Timing of emplacement of the Muskox intrusion: constraints from Coppermine homocline cover strata

Abstract

The Muskox intrusion is a Middle Proterozoic shallow-level mafic–ultramafic layered intrusion in the northwest corner of the Bear Province of the Canadian Shield. The intrusion was emplaced into the Lower Proterozoic Akaitcho Group and Hepburn metamorphic–plutonic complex of the Wopmay orogen and overlying Middle Proterozoic Coppermine homocline strata. To date, radiometric, paleomagnetic, and geochemical evidence has produced only equivocal results concerning the age of the Muskox intrusion relative to overlying Coppermine River Group basalts and Mackenzie diabase dykes. Stratigraphic redefinition of and structural relationships to the cover sequence of the intrusion provide the best available evidence for the age of the Muskox intrusion.Although previously inferred to be a pre-Dismal Lakes Group structure, the Canoe Lake Fault is here shown to cross-cut Dismal Lakes strata with west-side-down displacement equivalent to that documented to the south where it offsets the intrusion. This relationship, supported by detailed mapping that shows that the Muskox intrusion intrudes units 11 and 12 of the Dismal Lakes Group, demonstrates a post-Dismal Lakes age for the intrusion. This conclusion strongly supports the contention that the Muskox intrusion and Coppermine River Group are coeval and comagmatic.Syndepositional uplift and normal faulting within uppermost Dismal Lakes Group strata were most pronounced near the exposed roof of the intrusion, and are interpreted to have been related to emplacement of a small sill-like body that probably was the precursor to the main intrusive event. At least 70 m of sediments was deposited after this initial disturbance, but before extrusion of the Coppermine River Group basalts (which heralded the main emplacement event). This thickness of sedimentary strata implies a period of several hundred thousand years between the two igneous events.These results support earlier contentions that the Mackenzie igneous event, comprising emplacement of Mackenzie dykes, Coppermine River Group basalts, and the Muskox intrusion, was a coherent mafic igneous event that occurred approximately 1200 Ma ago, with the Muskox intrusion as a locus.