GEOCHRONOLOGICAL CONSTRAINTS ON THE LEGS LAKE SHEAR ZONE WITH IMPLICATIONS FOR REGIONAL EXHUMATION OF LOWER CONTINENTAL CRUST, WESTERN CHURCHILL PROVINCE, CANADIAN SHIELD

Abstract

The Legs Lake shear zone marks the southeastern boundary of an extensive region (>20,000 km2) of high-pressure (0.8–1.5+ GPa) granulite-facies rocks in the western Churchill Province, Canada. The shear zone is one of the largest exhumation-related structures in the Canadian Shield and coincides with the central segment of the ~2,800 km long Snowbird tectonic zone. The movement history of this shear zone is critical for the development of models for the exhumation history of the high-pressure region. We used electron microprobe U–Th–Pb dating of monazite with supplemental ID-TIMS U–Pb geochronology to place constraints on the timing of shear zone activity. Combining these and other data, we suggest that regional exhumation occurred during at least three distinct phases over an ~150 million year period. The first phase involved high temperature decompression from ~1.0 to 0.8–0.7 GPa shortly following 1.9 Ga peak metamorphism, possibly under an extensional regime. The second phase involved rock uplift and decompression of the hanging wall to 0.5–0.4 GPa during east-vergent thrusting across the Legs Lake shear zone at ca. 1.85 Ga. This phase was likely driven by early collision-related convergence in the Trans-Hudson orogen. The final phase of regional exhumation, involving the removal of 15–20 km of overburden from both footwall and hanging wall, likely occurred after ~1.78 Ga and may have been related to regional extensional faulting.