QUARTZ MICROSTRUCTURES AND C-AXIS PREFERRED ORIENTATIONS IN HIGH-GRADE GNEISSES AND MYLONITES AROUND THE MORIN ANORTHOSITE (GRENVILLE PROVINCE)

Quartz in deformed rocks from two large, high-grade shear zones around the Morin anorthosite (Morin terrane, Grenville Province) displays di stinctive microstructures, as well as c-axis preferred orientations. I n the west-dipping Morin shear zone, east of the Morin anorthosite, fo ur distinct quartz microstructures (types 1-4) are identified, based o n deformation features and grain size. The c-axis orientations are cha racterized by a single maximum near the stretching lineation and two m axima in type 1 microstructure: and by an asymmetrical, single girdle in type 2 microstructure. Quartz c axes show crossed-girdle pattern in type 3 microstructure. Both quartz microstructures and c-axis preferr ed orientations suggest that crystal-plastic slip and dynamic recrysta llization are the dominant deformation mechanisms. The asymmetry of c- axis orientations with respect to the mylonitic foliation, as well as the substructures developed in quartz, indicates a dextral sense of sh ear in the Morin shear zone. Type 4 microstructure, which developed in some gneisses and granulites, is interpreted to record influence of p ostdeformation annealing by which quartz c-axis orientations were part ially modified. In the north-northeast-trending, subvertical Labelle s hear zone that separates the Morin terrane from the Mont-Laurier terra ne, metamorphic assemblages and structural elements suggest that an ea rly, sinistral strike-slip deformation occurred under granulite-facies conditions. This was overprinted by a late downdip movement of the Mo nt-Laurier terrane under retrogressive conditions. Quartz in felsic gn eisses from this zone shows two types of microstructures: one is simil ar to type 4 from the Morin shear zone, the other is named type 5. Qua rtz c-axis orientations are complex and less systematic, due to overpr inting by two episodes of deformation and possible annealing. These co mplexities limit the utility of quartz microstructures and c-axis data in the structural analysis of the Labelle shear zone.