The synmetamorphic P-T-t path of granulite-facies gneisses from Torngat Orogen, and its bearing on their tectonic history

摘要

Granulite-facies gneisses in the Abloviak shear zone in the internides of the Torngat Orogen are characterized by subvertical shear fabrics and associated subhorizontal stretching lineations. P-T vectors, derived from individual samples of these gneisses by conventional geothermobarometry of equilibrium core, rim and replacement symplectite assemblages, yield evidence of over 3 kbars decompression associated with cooling of approximately 150°C. When the sample population is considered together, a P-T-t path involving over 5 kbars decompression and 250°C cooling is defined. Such paths are compatible with theoretical models of synmetamorphic uplift following doubling of crustal thickness during thrusting, and imply that transcurrent motion took place in tectonically thickened crust, and was coeval with uplift. In a regional context, the Torngat Orogen preserves evidence of the oblique collision of two Archean cratonic blocks, the Nain and Rae provinces, during the Early Proterozoic and their amalgamation with Laurentia.