Contractional uplift of deep crustal rocks along the Legs Lake shear zone, western Churchill Province, Canadian Shield

Abstract

The Legs Lake shear zone juxtaposes high-pressure (1.0+ GPa) granulite- and eclogite-facies rocks with low-pressure (~0.5 GPa) amphibolite- to granulite-facies rocks, and thus may represent an important crustal-scale, exhumation-related structure in the western Canadian Shield. Field mapping and structural and petrologic analysis document the deformation and metamorphic history of rocks within and adjacent to the shear zone. At least two important phases of deformation are recorded: (1) early oblique thrusting (D2) that placed high-grade rocks over lower grade rocks, and (2) more discrete and lower grade brittle–ductile normal faulting (D3) that may represent the later part of the exhumation history. The northwest-dipping shear zone consists of 5–8 km of mylonite in map view, which bounds the southeast margin of the East Athabasca mylonite triangle. High-pressure granulite-facies metamorphism (~750–850 °C, 1.0–1.2 GPa) occurred in the East Athabasca mylonite triangle at ca. 1900 Ma, prior to D2 juxtaposition with the adjacent low-pressure Hearne domain. Thermobarometry from Grt–Crd–Sil–Bt–Qtz metapelites in the Hearne domain suggests peak conditions reached 600–700 °C and 0.45–0.5 GPa, which are interpreted to have occurred late during D2. Published and preliminary U–Pb isotope dilution - thermal ionization mass spectrometry (ID-TIMS) zircon geochronology and electron microprobe monazite geochronology suggest that deformation in the Legs Lake shear zone coincided with the ca. 1830–1810 Ma terminal collision in the Trans-Hudson orogeny. Extensional faulting during D3 most likely occurred after ca. 1780 Ma. A multi-stage process of exhumation involving both thrust and normal-sense shearing, may serve as a model for the exhumation of other regionally extensive deep-crustal exposures.