Tectonic significance of the Virgin River shear zone of the Canadian Shield and implications for the origin of the Snowbird tectonic zone of Laurentia

The northwest-dipping Virgin River shear zone is part of the Snowbird tectonic zone, which divides the Churchill Province into Rae and Hearne subprovinces. Although Archean rocks are juxtaposed along most of the Snowbird tectonic zone, the Virgin River shear zone separates Paleoproterozoic intermediate orthogneiss from Archean orthogneiss of granitoid composition. Paleoproterozoic supracrustal rocks of the Virgin schist group core the shear zone. Kinematic indicators suggest that the oldest displacement in the northwest-dipping shear zone was reverse oblique. The shear zone was stitched by ca. 1830-1820 Ma granite plutons, which were deformed during reactivation of the structure. Very few modern age determinations are available for fundamental rocks internal to the Virgin River shear zone, with the exception being the youngest granite. Five rock samples were collected from within the structure for U-Pb zircon geochronology. Detrital zircon geochronology suggests that the previously undated metasedimentary rocks of the Virgin schist group were deposited after similar to 2150 Ma. A sample of leucotonalite collected metres from outcrops of Virgin schist group but with unclear contact relationships yielded a crystallization age of 3133 +/- 14 Ma and a Sm-Nd depleted mantle age of 3.48 Ga. This rock is likely an equivalent of the Paleoarchean to Mesoarchean Chipman tonalite that is exposed directly along strike to the northeast. A sample of diorite from the Virgin River shear zone has a crystallisation age of 1940 +/- 7 Ma with a 2.70 Ga Sm-Nd depleted mantle age. The sample could represent part of a previously unrecognised volcanic arc. Late tectonic granite that stitched the Virgin River shear zone has ca. 1820 and 1830 Ma crystallisation ages. A sample of granite that lies in the Snowbird tectonic zone beneath the Athabasca Basin to the northeast yielded an imprecise age of ca. 1833 +/- 31 Ma, consistent with the previous ages, and a 3.48 Ga Sm-Nd depleted mantle age. Given that the Virgin River shear zone cross cuts 1.940-1.925 Ga gneissic fabrics contained in Paleoproterozoic plutonic rocks of the Rae subprovince, the age of the shear zone must be younger than this fabric. The strain recorded along this boundary, coupled with the drastic change in geology across it, support the notion of a collisional Snowbird orogeny along the Virgin River shear zone at ca. 1.90 Ga.

Automated summary

The northwest-dipping Virgin River shear zone is part of the Snowbird tectonic zone, separating Paleoproterozoic rocks from Archean rocks. The oldest displacement in the shear zone was reverse oblique, with granite plutons deforming the structure. The age of the shear zone is younger than the gneissic fabrics contained within Paleoproterozoic plutonic rocks, indicating a collisional Snowbird orogeny around 1.90 Ga.