Pressure, temperature and fluid composition during amphibolite facies metamorphism of graphitic metapelites, Howard Ridge, British Columbia

ABSTRACT Graphitic metapelites from the Howard Ridge area, British Columbia, have been studied to estimate the pressure, temperature and fluid composition attending amphibolite facies metamorphism. Results from thermobarometric calculations indicate that P-T conditions of 610–625°C and 6.7kbar were reached during metamorphism. The equilibrium paragonite-quartz-albite-kyanite-H₂O gives significantly different estimates of XH₂O in the metamorphic fluid using different paragonite solution models. Estimates of XH₂O range from a maximum of 0.93 (Eugster et al., 1972) to a minimum of 0.29 (Chatterjee & Flux, 1986). H₂O estimates obtained using the Eugster et al. (1972) and Chatterjee & Froese (1975) solution models give similar results (i.e. 0.8 ± 0.1 versus 0.7 ± 0.1, respectively). Non-ideal mixing in the C-O-H system provides an XH₂O estimate of 0.74 at H₂O maximum conditions, 0.5 log units below the QFM buffer. The Chatterjee & Flux (1986) paragonite solution model provides unrealistically low estimates of XH₂O relative to other paragonite solution models, C-O-H equilibria, and published fluid inclusion and mineral equilibria data. Consistent estimates of fluid composition between C-O-H and mineral equilibria suggest that a H₂O-rich fluid attended metamorphism of graphitic metapelites at Howard Ridge.