Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening

U Pb ages have been obtained for the Muskox intrusion and the Mackenzie dyke swarm. The age of a pyroxenite from the layered series of the intrusion is 1270 ± 4Ma. Baddeleyite fractions from four widely-spaced Mackenzie diabases define a single discordia line with an upper intercept age of 1267 ± 2Ma. The dyke age of 1267 Ma provides a precise time-marker for much of the northwestern Canadian Shield. Mackenzie intrusive events were coeval with eruption of Coppermine River flood basalts in the Coppermine homocline. The short time-span, large volume and specific focus of Mackenzie igneous events suggest that magmatism occurred above a large hotspot caused by the presence of a mantle plume. We infer that magmatism was initiated when rifting breached a large domal uplift supported by the plume-generated hotspot. The uplift-subsidence record in the Coppermine homocline is predicted by numerical models for rifting above hot asthenospheric mantle leading to ocean opening. An array of five large gravity anomalies north of the homocline may outline a region of stretched continental crust extensively intruded by Mackenzie mafic igneous bodies.