Plagioclase-Garnet-Epidote Equilibria in Hornblende-Plagioclase Bearing Rocks from the Esplanade Range, British Columbia

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 [Formula: see text] 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 [Formula: see text] seems adequate to account for the increase in anorthite content in plagioclase with increasing metamorphic grade.