Pressure-temperature and evolution of fluid compositions of Al2SiO5-bearing rocks, Mica Creek, B.C., in light of fluid inclusion data and mineral equilibria

Abstract

Metamorphosed pelitic rocks from Mica Creek, British Columbia contain sillimanite, kyanite with minor fibrolite and andalusite-bearing quartz pods. Mineral equilibria were used to infer peak P-T conditions and fluid compositions in equilibrium with the solid phases. Fluid inclusions in three schist samples appear to be good indicators of conditions affecting those rocks during and after peak metamorphic conditions. In samples from two localities, fluid inclusions from schist and quartz-rich segregations have densities appropriate to the peak metamorphic conditions. The observed compositions for these fluids (low salinity with ≅12 mole % dissolved CO2) agree with calculated \({\text{X}}_{{\text{H}}_{\text{2}} {\text{O}}}\) values of 0.84 to 0.85, based upon paragonite-quartz-albite-Al2SiO5 equilibria. The fluids unmixed as the schists were uplifted and cooled; fluid inclusions trapped during this stage outline a solvus in the CO2-H2O-NaCl system. A later influx of fluids containing CH4 and N2 accompanied formation of andalusite-bearing plagioclaserich segregations. The restricted association of andalusite-bearing pods and low density fluids suggest a localized but pervasive fluid influx during uplift. Preservation of high density fluid inclusions during uplift and erosion, coupled with evidence for unmixing of H2O- and CO2-rich fluids on the solvus, provide constraints on the P-T uplift path.