PETROGRAPHY AND PHASE EQUILIBRIA MODELING OF MID-P 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 kilometers 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 in two types: sillimanite-rich rocks, with a high alumina index in AFM space and sillimanite-poor rocks with low alumina index and in which sillimanite is not part of the peak assemblage. Phase equilibria modeling with Thermocalc constrained the P–T field of the peak mineral assemblage(s) at 800–900 ºC and 6–11 kbar, with melt solidification in the range of 800–865 ºC 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.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author