The pyroxene granulites of the Mount Wright map area, Quebec - Newfoundland

An assemblage of pyroxene-bearing granulite facies rocks occurring in the Superior province of the Canadian Shield immediately north of the Grenville front in the Mount Wright area, Quebec, has characteristics similar to those of charnockitic rocks from other Precambrian areas of the world. These rocks vary from leucocratic varieties composed essentially of alkali feldspar and quartz with more than 75 per cent silica to mafic pyroxene-rich varieties with less than 50 percent silica. Most common varieties are quartz pyroxene-plagioclase granulites of intermediate composition and others with a granodiorite composition. The geological setting, mode of occurrence, distinguishing features, and characteristic minerals of these rocks are described as well as the petrography of the various types. As these high-grade rocks arc traced southward toward the Grenville front, they become progressively more recrystallized owing to later retrograde metamorphism related to the Grenville orogeny. Along the front the granulites have been converted to horn blende and biotite-bearing gneisses and schists similar to the almandine-amphibolite facies rocks within the Grenville province. The mineralogical changes involved in this retrograde metamorphism are described. Petrogenesis of the granulites is dealt with and the role of the variables, temperature, water vapour pressure, and rock pressure, is discussed in relation to the nature of the feldspars, the presence of hydroxyl-bearing minerals, and the fabric of the granulites. The status of charnockite nomenclature is considered in the light of the reinterpretation of the type area at Madras by Subramanian (1959) and the literature on the mode of origin of such rocks is reviewed. The authors propose a non-genetic classification in order to overcome previous confusion and place greater emphasis on chemical composition and the position of these rocks within the metamorphic facies concept.