The Cora Lake shear zone (CLsz) is a 4-6 km wide localized high-strain zone that bisects the polydeformed Athabasca granulite terrane, northern Saskatchewan. It also coincides with the geophysical trace of the Snowbird tectonic zone. The CLsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of >20 000 km 2 of high-pressure granulites. Most rocks have a strong mineral lineation plunging moderately to the southwest. The Northwestern subdomain (hangingwall) is characterized by ca. 2.6 Ga plutonic rocks that contain an early, subhorizontal gneissic layering (ca. 2.57 Ga) that was overprinted by large amplitude folds and a partitioned, but pervasive, axial planar, dextral, shear fabric at ca. 1.9 Ga. Thermobarometry suggests metamorphic conditions of ϳ0.9 GPa and ϳ750°C during both of the phases of tectonism. The footwall is predominantly underlain by the ca. 3.3-3.0 Ga Chipman tonalite, layers of intercalated mafic and felsic granulite, and the widespread 1.9 Ga Chipman mafic dyke swarm. Early subhorizontal layering in the footwall was also folded at ca. 1.9 Ga and transposed into a steeply dipping, northeast-striking axial planar shear fabric that corresponds with the metamorphic peak (1.1-1.2 GPa and 800-900°C). These distinct domains were juxtaposed across the CLsz, which contains a gneissic foliation striking 231°and dipping moderately to steeply to the northwest. Abundant sinistral-normal kinematic indicators are consistent with the distinctly lower pressures to the northwest. The shear zone is characterized by very fine grain sizes, despite its hightemperature assemblages including clinopyroxene and garnet. Thermobarometry from the CLsz displays progressive decompression of reworked footwall rocks with increasing mylonitization. In situ monazite geochronology indicates shearing at 1.89-1.87 Ga shortly after the granulite facies metamorphic peak. The anomalous sinistral kinematics of the CLsz, bracketed in time between periods of dextral shearing, can be explained by changing regional stresses during alternating convergent tectonics to the west and to the southeast of the Athabasca granulite terrane.Résumé : La zone de cisaillement de Cora Lake (CLsz) est une zone localisée de fortes déformations; elle a une largeur de 4 à 6 km et elle divise en deux parties le terrane de granulite polydéformé d'Athabasca, dans le nord de la Saskatchewan. Cette zone coïncide aussi avec la trace géophysique de la zone tectonique Snowbird. La CLsz représente une importante discontinuité lithotectonique et thermobarométrique dans l'affleurement de plus de 20 000 km 2 de granulites de haute pression. La plupart des roches ont une forte linéation minérale plongeant modérément vers le SO. Le sous-domaine nord-ouest (l'éponte supérieure) est caractérisé par des roches plutoniques d'environ 2,6 Ga qui contiennent un rubanement gneissique précoce subhorizontal (ϳ2,57 Ga) lequel a été surimprimé par des plis à grande amplitude et une texture de cisaillement envahissante, partitionnée, à axe planaire ...

a b s t r a c tThe deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8e1.0 GPa, 725e850 C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4e0.6 GPa, 550e650 C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolutionprecipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

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