Two Transects Across the Grenville Front, Killarney and Tyson Lake Areas, Ontario

Abstract

The Grenville Front marks the northwest limit of the Grenville Province. It is a tectonite front of major significance, separating older structural provinces of the Canadian Shield from the 1.3–1.0 Ga Grenville orogen to the southeast. An historical perspective of the whole front is given, followed by a more detailed appraisal of relationships across the front in Ontario. Of the various relationships found along this 200 km segment of the front, two have been chosen for illustration on field excursions: 1, a section along the north shore of Georgian Bay, where a 1740 Ga granite and rhyolite complex lies between the Grenville orogen and folded Huronian sedimentary rocks of the Southern Province and is involved in Grenvillian tecton- ism; 2, an oblique section, 10 to 20 km inland from Georgian Bay, which demonstrates abrupt changes in structural style and metamorphic grade on entering the Grenville Province. In both sections, severe modification within the Grenville orogen has been superimposed on rocks that were already deformed and metamorphosed, leading to extremely complex geological relationships. The only previously unaffected rock unit is diabase of the 1.24 Ga Sudbury dyke swarm, the effects of the Grenvillian orogeny on which is examined in some detail. The evidence presented on the field excursions supports the contention that the northwest margin of the Grenville orogen is a strongly uplifted, compressed zone in which progressively deeper crustal levels are exposed at the present surface.