Metamorphic constraints on the evolution of the gneisses from the parautochthonous and allochthonous polycyclic belts, Grenville Province, western Quebec

Abstract

The tectono-metamorphic history of polycyclic "grey gneisses" located in the central Grenville Province of western Quebec has been constrained along a transect perpendicular to the length of the Grenville Orogen. Two terranes, the Réservoir Dozois terrane (RDT) and the Réservoir Baskatong terrane (RBT), were recognized from their structural, lithological, and geochronological characteristics. This subdivision has been confirmed by application of geothermobarometric techniques to appropriate mineral assemblages.The RDT is the southern extension of the parautochthonous belt of the Grenville Province, which in this area is composed of Archean rocks of upper-amphibolite grade. During the Grenvillian Orogeny, northwest-directed thrusting resulted in the tectonic burial of this terrane as a single tectonic unit, in contrast with the northern part of the parautochthonous belt, where several slices were imbricated against the Grenville Front. Maximum P–T conditions in the RDT (850 MPa, 720 °C) were likely Grenvillian and were followed by pervasive retrogression down to the hornblende–epidote subfacies. Locally, the RDT is overlain by remnants of thrust slices composed of monocyclic metasedimentary rocks that were deformed and metamorphosed in the granulite facies during the Grenvillian Orogeny.To the southeast, the RBT is an allochthonous or exotic terrane probably of Proterozoic age. It also experienced tectonic burial by thrusting (1030 MPa, 710 °C) during the Grenvillian Orogeny, whose thermal climax (790 °C) coincided with charnockite emplacement during decompression to 850 MPa.These two terranes are separated by a narrow strip of sheared rocks, the Renzy shear belt (RSB), which comprises mafic and ultramafic rocks subjected to high P and T (975 MPa, 745 °C). In view of the significant discrepancy between the metamorphic histories of the two terranes separated by the RSB, major tectonic transport has to be envisaged along this zone.