A lower crustal perspective on the stabilization and reactivation of continental lithosphere in the western Canadian shield

Abstract

New geochronological, thermochronological, geological and isotopic data from an extensive (> 20,000 km²) exposure of high-pressure granulites (0.8 to > 1.5 GPa, >750 ⁰C) in the East Lake Athabasca region of the Snowbird tectonic zone provide important constraints on the stabilization, reactivation and exhumation of continental lithosphere in the western Canadian Shield. The exhumed lower crust of this craton comprises several disparate domains that preserve a complex record of tectonic, magmatic and metamorphic processes from formation to exhumation. U-Pb zircon geochronology documents two episodes of metamorphic zircon growth at 2.55 Ga and 1.9 Ga, linked with two high-pressure granulite facies assemblages preserved in Chipman domain mafic granulites. The intervening 650 m.y. of relative quiescence implies a period of lithospheric stability during which the granulites continued to reside in the deep crust. Disruption of the stable Archean craton at 1.9 Ga broadly coincides with the assembly of the Laurentian supercontinent. The correlation of 1.9 Ga mafic magmatism and metamorphism in the Chipman domain with contemporaneous mafic magmatism along > 1200 km strike-length of the Snowbird tectonic zone indicates that regional asthenospheric upwelling was an important aspect of this reactivation event.

(cont.) UL-Pb (titanite, apatite, rutile), ⁴⁰Ar/³⁹Ar (hornblende, muscovite, apatite) and (U-Th)/He (zircon, apatite) thermochronometry documents the cooling history of domains in the East Lake Athabasca region during the 200 m.y. multistage history of unroofing following 1.9 Ga metamorphism. Linkage of reconstructed temperature-time histories with existing pressure-temperature-deformation paths reveals spatial and temporal heterogeneity in exhumation patterns, with domain juxtaposition during episodes of unroofing separated by intervals of crustal residence. Low temperature (U-Th)/He zircon and apatite dates are the oldest reported for terrestrial rocks, and confirm the protracted residence of rocks at shallow (< [or equal to] 2 km) crustal depths following the re-attainment of a stable lithospheric configuration in the western Canadian shield at ca. 1.7 Ga.