The fabric of the granulites of Mont Tremblant Park, Quebec

Abstract

The fabric of the highly deformed and recrystallized gneisses formed under granulite facies conditions of metamorphism are described from Mont Tremblant Park, Quebec, Canada. These gneisses are termed granulites and are classified into quartzofeldspathic and hypersthene varieties.The term 'cataclastic-gneissose' fabrics is used to imply varying degrees of deformation and recrystallization without micro-brecciation or cataclasis 'sensu stricto' and a classification of these various fabrics is presented. The modifications in these granulites are progressive and are assumed to have the following sequence:[Formula: see text]During the evolution from mortar to granoblastic types, the minerals become flattened and increasingly elongated, while the individual recrystallized grains increase in size. The same deformation produced flaser and granoblastic gneisses in the quartzofeldspathic granulites and mortar and augen gneisses in the hypersthene granulites.The mineral relicts (porphyroclasts) in the mortar and augen gneisses are highly deformed and display evidence of strain, such as undulatory extinction, deformation bands, polygonization, mechanical twins, and bent cleavages. These mortar and augen gneisses display a poor preferred orientation of quartz c axis.In the recrystallized rocks the grains form a clear, fresh, unstrained polygonal mosaic and in the case of quartz, long leaf-like uniform plates are produced. In the flaser and especially the granoblastic gneisses no signs of strain remain and the recrystallized fabric consists of quartz plates alternating on a micro-scale with bands of polygonal fresh feldspar and granular streaks of dark minerals. A petrofabric diagram from a granoblastic gneiss shows a marked preferred orientation of quartz c axis.A review of the relevant literature on the natural and experimental investigations strongly supports the hypothesis that the fabrics of the granulites are produced as a result of the progressive deformation and recrystallization of the mineral constituents.