The Archean gneiss complex of northern Labrador. 2. Mineral ages, secondary isochrons, and diffusion of strontium during polymetamorphism of the Uivak gneisses

Abstract

Strontium isotope systematics of minerals from the ca. 3600 Ma old Uivak gneisses and of compositionally layered Uivak I gneisses, which define secondary isochrons, are the result of Sr isotopic homogenization during thermal events ca. 2800, ca. 2500, and ca. 1800 Ma ago. The strontium isotope data provide information about the thermal history of the Archean gneiss complex in northern Labrador, the scale of Sr isotopic equilibration, and mechanisms of Sr diffusion during polymetamorphism.The mineral data indicate that biotite and amphibole were both partially open to Sr diffusion during the Hudsonian Orogeny ca. 1800 Ma ago. However, cooling rates and closure to diffusion of Sr in these phases varied throughout the terrane. Potassium feldspars were largely closed to diffusive loss of Sr during the 1800 and 2500 Ma events, and they record evidence of a ca. 2800–2900 Ma thermal event.Ages defined by secondary isochrons correlate with variations in the widths of the gneissic layers and correspond with geologically established events in the Nain and Churchill Provinces. The secondary isochrons developed as a result of local isotopic equilibration on different scales during these thermal events. The variable scales of equilibration reflect the effects of different Sr diffusion mechanisms. As a melt phase was not produced in the gneisses at the times indicated by the secondary isochrons, diffusion must have occurred under subsolidus conditions. Temperatures were therefore too low for extensive volume diffusion to have taken place. Exchange of Sr isotopes over distances greater than 5–10 cm must have involved grain boundary diffusion or some form of infiltration metasomatism.