The Offset Dykes in the North and East Range of the Sudbury impact structure, Canada: A synthesis

Abstract

The Offset Dykes of the ∼1.85 Ga ∼200 km-diameter Sudbury impact structure formed by the injection of impact melt from embayment structures in the Sudbury Igneous Complex (SIC) – the ∼3–5 km thick impact melt sheet – into the footwall of the crater. Despite having been recognized over a century ago and being the sites for several world-class ore deposits, the timing and emplacement mechanism of the Offset Dykes remains debated. In this contribution we document the geological, geochemical, and mineralogical relationships in the North Range Offset Dykes – the Hess, Trill, Ministic, Cascaden, Pele, Foy, Parkin, Whistle, and Rathbun dykes – and the implications for their injection, crystallization, and differentiation history.The Offset Dykes are locally referred to as “quartz diorite” and typically comprise a groundmass of plagioclase, amphibole, and biotite, with varying amounts of pyroxene, quartz, oxides, and sulfides. While it is common for the centre of the dykes to be enriched in sulfides and lithic inclusions (“IQD”) and with inclusion- and sulfide-poor margins (“QD”), the Pele dyke is entirely inclusion-free, and is considerably more evolved and depleted in Ni and Cu relative to the other North Range Offset Dykes. A unit of recrystallized breccia termed “metabreccia” is commonly entrained as inclusions within the QD and IQD, with a greater abundance near the embayment structures and the SIC.From observations of other impact structures, it is apparent that the base of the impact melt sheet would be laden with clasts early on. Our synthesis thus leads to a model of formation where an injection of clast- and sulfide-rich melt is separated into a clast-rich centre and a clast-poor margin via a process known as flow differentiation. After some differentiation of the melt sheet had occurred, ongoing tectonism in the region opened fracture systems into which chemically evolved clast-poor melt, formed after sulfide saturation of the melt sheet, was emplaced to form the Pele Offset Dyke. We note that the presence of quenched margins along some sections of some Offset Dykes requiring an injection of sulfide-free and clast-free melt is not readily addressed in the model presented here and should be a focus for future investigations. The reason for the major differences between the North Range Offset Dykes discussed here, and their counterparts in the South Range, also remain unanswered.