The Muskox intrusion: Overview of a major open-system layered intrusion and its role as a sub-volcanic magma reservoir in the Mackenzie large igneous province

Abstract

The Muskox intrusion in Nunavut, Canada, is one of the archetype mafic layered intrusions globally. It represents a sub-volcanic magma reservoir to flood basalts of the 1.27 Ga Mackenzie large igneous province (LIP), which also includes the coeval Coppermine River flood basalts and Mackenzie dike swarm. Discovered in 1956, the Muskox intrusion was the target of a scientific drilling program in 1963 by the Geological Survey of Canada that yielded an unprecedented composite section of a terrestrial layered intrusion. The intrusion, which is funnel-shaped in cross section and forms a shallowly northward-plunging elongate body, comprises a keel dike, marginal zone, layered series, and roof zone. The 150–500 m-wide and 60 km-long (as exposed) gabbronoritic keel dike, which is not a direct feeder to the Muskox intrusion, is essentially a wide Mackenzie dike constituting early injections of crystal-rich magmas prior to emplacement of the main components of the Muskox intrusion. The 100–200 m-thick marginal zone, defined by a thin interval (<10 m) of contaminated granophyre-bearing gabbronorite at the contact, followed inward by uncontaminated feldspathic olivine-orthopyroxene cumulates (peridotites), formed incrementally during progressive inputs of magma that in turn produced the layered series. The 1800 m-thick layered series is dominated by olivine-rich ultramafic cumulates (dunite, olivine clinopyroxenite) with the upper part containing more variable and fractionated ultramafic-mafic cumulates (peridotite, websterite, orthopyroxenite, gabbronorite). Rocks of the layered series are grouped into four megacycles, based on differences in the relative crystallization order of primocrysts, and 25 cyclic units are recognized with each representing the partial crystallization sequence of a single sill-like injection of magma. The presence of fine-scale layering, layer disruption, and irregular structures restricted to the uppermost layers of megacycles is evidence for dynamic processes that mark the onset of major magma filling stages and new megacycles. Interstitial granophyre patches in gabbronoritic cumulates from the uppermost cyclic units may record the products of silicate liquid immiscibility in these more differentiated rocks. Feldspathic rocks without cumulus textures (granophyre-bearing gabbros, mafic granophyres, granophyres ± brecciated roof rocks) form a complex heterogeneous roof zone, produced by localized melting of the overlying roof, that formed a gravitationally stable, buoyant Si-rich cap above the layered cumulates below. Accommodation space for the development of the nearly 2 km thickness of ultramafic-mafic cumulates, exposed over 60 km and potentially extending >100 km to north in the subsurface, was provided by downward displacement along reactivated basement faults during each major magma injection. A newly recognized 250–300 m-thick “basal zone” of feldspathic peridotites and overlying magmatic breccias that outcrops just north of the Coppermine River may represent the initial stage of Muskox magmatism above the previously consolidated keel dike. Cumulates from the layered series correlate geochemically with a distinctive suite of basaltic andesites from the lowermost 400 m of the 1000 m-thick September Creek member of the Copper Creek Formation, the stratigraphically lowest part of the regionally extensive >3 km-thick Coppermine River Group flood basalts. The Muskox intrusion is interpreted as a high-level magma reservoir where crystals (mostly olivine) accumulated following transit of crustally contaminated basaltic magmas early in the emplacement sequence of the Mackenzie LIP. Eruption of subsequent Coppermine River basalts (upper September Creek, Stony Creek, Burnt Creek members) utilized alternate conduit systems and extensive shallow to mid-crustal magma reservoirs to the north underlying the apical graben that represent stacks of amalgamated and differentiated cumulates up to 10 km thick. The Mackenzie LIP, with a low-latitude, near-equatorial setting in the Mesoproterozoic, would have been associated with significant global environmental impacts due to volatile release (e.g., CO2, SO2) from the voluminous flood basalts (>1.5 × 106 km3) and widespread degassing from their plutonic counterparts at depth, including a thick magma underplate in the lower crust. Although a geochronological framework does not yet exist for the Muskox intrusion and Mackenzie LIP, a wealth of potential dating targets from all magmatic components, intrusive and extrusive, has been identified. The Muskox intrusion is an ideal natural laboratory to test hypotheses concerning the evolution of large, open-system magma reservoirs in the Earth's crust and linkages to contemporaneous flood basalts, dike swarms, and paleoenvironmental changes.