LOCALIZATION, EXTREME GRAIN SIZE REDUCTION, AND SMALL EQUILIBRIUM VOLUMES IN A HOT LOWER CRUSTAL ULTRAMYLONITE, CORA LAKE SHEAR ZONE, NORTHERN SASKATCHEWAN

Mylonitic shear zones are common features in granulite terrains. These are regions of strain localization that likely reflect strength heterogeneity within the deep crust. Although strain is localized, extreme grain size reduction is not predicted in high-grade shear zones because elevated temperatures generally result in annealing and grain coarsening. The Cora Lake shear zone (CLsz), Athabasca Granulite Terrain, Canada provides an opportunity to test assumptions about grain-size reduction in high-grade shear zones. The CLsz formed during the waning phases of granulite grade metamorphism within the lower crust. This mylonitization is characterized by the reduction of feldspar and clinopyroxene grains to the <10 μm scale, producing an almost-glassy texture. Compositional analyses of major fabric forming phases suggest chemical disequilibrium even on the scale of tens of micrometers. Due to the fine-grained nature of this shear zones, late Fe-Mg diffusional exchange is a crucial consideration for thermobarometric analysis. In particular, phases such as clinopyroxene and garnet need to be carefully evaluated with respect to their spatial distribution so that the influence of post kinematic diffusional exchange may be addressed. Thermobarometric calculations indicate mylonitization occurred at ca. 750°C with decreasing pressure (1.06 - 0.82 GPa) with increasing strain, which is consistent with the localization of deformation during sinistral-normal sense motion along the CLsz. This observation requires that strain rates were high enough to be grain size insensitive, and that localization persisted such that grain sizes were reduced into a diffusion flow deformation regime. However, localization through time would progressively leave portions of the shear zone inactive. With temperatures still elevated, static recovery in these abandoned regions would be expected to promote coarsening. In the CLsz, static coarsening may have been inhibited by the unusually anhydrous nature of the rocks and potentially CO₂-rich conditions. Electron backscattered diffraction analysis of silicates across the transect is underway, and should provide data regarding deformation mechanisms occurring within the compositionally heterogeneous material that formed and evolved in the lower continental crust.