THE CORA LAKE SHEAR ZONE, ATHABASCA GRANULITE TERRAIN: AN EXHUMED DEEP CRUSTAL TERRANE BOUNDARY

The Cora Lake Shear Zone (CLsz) forms the boundary between two major structural domains in the Athabasca granulite terrane, northern Saskatchewan. It is interpreted to have developed within the lower crust of the Canadian Shield and thus may serve as a rare exposed example of a lower-crustal high-strain zone and tectonic discontinuity. This structure defines the boundary between a northwestern domain of the Athabasca granulite terrane that is primarily underlain by ~2.6 Ga plutonic charnockitic gneisses (Mary batholith), and a southeastern domain that consists of dominant ca. 3.2 Ga tonalitic straight gneiss, subordinate amounts of felsic and mafic granulites, and the widespread 1.9 Ga Chipman dyke swarm. The contact between the two domains is gradational across the shear zone, with distinctive meter to decimeter wide sheets of felsic and mafic granulite. Mafic granulites consist of abundant garnet, cpx, plagioclase, +/- hornblende. Felsic granulites consist of coarse garnet, kspar, plagioclase, and quartz, +/- kyanite. The CLsz is a ~5 km wide, subvertical, SW striking, mylonitic to ultramylonitic shear zone. Stretching lineations trend approximately 230°, with a shallow (<25°) plunge. Consistent sinistral indicators include both delta and sigma feldspars, garnet, and cpx, abundant S-C fabrics, and shear bands. On the E and W margins and in low strain domains within of the shear zone, an earlier S₁ gneissic layering/fabric is folded with axial planes parallel to regional shear zone foliation (~237° strike) and fold axes coincident with stretching lineations. Within a several km-wide core zone, grain sizes are extremely fine, and ribbons of quartz, feldspar and even pyroxene are unannealed. This is surprising for a granulite facies shear zone and may result from extremely H₂O poor conditions. The presence of an S₂ fabric in the Chipman mafic dykes suggests that at least one period of deformation occurred after 1.9 Ga. However, it is likely that an older shearing event occurred along the zone. At 2.6 Ga, the zone may have represented a boundary between a weak, flowing, NW domain, and a strong, restitic, SE domain, representing the edge of an actively flowing lower crust. All this considered, the CLsz provides an opportunity to study reactivated deep crustal boundary and intense fabrics formed in the deep crust.