Quartz-sillimanite knots and metamorphism at Southern Indian Lake, Manitoba

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Cranstone, John Ronald
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All outcrops in the area surrounding the northern portion of Southern Indian Lake were mapped during the summers of 1969 and 1970, and a report with accompanying geological maps has been prepared for the Manitoba Mines Branch. Quartz-muscovite-microcline-plagioclase-sillimanite nodules occurring in five outcrops of Sickle-type metasediments were mapped and sampled in detail. Twenty thin sections were studied and six chemical analyses done. The almost spherical knots are late or post tectonic, formed subsequent to two periods of folding. The metamorphic grade reached during the first period of folding (intense, isoclinal) is uncertain, however, extensive recrystallization occurred. The metamorphic grade reached during a second period of deformation (north-east trending) was the upper amphibolite facies of the low or medium (cordierite bearing) pressure facies series. Actual knot formation presumably was by simple metamorphic differentiation and segregation in beds of a suitable composition during increasing metamorphism or, less likely, was the result of potash realkalization of quartz-sillimanite knots formed in an earlier dealkalization event. Quartz muscovite knots occurring elsewhere in the Lynn Lake District appear to have formed at somewhat lower metamorphic grade.