Plagioclase–clinopyroxene–garnet–quartz equilibria and the geobarometry and geothermometry of garnet amphibolites from Mica Creek, British Columbia

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Abstract
Metabasites form a small but significant volume of the Upper Precambrian Horsethief Creek Group near Mica Creek, British Columbia. Sillimanite zone assemblages in metabasites are: hornblende–plagioclase–ilmenite ± quartz ± garnet ± clinopyroxene ± sphene ± calcite. The incoming of garnet–clinopyroxene in metabasites very nearly coincides with the kyanite–sillimanite isograd. Fe–Mg distributions between coexisting garnet and clinopyroxene suggest a temperature range of 650–705 °C (Ellis and Green calibration). The higher temperature estimates disagree with the upper thermal stability limits of muscovite–plagioclase–quartz–sillimanite from adjacent pelitic rocks. Pressure estimates based upon clinopyroxene–garnet–plagioclase–quartz equilibria are 5.6 ± 0.3 and 7.2 ± 0.3 kbar (560 ± 30 and 720 ± 30 MPa), respectively, using two different calibrations of this geobarometer. These estimates bracket the pressure estimated from garnet–plagioclase–Al2SiO5 equilibria in pelitic rocks.

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