The Sudbury Igneous Complex: Viscous emulsion differentiation of a superheated impact melt sheet

Abstract

The Sudbury Igneous Complex of Ontario, Canada, is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1.85 Ga ago. The transient cavity and melting zone reached the Moho and instantly (∼2 min) relaxed to form a more familiar large, shallow crater holding a thick, superheated (∼1700 °C) melt sheet covered by ∼2 km of breccia. There is little about the resulting bimodal igneous complex that resembles crystallization of well-known sheet-like bodies of similar composition. Yet, the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions, suggesting an intimate common parentage from the surrounding crust. This petrogenetic enigma is explained here as a natural, unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion, which we take as the high-temperature equivalent of breccia. A wide spectrum of viscously discrete, interdispersed parcels of mafic and felsic liquids, reflecting the compositional heterogeneity of the target crustal materials, formed the emulsion. Within days to months, the emulsion components separated according to their relative densities into a bimodal norite-granophyre assemblage that formed the basic structure of the present Sudbury Igneous Complex. There is clear evidence of this emulsion in the earliest dikes (i.e., offsets), which likely give the earliest state of the nascent melt sheet. Immediately following emulsion separation, the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer. These convective motions homogenized and rapidly cooled the magma to the liquidus temperatures, whereupon convection ceased. The pattern of convection was pinned in place by the embayment topography of the crater floor, which in turn played a pivotal role in directing sulfide deposition into the embayments. All further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins. There is clear evidence of solidification from the floor upward and the roof downward. Minimal differentiation and compositional modification took place throughout cooling and solidification. Nevertheless, during the solidification stage, granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying Onaping Formation became unstable and entered the melt sheet, and the partially melted remnants collected at the interface between the norite and granophyre. Some interstitial melt from the norite also percolated upward, and, altogether, the blocks and melt produced the distinctive chemical and physical characteristics of the unusual Transition Zone. The Sudbury melt sheet is, in essence, a full-scale magmatic experiment. The conditions of formation, relative to any other large terrestrial magma, are “precisely” known. Thus the clear lack of any significant modal layering, the overall homogeneous nature of each unit, and the lack of any significant chemical differentiation through crystal fractionation establish Sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers.