Metamorphic constraints on the tectonic evolution of the allochthonous monocyclic belt of the Grenville Province, western Quebec

Abstract

The tectono-metamorphic evolution of Grenvillian monocyclic terranes in western Quebec has been constrained along a northwest–southeast transect from Réservoir Cabonga to the Morin anorthosite by the study of garnet zoning and the application of geothermobarometric techniques to appropriate mineral assemblages. In all three terrenes examined (Réservoir Cabonga, Mont-Laurier, and Morin terranes), the results are consistent with Grenvillian metamorphism induced by thrusting. Additional magmatic heat may have been provided by the cooling of igneous rocks related to the Morin anorthosite complex.After the peak of the Grenvillian metamorphism, the monocyclic rocks were sliced, and three Grenvillian crustal levels, sharing some of their lithologic elements but having contrasting Grenvillian peak-metamorphic conditions, were subsequently juxtaposed. The deepest crustal level exposed in the transect (level A) comprises the Réservoir Cabonga terrene and the margins of the Mont-Laurier terrene. This level consists of metasediments of the "Grenville Supergroup," garnet amphibolites, and synmetamorphic plutonites whose emplacement predated the thrusting. Metamorphic conditions in level A are estimated at 725–875 MPa and 725–800 °C. Level C, which is exclusively in the central part of the Mont-Laurier terrene, comprises metasediments of the Grenville Supergroup that were metamorphosed at 425–600 MPa and 625–725 °C. Synmetamorphic igneous rocks of the Morin plutonic complex in the Morin terrane were metamorphosed at 600–800 MPa and 650–775 °C, probably in an intermediate crustal level B, which is missing in the Mont-Laurier terrane. It is thus suggested that juxtaposition of the Mont-Laurier terrane and the Morin terrane along the Labelle shear belt occurred after the peak of metamorphism.A comparison of the metamorphic evolution of these monocyclic terranes with that of their adjacent polycyclic counterparts suggests that (i) the Réservoir Cabonga terrane was thrust over the parautochthonous Réservoir Dozois terrane after the peak of metamorphism and (ii) level A rocks of the Mont-Laurier terrane and the Réservoir Baskatong terrane were juxtaposed during the peak of metamorphism.