock isotopic evidence for late Archaean ages for these gneisses, some workers have used field criteria to correlate them with the early Archaean (> 3.5 Ga) Amitsoq gneisses of the Godthåbsfjord area farther north and explained the isotopic characteristics in terms of various secondary element migrations. Ages obtained on well-preserved igneous zircons from the Tinissaq gneiss suggest a 2922 ± 7 Ma (2σ) emplacement age for the igneous precursor to the gneiss, ruling out any correlation with the Amitsoq gneisses. A younger generation of igneous zircons may have formed at 2852±9 Ma (2σ), interpreted as the time of a migmatization event. All igneous zircons recrystallized to varying extent as massive U-poor zircons during granulite-facies metamorphism at 2819±23 Ma (2σ). Younger granitoid gneisses that break up the 2922-Ma gneiss were most probably emplaced approximately contemporaneously with this recrystallization event. There is little direct geological correlation across the amphibolite-granulite facies boundary between Tinissaq and Nuuk (Godthåb), in agreement with recent suggestions that the boundary is a major tectonic break.