| Summary: | NOTE: Text or symbols not renderable in plain ASCII are indicated by [.]. Abstract is included in .pdf document. The Grenville Front forms the boundary between the Superior and Grenville provinces, which differ greatly in structural trends and in grade and age of major metamorphism. The Front has been claimed to be a metamorphic transition, a regional fault zone, or a combination of the two. The Lake Timagami area lies athwart the Front and is favored by good outcrop and an unusually complete geological section. Detailed mapping, petrographic and chemical studies permit tentative conclusions which are validated by Rb-Sr isochron analyses. Early Precambrian Keewatin-type metagreywacke and metavolcanic rocks form a steeply dipping, easterly trending belt intruded successively by quartz diorite and granite. To the north, these rocks are overlain with marked unconformity by flat lying virtually unmetamorphosed Huronian strata; diabase intrudes the Huronian and older rocks. Pre-Huronian "Buchan" metamorphism of the Keewatin-type rocks predated, but was probably related to, the emplacement of the granite. Later metamorphism affected the granite and older rocks and probably the Huronian sediments and the diabase, but its macroscopic effects are only visible - by definition -south of the Grenville Front. Here one finds a migmatitic terrane in which the probable equivalents of the metagreywacke quartz diorite and granite can be distinguished. In the west, the transition into this terrane is unfaulted, but to the east it is largely cut out by a northeasterly trending fault system. Within this terrane, the late metamorphism produced lithologies, metamorphic grade and structures typical of the northwestern part of the Grenville province, and for this reason the metamorphism was considered to be of Grenville age. X-ray fluorescence analyses were used to establish the range of chemical composition of the metagreywacke and the apparently equivalent schist south of the Front. Comparison of these ranges suggests that the late ...
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