A note on the significance of the assemblage calcite-quartz-plagioclase-paragonite-graphite

The biotite zone assemblage: calcite-quartz-plagioclase (An₂₅)-phengite-paragonite-chlorite-graphite, is developed at the contact between a carbonate and a pelite from British Columbia. Thermochemical data for the equilibrium paragonite+calcite+2 quartz=albite+ anorthite+CO₂+H₂O yields: $$log f{text{H}}_{text{2}} {text{O}} + log f{text{CO}}_{text{2}} = 5.76 + 0.117 times 10^{ - 3} (P - 1)$$ for a temperature of 700°K and a plagioclase composition of An₂₅. By combining this equation with equations describing equilibria between graphite and gas species in the system C-H-O, the following partial pressures: (P{text{H}}_2 {text{O}} = 2572{text{b, }}P{text{CO}}_2 = 3162{text{b, }}P{text{H}}_2 = 2.5{text{b, }}P{text{CH}}_4 = 52.5{text{b, }}P{text{CO}} = 11.0{text{b}}) are obtained for (f{text{O}}_2 = 10^{ - 26}) . If total pressure equals fluid pressure, then the total pressure during metamorphism was approximately 6 kb. The total fluid pressure calculated is extremely sensitive to the value of (f{text{O}}_2) chosen.