La zone tectonique du front de Grenville à l'est de Louvicourt, Québec : exhumation de la croûte archéenne pendant l'orogénie grenvillienne

Abstract

The Grenville Front tectonic zone, East of Louvicourt in Quebec, defines a tectonometamorphic domain marked by the Grenville Front oriented North 50°, that is locally crosscut by a North 30° reverse fault, the Matchi-Manitou Fault. These tectonic accidents separate the central parts of the Superior Province to the northwest from the Grenville Province to the southeast. The Archean age, high-grade polymetamorphic sequences of the Grenville Province consist of paragneisses and migmatitic quartzofeldspathic gneisses associated to coarse-grained, anatectic pegmatitic veins, the whole containing shreds of two pyroxenes-bearing mafic gneisses. The metamorphic disequilibrium textures, the chemical analyses of the mineral phases, and the whole-rock chemistry of these sequences indicate that a strong crustal thickening, marked by the incorporation of shreds of volcanic rocks in the metasedimentary sequences, led to the granulitization of the lithologies and the quartzofeldspathic pegmatitic vein formation by fluid-absent partial melting of the surrounding migmatitic quartzofeldspathic gneisses under 750–825 MPa and 675–745 °C conditions. Subsequently, a higher pressure and slightly lower temperature episode (920–990 MPa and 625–750 °C) is at the origin of a development of secondary coronitic garnet around primary garnet. A late fluid is responsible for final retrograde reequilibrations in all the lithologies. All these successive events probably occurred during the Archean ages. During the Grenvillian orogeny (1.0 Ga), the thrusting movement with a strong sinistral component of the Grenvillian Province onto the Superior Province is characterized: (i) to the east of the Grenville Front, by the exhumation and the rotation of 9–12 km thick sequences of the Grenville Province; (ii) to the west of the Grenville Front, by the development of a 705–845 MPa and 570–605 °C medium-grade metamorphism that overprints the 400–565 MPa and 600–660 °C regional metamorphism of Kenorean age. Both metamorphic episodes define a 1–6 km width band made of biotite and garnet-bearing paragneisses.