The Grenville Front south of Val-d'Or, Quebec

Abstract

The tectonometamorphic history of rock assemblages is unravelled throughout a 50 × 30 km strip across the Grenville Front, the boundary between the Superior and the Grenville Provinces in the Canadian Shield. This was accomplished by applying geothermobarometric techniques to zoned garnets. Systematic analysis of garnetiferous assemblages revealed that: 1. (1) primary garnet (grtI) cores grew during a high- T/low- P metamorphic episode, and 2. (2)secondary garnet (grtII) cores as well as most of the grtI rims grew during a low low- T/high- P metamorphic episode. The two sets of P- T conditions can by comparison with available geochronological data be assigned to the Kenoran (late Archaean) and Grenvillian (mid-Proterozoic) orogenies, respectively. A prominent ductile shear zone traverses the 50 × 30 km strip and divides the area southeast of the Grenville Front into two regions with contrasted tectonometamorphic behaviour. The eastern region is composed of migmatitic and charnockitic gneisses representing intermediate to lower levels of the Archaean crust. The western region is composed of migmatitic metasediments which show no evidence of significant crustal thickening during the Grenvillian Orogeny. Along a transect across the Grenville Front (eastern region), significant offsets of Grenvillian paleopressure levels indicate that the thickened crust is a stack of imbricated slices with vertical components of displacement in the order of several kilometres. After compensating for the Grenvillian movement along the appropriate shear zones, offsets remain between some of the Archaean paleopressure levels, suggesting that delamination of the Archaean crust was initiated prior to the peak of Grenvillian metamorphism. The thrust slices thus identified in the eastern region have thicknesses of a few kilometres but they could be much thinner in the western region. Here the Grenville Front is ill-defined, and lies in a zone of gradual transition between the Pontiac Group metasediments of the Superior Province and the Lac Lambert migmatites of the Grenville Province. The overall deformations of the two regions southeast of the Grenville Front are incompatible and cause the development of an intervening shear zone. Although the two regions experienced different tectonometamorphic histories, they both belong to the Parautochthonous Belt of the Grenville Orogen.