Abstract

Hornblende-plagioclase bearing rocks occur as layers and lenses within metasedimentary rocks of the Esplanade Range, British Columbia (51°31′ N, 117°27′ W). A typical assemblage in the hornblendic rocks is quartz-plagioclase-garnet-epidote-ilmenite ± sphene ± calcite. Plagioclase from the beginning of the garnet zone has an average composition near An35, whereas plagioclase from well within the staurolite-kyanite zone has an average composition near An95. Garnet and epidote also show variation in composition with increasing metamorphic grade.From structural and stratigraphic arguments and from the presence of kyanite in pelitic rocks, we estimate a minimum load pressure during metamorphism of about 4 kb.Using thermochemical and experimental data on the following reactions:1) 4 Zoisite + quartz = 5 anorthite + 1 grossular + 2 water;2) 2 Zoisite + CO2 = 3 anorthite + calcite + water;3) Calcite + rutile + quartz = sphene + CO2;4) Calcite + quartz = wollastonite + CO2, andassuming an ideal solution model for the solids, we can construct a set of curves for the above reactions on a graphic diagram, each curve representing a different set of solid compositions.A temperature variation of approximately 150 °C from the beginning of the garnet zone to well within the staurolite-kyanite zone and a fluid composition of graphic seems adequate to account for the increase in anorthite content in plagioclase with increasing metamorphic grade.

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