Isotopic evidence for Neoarchean continuity across the Snowbird Tectonic Zone, western Churchill Province, Canada

Abstract

The Snowbird Tectonic Zone (STZ), the ca. 2800km-long boundary between Rae and Hearne domains of the western Canadian Shield, remains of uncertain tectonic significance, but is currently interpreted as a ca. 1.9Ga suture. The eastern Rae domain is well studied and underlain by a >20,000km2 region of high pressure granulites, containing ca. 2.60Ga arc rocks, and granulite to amphibolite-grade structures, which formed at ca 2.57Ga (D1/M1) and 1.90Ga (D2/M2). The 1.85Ga Legs Lake shear zone juxtaposed the high-P granulites of the eastern Rae domain structurally above lower amphibolite- to greenschist-grade supracrustal gneisses of the Hearne domain. Zircon U-Pb and Hf isotopic analyses were acquired from igneous and metasedimentary rocks across the STZ and Rae-Hearne boundary to assess the validity of the suture model, with specific focus given to the >5km wide Legs Lake shear zone. Results indicate that both 2.60–2.55Ga magmatism and deformation continue unimpeded across the STZ and Rae-Hearne boundary. Furthermore, Hf isotopic data from igneous zircon from 2.60Ga plutonic rocks across the STZ display little variation in εHf indicating that no major crustal-break corresponds to the Rae-Hearne boundary, or any of the shear zones that parallel the STZ on the ground. Older plutonic rocks show a greater variability in εHf, which may indicate a disparate origin. Grade, intensity, and timing of Paleoproterozoic features vary along the transect and display little continuity across the Rae-Hearne domain boundary. In conjunction with existing field and isotopic data, we interpret that the Rae and Hearne domains were together by 2.60Ga, and that the STZ was host to repeated Paleoproterozoic reactivation driven by the latter stages of Taltson-Thelon orogenesis, final indentation of the Slave craton into the Rae domain, and accretionary processes on the southern periphery of the Hearne domain from 1.93 to 1.84Ga. Furthermore, assessing the viability of major shear zones within high grade terranes as potential sutures is complicated by the lateral and vertical heterogeneity of the crust juxtaposed by them.