Tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston–Mudjatik Transition Zone, Saskatchewan

Abstract

The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (∼1.7 Ga) within the Wollaston–Mudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly–McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1–D1 and M2–D2. M1–D1 is associated with the development of a gently dipping foliation striking N90°–N100° and a southward decrease in peak pressures from up to 10 kbar (1 kbar = 100 MPa) in the Cochrane River area down to 6 kbar in the Wolly–McClean exploration drilling project. The M2–D2 event is responsible for the main northeasterly trend of the WMTZ that developed in a sinistral transpressional tectonic regime during the final oblique collision of the Trans-Hudson Orogeny. Thermobarometric estimations on M2–D2 assemblages show that the studied area was reequilibrated at about 4–5 kbar and 750–825 °C. The basement has thus been affected by a differential isothermal decompression event between D1 and D2 that allowed the juxtaposition of the deepest northeastern domains and the Wolly–McClean exploration drilling project, at the same structural level. These results suggest that the basement exposed to the northeast of the Athabasca Basin is not an analog of the basement located beneath the eastern Athabasca Basin where uranium-enriched granitic pegmatites and granites are known. We also suggest that uranium-enriched melts produced during the early M1–D1 stage of partial melting in the deep crust were transferred to the midcrust, owing to D2 shear zones, where they have differentiated to produce uranium-bearing pegmatites.