A P-T-TIME-DEFORMATION PATH FOR EXPOSED DEEP CONTINENTAL CRUST IN THE SNOWBIRD TECTONIC ZONE, WESTERN CANADIAN SHIELD

Exposed lower crustal granulites in the East Lake Athabasca region of the western Canadian Shield contain a rich history of metamorphism and strain. At least two granulite-facies events at ≥1.0 GPa, separated by 500 m.y., suggest a long record of residence within the deep crust. We present results from mafic and felsic granulites that allow construction of a comprehensive P-T-time-deformation path for one domain within this high-grade terrane. Mafic granulite preserves early Grt+Cpx (~1.3 GPa, 850 °C) in cm-scale low strain lenses that developed from an original Opx-bearing igneous assemblage. The mylonitic Hbl-bearing matrix developed during a subsequent period of deformation accompanied by cooling and rehydration, and then records a second prograde metamorphic event indicated by the breakdown of Hbl to Opx+Cpx at ca. 1.0 GPa. High precision U/Pb ID-TIMS zircon geochronology from these mafic granulites defines a chord with a well-defined upper intercept at 2.55 Ga and a lower intercept at 1.9 Ga. The older date best constrains the age of the earliest metamorphic event in the region whereas the lower intercept is consistent with independent constraints for 1.9 Ga high-pressure partial melting of mafic dikes elsewhere in the domain. Textural observations, X-ray mapping, and integrated in situ EMP and ID-TIMS geochronology from felsic granulite (Grt+Sil+Ksp+Pl+Qtz) indicate several distinct monazite generations at 2.55 Ga, episodically between 2.50 and 2.0 Ga, 1.9 Ga, and 1.85 Ga. Inclusions in garnet are the oldest and highest Y monazite, which is consistent with initial growth of garnet at 2.55 Ga. All generations with 2.5-1.9 Ga dates contain significantly less Y suggesting growth in the presence of a relatively inert garnet-bearing assemblage. The youngest monazite generation (again high Y) is directly linked to synkinematic Grt-consuming reactions in retrograded felsic granulites (Grt-Crd-Bt-Sil, 0.4-0.5 GPa) that directly experienced post-1.9 Ga deformation within a major domain-bounding shear zone. These data represent a ~700 m.y. record including the origin, several periods of potential deep crustal reactivation (e.g., deformation, metamorphism, fluid-flow), and the ultimate exhumation of this terrane.