THE MUSKOX INTRUSION: ARCHITECTURE OF A LARGE, SUB-VOLCANIC, OPEN-SYSTEM MAGMA RESERVOIR TO CONTINENTAL FLOOD BASALTS OF THE MACKENZIE LARGE IGNEOUS PROVINCE

Continental flood basalts, the largest effusive eruptions on Earth, are the end-product of extensive crustal processing of high flux mantle-derived magmas during short-lived events related to the emplacement of large igneous provinces (LIPs). The immense 1.27 Ga Mackenzie LIP in northern Canada includes the >4.2 km-thick continental flood basalts of the Coppermine River Group, the Muskox layered ultramafic-mafic intrusion, and the regionally extensive Mackenzie dike swarm. The vast majority of the Coppermine River volcanic rocks are fractionated basalts (<9–10 wt% MgO) with mineralogical and geochemical evidence for prior crystallization of olivine ± clinopyroxene ± plagioclase. The 1.8 km-thick, funnel-shaped Muskox intrusion, exposed over 60 km and extending >100 km to the north in the subsurface, represents a high-level reservoir for the storage of a component of this crystal cargo. The intrusion is exposed from its base, where dunites ± olivine clinopyroxenites of the layered series overly a “basal zone” of feldspathic peridotites and magmatic breccias and an earlier gabbronoritic keel dike, to its top, where the uppermost cumulates are granophyre-bearing gabbronorites capped by a heterogeneous granophyric roof zone that represents a layer of buoyant, gravitationally stable crustal melt. The Muskox intrusion grew through a process of vertical stacking and lateral accretion of multiple sheet-like bodies in a magma reservoir that underwent periodic recharge-eruption coupled with progressive floor depression along reactivated basement faults during an interval of sustained high magma supply. During each new injection, residual magma was erupted from the Muskox reservoir and now forms a distinctive suite of contaminated basaltic andesites found only in the lowermost 400 m of the exposed Coppermine River flood basalt sequence. Magmas of subsequent eruptions utilized alternate conduit systems that include intrusions beneath the apical graben, extensive mid-crustal reservoirs, and a thick magma underplate in the lower crust. The sub-volcanic Muskox intrusion is interpreted as one of many interconnected magma bodies, each formed incrementally, and distributed as a magma plumbing network through the crust that fed the voluminous continental flood basalts (>1.5 x 10⁶ km³) and contemporaneous dike swarms of the Mackenzie LIP.