Analysis of parageneses of metapelite gneisses of the Okhotsk granulite complex by minimization of Gibbs thermodynamic potential

Abstract

Abstract The problem of modeling of real parageneses has been solved by minimization of Gibbs thermodynamic potential for metapelites of the Okhotsk granulite complex. Model mineral assemblages completely reproduce the composition of minerals and their modal contents in the studied rocks. This fundamental fact directly verifies the solution of the problem, proving the validity of the principle of local equilibrium in the studied assemblages and the agreement of all thermodynamic data accepted on the modeling. The pressure and temperature during the metamorphism of granulites of the Okhotsk complex, estimated by modeling, are 5.2–7.0 kbar and 620–770 °C, which corresponds to the near-boundary conditions of the amphibolitic and granulitic facies. Model mineral assemblages similar to real parageneses in the composition of minerals and their modes can be successfully obtained with the Selektor software under conditions of both inert and moving water. The composition of the external metamorphic fluid and the approximate weight ratio of fluid to rock have been determined. The oxidation potential of this fluid is similar to the potential of oxygen at the buffer C–CO–CO2 if the fluid/rock ratio is 0.03–0.30 and the low partial pressure of water varies from 1.80 to 0.35 kbar. The Okhotsk metamorphic complex is not an analog of the granulites of the southern Aldan Shield, because considerably higher pressure and temperature are typical of the latter.