Metamorphic petrology of the Gillam area, Manitoba
Elphick, Stephen Conrad
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.