The Bear River dykes (1265–1269 Ma): westward continuation of the Mackenzie dyke swarm into Yukon, Canada

Abstract

The 1.27 Ga Mackenzie dyke swarm, the largest on Earth, radiates from a point in the western Canadian Arctic and extends across much of the Canadian Shield. Possible western extensions of the swarm are largely obscured by younger sedimentary cover. Paleoproterozoic inliers in northern Yukon host the Bear River dykes (BRD), herein dated by U–Pb baddeleyite and zircon methods at 1268.5 ± 1.5 Ma and 1264.6 ± 1.2 Ma. The BRD share similar geochemical and Nd-isotopic characteristics with the Mackenzie dykes and the coeval Coppermine River basalts, and are regarded as products of the same plume. Hydrothermal activity at ∼1270 Ma in breccia of the Nor mineral occurrence, northwest of the BRD, was probably generated by BRD at depth. The BRD generally strike northwest, approximately 90° to the orientation predicted by the radiating dyke model. This difference may be explained by an anomalous local stress field at the time of BRD emplacement, or reorientation of BRD during subsequent events of Cordilleran deformation. Using the Nor mineral occurrence as the westernmost locale of Mackenzie dyking, and assuming a model of uniform dyke radiation, the arc of the Mackenzie swarm is at least 50° greater than previously recognized, bringing the total arc of dyke radiation to >150°.