Metamorphic petrology of the Gillam area, Manitoba

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Elphick, Stephen Conrad
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Following mapping of the Gillam area, Northern Manitoba, by Dr. I. Haugh and S.C. Elphick, during the summer of 1968, and publication of a preliminary report in the Manitoba Mines Branch Geological Paper 3/68, further work was done on samples by Mr. Elphick. This was an examination of the petrology and mineralogy of the gneisses, using 300 thin-sections and 150 slabs stained for potash feldspar. Plagioclase compostions were measured in 80 thin-sections. 40 samples from various areas were point counted for modal mineral composition. With these data the original map was revised, a more detailed stratigraphic sequence drawn up, and a plagioclase anorthite content distribution map drafted. Two periods of folding are recognisable both on geologic and aeromagnetic maps. The earlier period involved the formation of west-north-west, near isoclinal folds, with axial planes steeply dipping to the north. Metamorphism produced sillimanite, under moderately low pressure conditions (Abukuma type). The second metamorphic event involved formation of broad Z-folds, axial planes trending north-east, refolding the earlier isoclines Metamorphic grade was lower than the first event, and was of lowest sillimanite or greenschist facies. During the first episode iron was a mobile constituent, as proven by various textural relations displayed by the magnetite. Calcium distribution in plagioclase was established during this period, and results indicate strict dependence of plagioclase-anorthite-content on bulk calcium content of each gneiss.