PLAGIOCLASE FABRIC DEVELOPMENT IN THE MORIN SHEAR ZONE, QUEBEC, CANADA

Lower crustal rheology plays an important role in plate-scale tectonic processes, especially during stress transfer associated with earthquakes. The deformation behavior of feldspar is critical because it is a volumetrically important mineral in the lower crust. We report field-based and microstructural work from the Morin Anorthosite, Quebec, Canada, which is deformed on the eastern limb by the Morin shear zone (MSZ). The MSZ is a 20 km wide zone of granulite facies deformation; approximately 10 km of the MSZ deforms the eastern margin of the Morin anorthosite. Tectonites associated with MSZ deformation record a gradient in fabric development including: 1) magmatic fabrics; 2) weakly foliated; 3) moderately foliated; and 3) strongly foliated and lineated fabrics. The foliation generally dips gently to moderately to the west, but in some areas is east dipping. Lineation is consistently strike-parallel (~NS and horizontal). Strongly foliated samples are found throughout the MSZ. Pockets of moderate to weakly foliated sites in the MSZ are located on the north-central side. Variable fabric development indicates an associated strain gradient that we interpret as a system of anastomosing high strain zones surrounding blocks recording lower strain. The fabric gradient generally correlates with a change in deformation temperature. Ti-in-quarts thermometry found that the weaker foliated fabrics tend to record the the highest temperatures (~718^o C) while the stronger foliated fabrics are associated with the lower temperatures (~618^o C). Optical microscopy reveals differing microstructures characteristic of feldspar dynamic recrystallization by bulge nucleation and grain boundary migration. Electron backscattered diffraction analysis shows increasing fabric intensity is associated with grain size reduction, increased shape preferred orientation, and changes in crystallographic preferred orientation topology. These microstructural data indicate variability in active slip system and dominant deformation mechanisms of feldspar associated with fabric development in the field area. Dynamically recrystallized grain sizes range between 65 and 359 microns, which correspond to the piezometric differential stresses of approximately 51 and 16 MPa, respectively.