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