Tectonic Framework of the Grenville Province in Ontario and Western Quebec, Canada

Abstract

The southwestern Grenville Province embraces three subprovinces of different character. Northwesternmost, the Grenville Front Tectonic Zone (GFTZ) exhibits ductile deformation in gneisses with margin-parallel structures inclined toward the orogen. At its northwest limit (Grenville Front), steeply southeast-dipping mylonitized rocks abut Archean supracrustal and plutonic rocks overlain by Lower Proterozoic (Huronian) sediments. Southeastward, gneisses in the Central Gneiss Belt (CGB) include younger deformed and metamorphosed plutonic rocks (1.5–1.35 Ga). Curved shear belts (∿1.1 Ga) outline distinct lithotectonic domains. Farther southeast the Central Metasedimentary Belt (CMB) is underlain by Grenville Supergroup marble, metavolcanic and other metasedimentary rocks, intruded by plutons either pre- (∿1.2 Ga) or syntectonic (1.15–1.0 Ga) with respect to Grenvillian Orogeny. A major, shallowly southeast-dipping tectonic zone separates the CMB and CGB. Shortening across the Province is implied by abundant evidence for northwestward overriding found in high strain zones. Granulite facies distribution points to a formerly much thicker crust. Grenvillian supracrustal and plutonic rocks are lacking in the CGB and GFTZ, in which Grenvillian Orogeny is registered solely as a major tectonothermal event. Whether the northwest CMB margin is a decollement or a telescoped unconformity remains undetermined.