OUTWARD EMPLACEMENT OF THE FOY OFFSET DYKE AT THE SUDBURY IMPACT STRUCTURE, CANADA - EVIDENCE FROM THE VARIATION IN INCLUSIONS ALONG STRIKE

Dykes have been found in association with impact craters on Earth, the Moon, and Mars, the largest of which are the so-called Offset Dykes found around the Sudbury Impact Structure in Ontario, Canada. Despite being mined for over 100 years for their rich Ni-Cu-PGE deposits, the origin of these Offset Dykes - and the origin of impact-related dykes in general - is poorly understood. A statistical analysis of the variation in inclusions found along strike of the Foy Offset Dyke at the Sudbury structure has provided insight into its formation.

The Offset Dykes occur radially and concentrically around the Sudbury Igneous Complex (SIC), which is a ~3 km thick differentiated impact melt sheet. They are composed of quartz diorite and often contain inclusions of the regional host rocks. The dykes vary in length from 1 km to greater than 60 km and can reach widths of over 400 m. This study focuses on the Foy, the longest of the radial dykes which extends 37 km along strike northeast of the SIC. Its host rocks include the Levack Gneiss Complex to the south, Archean granitoids to the north, and various mafic dyke swarms common to the region. Fieldwork was completed at three sites along strike of the dyke: the Nickel Offset Mine, the intersection of the Hess and the Foy Offset Dykes, and Foy North ~ 3km northeast of the intersection.

With increasing distance away from the SIC, the number of gneiss clasts decreased, as did the total number and size of the inclusions. Additionally, quartz diorite inclusions were most common at the intersection of the Foy and Hess, as well as just north of the intersection. Across the width of the dyke there was no significant variation in clast lithology. However, the number of clasts consistently displayed a bell-curve shaped distribution across the width of the dyke, with the centre containing the largest of the clasts.

The decrease in gneiss clasts to the north and the higher frequency of large clasts close to the SIC suggest a strong northward emplacement of the dyke from the SIC. This flow may have steadily lost kinetic energy as evidenced by the decreased number and size of clasts to the north. The high frequency of quartz diorite inclusions at - and north of - the intersection supports the northward emplacement.