A COMPARISON OF FOUR TRANSECTS THROUGH THE NORTH RANGE METAMORPHIC AUREOLE OF THE SUDBURY IGNEOUS COMPLEX, SUDBURY, ONTARIO

The Sudbury Igneous Complex (SIC), within the 250km diameter Sudbury Impact Structure, was formed at 1.85 Ga due to the hypervelocity impact of a meteorite or comet into Archean and Proterozoic target rocks of the Canadian Shield. The SIC is currently exposed as an elliptical 60 x 30 km, 2.5 km thick erosional remnant of the original impact melt sheet. Surrounding the SIC, to a maximum radius of 1.5 km, is a metamorphic aureole that comprises an innermost zone metamorphosed to pyroxene hornfels facies, a zone subjected to hornblende hornfels facies, and an outermost zone metamorphosed in the albite-epidote hornfels facies.

The metamorphosed rocks in this study are the North Range footwall rocks of the SIC, that are predominantly composed of Levack Gneiss, Cartier Granitoid and a remnant gabbro - anorthosite complex. Prior to the impact, these rocks had already undergone polyphase tectonometamorphism. An initial burial event produced granulite facies assemblages, which were then retrogressed to amphibolite facies during their subsequent uplift. The Penokean Orogeny, at 1.9 to 1.7 Ga, caused the NE - SW trending shortening now observed through the centre of the crater. The Levack Gneisses were retrogressed further, to P-T conditions within the greenschist facies, due to Proterozoic faulting and shearing events.

Field mapping and sampling in the North Range metamorphic aureole reveals that some transects show deformation and/or alteration. In two transects, near Mosquito Lake and the Bowell Township, evidence of faulting subparallel to the footwall-SIC contact is observed. Analysis of samples from these faulted transects indicates that some, or all, of the isograds are missing. This suggests that a number of discrete thrust faults are developed at, or close to, the footwall-SIC contact. In the Whistle - Parkin transect, hydrothermal fluids have obliterated almost all traces of the metamorphic aureole. A comparison of isograd maps and mineral textures and assemblages will be made between these transects, and unfaulted and unaltered transects. This will provide further insight into the nature and extent of tectonometamorphism that has affected the Sudbury Region.