Contrasting zoning profiles in high-grade garnets: evidence for the allochthonous nature of a Grenville province terrane

Abstract

In the central part of the Grenville Province, Proterozoic metasediments of the Réservoir Cabonga Terrane arc underlain by gently dipping Archean migmatites. A major mylonite zone, located within the migmatites, is the base of a large thrust sheet made up of the Réservoir Cabonga Terrane and a migmatitic sole. Garnets from meta-aluminous assemblages in Proterozoic metasediments, mylonites and migmatites display contrasting types of Ca zoning patterns. Garnets with Ca-rich cores are found along the western edge of the Réservoir Cabonga Terrane, whereas garnets with flat Ca profiles are found in the remaining Proterozoic metapelites, in the mylonites and in the Archean migmatites. Considering samples in which an infinite reservoir of Ca ions is warranted by abundant plagioclase in the matrix, pressures well in excess of 1200 MPa, at temperatures between 700° and 800°C are indicated for a metamorphic event recorded in the cores of garnets from the western edge of the Réservoir Cabonga Terrane. The inner rim of garnet records pressures of around 1000 MPa for the same temperature range. In homogeneous garnets, a single P-T set, around 800 MPa and 750°C, is recorded in the migmatites. Some of the mylonites initiate their recrystallization under relatively high pressures, which is continued during decompression, and record the lowest P-T values. These metamorphic signatures suggest that the Proterozoic metapelites were already metamorphosed under relatively high pressure when emplaced on to Archean migmatites and that the entire area was subsequently subjected to metamorphism at a lower pressure.