A large portion of the hinterland of the central Grenville Province is characterized by mid-pressure granulite-facies metamorphic rocks. In the Manicouagan - Lac du Milieu region of Quebec, aluminous gneisses derived from hydrothermally altered felsic protoliths provide a record of anatectic processes and of the pressure-temperature (P-T) evolution during orogenesis. Samples collected in areas separated by several tens of kilometres consist of garnet + biotite + quartz + K-feldspar +/- plagioclase +/- sillimanite, with retrograde cordierite in some, and most display microstructural evidence of partial melting. However, they have a wide range of bulk compositions and textures, and may be grouped into two types: sillimanite-rich rocks, with a high alumina index and sillimanite-poor rocks with low alumina index and in which sillimanite is not part of the peak assemblage. Phase equilibria modeling with THERMOCALC software constrained the P-T field of the peak mineral assemblage(s) at 800-900 degrees C and 6-11 kbar (1 kbar = 100 MPa), with melt solidification in the range of 800-865 degrees C and 6-8 kbar. The presence of sillimanite inclusions in garnet, and the scarcity of retrograde cordierite are consistent with moderate dP/dT gradient "hairpin" P-T paths. The data suggest that this part of the mid-P hinterland in the central Grenville experienced a rather uniform high-temperature metamorphic evolution, with large temperature variations relative to pressure. This is consistent with the thermal evolution of middle crust beneath an orogenic plateau.
