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The metamorphism and alteration of the basement rocks in the Carswell circular structure, Saskatchewan Herring, Bernard Geoffrey

Abstract

This thesis reports on a petrographic study of the metamorphosed Precambrian basement rocks exposed in the Carswell Circular Structure in the Athabasca sandstone of northern Saskatchewan. A descriptive classification of the rocks present is set up, and comprehensive illustrations are provided for the benefit of subsequent workers in the area. The basement rocks consist of gneisses and pegmatites, and have been metamorphosed to the granulite facies, with subsequent partial retrogression to upper amphibolite facies. This retrogression evidently resulted from increased availability of water, and may be related to the emplacement of pegmatites. A study of Mg-Fe partitioning between coexisting proxenes indicates that prograde metamorphism occurred at a temperature of about 800°C, consistent with the granulite facies. The basement rocks are highly altered by low-temperature hydration. Geochronologic studies suggest that this alteration occurred at about one billion years before present, and is therefore distinct from prograde metamorphism (two billion years) and from the Circular Structure "event" (480 million years). The alteration is considered to have been related to uranium mineralisation, and a possible model for the emplacement of mineralisation is discussed.

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