Petrography and phase equilibria modeling of mid-pressure aluminous gneisses derived from hydrothermally altered protoliths, Grenville Province, Canada

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 °C and 6–11 kbar (1 kbar = 100 MPa), with melt solidification in the range of 800–865 °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.