Abstract
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:
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.
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Ghent, E.D., Jones, J.W. & Nicholls, J. A note on the significance of the assemblage calcite-quartz-plagioclase-paragonite-graphite. Contr. Mineral. and Petrol. 28, 112–116 (1970). https://doi.org/10.1007/BF00404993
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DOI: https://doi.org/10.1007/BF00404993