Multiple Metamorphism Along the Grenville Front, North of Georgian Bay, Ontario

Four periods and kinds of Precambrian metamorphism are recognized in a single small map area astride the Grenville Front north of Georgian Bay, Ontario. They are, from oldest to youngest: (1) low-grade Hudsonian (Penokean) regional metamorphism of Huronian sedimentary rocks and later, Nipissing? intrusive rocks; (2) moderately low-grade (northwest of the Grenville Front) to high-grade regional metamorphism and generation of granites of Killarnean age associated with development of the Grenville Front and related slip planes; (3) slightly later contact metamorphism about intrusive Killarnean granites, resulting in porphyroblastic growth of andalusite, cordierite, chlorite, muscovite, and perhaps garnet; and (4) nonpervasive post-Keweenawan? retrogressive metamorphism and cataclasis, resulting most commonly in cataclasites derived from the Killarnean granites.