The nature of the Grenville Front near Lake Timagami, Ontario

• Main Author: Grant, James Alexander
• Format: Thesis
• Language: English
• Published: California Institute of Technology 1964
• Subjects: Geological and Planetary Sciences; Lake Timagami; Grenville Front; metavolcanic rocks; metagreywacke rocks; lithology; Huronian strata; Canada; Hobbs; Buchan; Keewatin
• Online Access: https://dx.doi.org/10.7907/bhp6-te58; https://resolver.caltech.edu/CaltechETD:etd-10152002-114122

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 metamorphism was isochemical and that this correlation is permissible. The tentative correlations from the Superior province into the Grenville province and the Grenville age of the late metamorphism cannot be proved conclusively by the mapping, or by the petrographic and chemical studies. However, the Rb-Sr analyses provide convincing evidence that these conclusions are correct. (a) Whole rock samples of granite from the Superior and Grenville provinces define an isochron. This substantiates the correlation of these as comagmatic granites. The age derived from the isochron is 2220 m.y. ? 70 m.y. and the corresponding initial (Sr[superscript 87]/Sr[superscript 86])[...] = 0.703 ? 0.001. (b) Mineral isochrons indicate virtually complete strontium isotopic equilibration south of the Front and partial equilibration north of the Front between 0.9 b.y. and 1.1 b.y. This is considered proof that the late metamorphism was indeed of Grenville age. (c) The combined data from the whole rock and mineral isochron studies prove beyond all reasonable doubt that granite of the Superior province, with a primary age of approximately 2.2 b.y., was subjected to the Grenville orogeny at approximately 1 b.y. (d) The whole rock analyses place a new maximum limit of 2.3 b.y. on the time of deposition of the Upper Huronian sediments. The major conclusion from this work is that rocks of the Superior province were "cannibalized" during the Grenville orogeny and can be traced into the Grenville province. The Grenville Front is a metamorphic transition of Grenville age locally disrupted by faulting, and approximately defined by the southeastward transition from greenschist to amphibolite facies. (Plate 1, the geological map of the Lake Timagami area is essential to this thesis. It has been published (Map #2048, Vogt-Hobbs area) by the Ontario Department of Mines, Parliament Buildings, Toronto 2, Ontario, Canada, from whom it may be obtained.)