USING DEFORMATION AND METAMORPHISM IN HIGH STRAIN ZONES TO TRACK THE EXHUMATION HISTORY OF DEEP-CRUSTAL ROCKS IN THE CANADIAN SHIELD

Exhumation-related structures, retrograde metamorphic reaction, and isotopic cooling paths record the exhumation path of deep-crustal rocks. Deformation and metamorphism are commonly related in that retrograde reactions are typically best developed in high-strain zones that were active during the exhumation process. We are studying the regional exhumation history of deep-crustal rocks within the Snowbird Tectonic Zone (Canadian Shield) using detailed field, microstructural and petrologic analysis, high-resolution X-ray compositional mapping, and in situ electron microprobe monazite dating in order to develop comprehensive P-T-t-D paths. The East Lake Athabasca region, consisting of a network of high strain zones, is a broad exposure of high-pressure (1.0+ GPa) granulite-facies rocks exhumed during 1.9-1.75 Ga. The Legs Lake shear zone, considered the main exhumation-related structure in the region, is a 5 km wide zone of amphibolite-facies mylonite that accommodated at least 20 km of uplift of deep-crustal rocks via thrusting over mid-crustal rocks (<0.5 GPa) in its footwall. Pelitic gneiss in the sheared hangingwall contains early garnet partially replaced by retrograde Crd+Bt. The retrograde assemblage yields conditions near 0.5 GPa indicating substantial exhumation during shear zone development. X-ray maps reveal thin Y- and HREE-rich monazite rims (~10 mm) that are texturally linked to the garnet porphyroblasts. The rims yield microprobe dates of 1850-1800 Ma and are interpreted to have grown during garnet consumption thus constraining the age of the metamorphic reactions. A second subparallel but dominantly strike-slip high-strain zone occurs in the hangingwall of the Legs Lake structure. Here, mafic granulites preserve an early assemblage of Grt+Cpx in low-strain lenses. The mylonitic Hbl-bearing matrix assemblage is interpreted as the result of cooling and decompression followed by a second prograde heating event. No evidence of the second heating event is present in the low-strain lenses suggesting that deformation facilitated reactions during both the initial retrograde path and subsequent reheating. Although kinematically distinct, these high strain zones record different components of decompression and thus together, provide a more complete exhumation history.