The Nature and Origin of the Granulites of Mont Tremblant Park, Quebec

Abstract

The Mont Tremblant Park granulites can be divided into two main groups: pink, quartzofeld-spathic rocks and green-gray hypersthenic rocks. The quartzofeldspathic granulites are interlayered with garnet-sillimanite gneisses and locally occur as cores of folds or large massifs. More commonly, they are regionally interbanded with the hypersthene granulites. All the granulites are intruded by the quartz mangerites of the Morin anorthosite suite. Geochemical considerations based on ternary variation diagrams and relationships in the albite-orthoclase-anorthite system show that the two groups of granulites have different compositional trends, converging at the extreme granitic end. Mineralogical and geochemical data are consistent with granulite facies conditions of metamorphism. The composition of the granulites and their field relationships are consistent with their postulated derivation from a graywacke-shale geosynclinal sequence. Under granulite facies conditions, partial anatexis formed granitic melts. This anatectic differentiation caused the graywackes to attain a composition appropriate to form hypersthenic rocks and the shales to produce quartzofeldspathic rocks with residual sillimanite-garnet layers. These rocks were deformed during the intrusion of the anorthosite suite, and the characteristic fabrics of the granulites were produced. Volatiles given off by late-stage granite magmas caused retrograde effects at contacts.