The case for a distinct Taltson orogeny: Evidence from northwest Saskatchewan, Canada

Abstract

The Taltson magmatic and Thelon tectonic zones have historically been considered parts of a single orogen recording collision between the Slave craton and Buffalo Head terrane to the Rae craton between 2.0 and 1.9Ga. The Taltson–Thelon connection stemmed from recognition that the Taltson magmatic zone coincides with the Taltson aeromagnetic high that extends northward and merges with the Thelon aeromagnetic high. Analysis of geophysical data in this study, however, coupled with recent mapping, indicates that this magnetic high is underlain in large part by rocks of Taltson basement complex. In addition, both basement rocks and Taltson-age plutons can be traced eastward into northern Saskatchewan, below Athabasca Basin cover, and reappear/re-emerge to the south where they are truncated by the Virgin River shear zone (Snowbird tectonic zone). This paper presents new field, geochronological and geochemical data for the Lloyd domain of Saskatchewan bearing on the extent and nature of the Taltson orogen. The Lloyd domain is underlain mainly by magnetic dioritic–granodioritic orthogneisses and subordinate supracrustal rocks. It also hosts the Clearwater anorthosite complex, a ∼300km2, northeast-elongate gabbro-anorthosite intrusion about 15km northwest of the Virgin River shear zone. While previous geochronology established that some Lloyd domain orthogneisses are of Taltson (i.e., 1.98Ga) age, geochronology presented herein has identified an appreciably older (ca. 2460Ma) component. Geochemical study indicates that these older metaplutonic gneisses are distinguishable from younger (Taltson-age) ones based on trace element characteristics, but the true extent of basement rocks is impossible to evaluate. U–Pb geochronological data from the Clearwater anorthosite complex are complicated but allow interpretation that it was emplaced into this older basement complex at ca. 2.1Ga and was subsequently metamorphosed during Snowbird orogenesis at ca. 1.9Ga. Geochemical study also indicates that the Taltson-age quartz dioritic to granodiorite suite in Lloyd domain is chemically related but less evolved than the younger dominantly granitic, Taltson magmatic zone plutons of Alberta and N.W.T. The two are interpreted to be related and reflect gradual evolution of a subduction-related continental magmatic arc, challenging previous models invoking intra-plate magmatism. Collectively, these data, along with regional geological relationships, indicate that the Taltson orogen originally trended northwest–southeast and is a unique tectonic entity, distinct from the Thelon tectonic zone. Two alternative models are provided for Taltson orogeny. In the first, the Thelon orogeny develops along the margin of the Slave craton due to accretion of the Buffalo Head terrane between 2.0 and 1.95Ga. In the meantime, the Taltson arc was built on the southwest margin of the Rae province as the now combined Slave craton and Buffalo Head terrane approached. Terminal collision of the Rae and Slave culminated with high-grade metamorphism between 1.94 and 1.93Ga. In the second, the Slave craton was attached to the Rae craton far in advance of the Taltson orogeny, perhaps during the ca. 2.5–2.3Ga Arrowsmith orogeny. In this model, the Taltson orogeny developed on the southwest margin of the amalgamated Rae-Slave craton due to accretion of the Buffalo Head terrane and the Thelon tectonic zone represents an intracontinental zone of reworking.