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

Abstract

Lower crustal rheology plays an important role in plate-scale tectonic processes. Understanding the deformation behavior of feldspar is critical because it is a volumetrically important mineral in the lower crust. I report field-based and microstructural work from the Morin anorthosite of Quebec, Canada, where it is deformed by the Morin shear zone (MSZ). The 20 km wide MSZ deformed at granulite facies conditions; approximately 10 km of the MSZ overlaps with the eastern portion of the Morin anorthosite. The foliation generally dips gently to moderately to the west, though in some areas it dips east. Lineation is consistently strike-parallel (~NS and horizontal). MSZ tectonites record a gradient in fabric development, in which the protolith is overprinted by weakly to strongly developed deformational fabrics. Fabrics observed within the MSZ are labeled as: Protolith fabrics; Type 1; Type 2; and Type 3 fabrics, with fabric development increasing from protolith to type 3. Microstructures across this gradient record dynamic recrystallization of feldspar by subgrain rotation in the type 1 fabrics and bulge nucleation in the type 3 fabrics. Type 3 fabrics are found throughout the MSZ, whereas regions containing type 1 and 2 fabrics are typically located on the north-central side of the MSZ. The fabric gradient generally correlates with a change in deformation temperatures recorded by Ti-in- quartz thermobarometry. Analyses indicate that the protolith fabrics record the highest temperatures (~720o C) and type 3 fabrics record the lowest temperatures (~630o C). Electron backscattered diffraction analysis of anorthite demonstrate fabric intensity is associated with grain size reduction, increased shape preferred orientation, and changes in crystallographic preferred orientation patterns. Dynamically recrystallized grain sizes range from 65 to 359 microns, corresponding to piezometric differential stresses of approximately 51 (type 3) and 16 MPa (protolith fabrics), respectively.

The Morin shear zone records top-to-the-N kinematics resulting from deformation below an “orogenic lid” of the Grenville orogeny. The variable fabric development is interpreted to records a system of anastomosing high strain zones surrounding lower strain blocks. This system is inferred to have formed as the Morin shear zone progressively localized from a wide shear zone at higher temperatures into an anastomosing shear zone at lower temperatures.