U–Pb ages of single zircons within "Upernavik" metasedimentary rocks and regional implications for the tectonic evolution of the Archaean Nain Province, Labrador

Abstract

Detrital zircons and their postdepositional overgrowths from three units of the "Upernavik" supracrustal association in the northern (Saglek) block of the Archaean Nain Province have been dated with the ion microprobe SHRIMP. In one unit, from the granulite-facies area in inner Saglek Fiord, the zircon population is dominated by early Archaean grains thought to be derived from the Uivak gneisses. Recrystallization and growth of new zircon within this metasediment took place during granulite-facies metamorphism at 2761 ± 12 Ma (2σ), which is also a younger limit on the age of deposition.In a second unit, from the amphibolite-facies area in outer Saglek Fiord, detrital zircons have predominantly mid- and late Archaean ages. The mid-Archaean zircons are comparable in age to the 3235 Ma Lister gneisses. The ages of the late Archaean detrital zircons (2800–2960 Ma) do not correspond to any known rock complex in the Saglek block, but plutonic rocks associated or correlative with the ca. 2840 Ma Kanairiktok Plutonic Suite of the southern (Hopedale) block are a possible source for many of the grains. Overgrowths were dated at 2690–2730 Ma in this sample.A third metasedimentary unit from the Okak Bay area, 100 km south of Saglek Fiord, also contains detrital zircons with ages comparable to that of the Lister gneisses (3235 Ma). The age of recrystallization and zircon overgrowths was dated at ca. 2560 Ma in this sample. A single grain dated at ca. 2780 Ma is considered most likely to be detrital, which would imply an age of deposition between ca. 2780 and 2560 Ma for this unit.The results show that although late Archaean depositional ages are possible for all three units, the "Upernavik" supracrustal association is composite and sediments in different units have widely different sources and metamorphic histories. These conclusions support a new model for the Nain Province according to which separate terranes were tectonically juxtaposed in the late Archaean. In this model, the age of plutonic and supracrustal rocks and their metamorphic histories prior to juxtaposition differ from one terrane to another.