Thermodynamic modelling of the phengite geobarometry

摘要

Using the updated models of activity-composition relations for solid solutions, P-Tpseudosections and Si isopleths of phengite in the system KFMASH (K_2O-FeO-MgO-Al_2O_3- SiO_2-H_2O) are calculated with the software THERMOCALC 323 for three bulk compositions of the average, Al-rich and K-rich metapelites. The Si isopleths of phengite in the calculated pseudosections are highly dependent on mineral assemblages. In the chlorite- and biotite-bearing di- and trivariant fields, and in the garnet + kyanite-bearing trivariant fields, the phengite can be used as a good geobarometer. But in the carpholite-present and chlorite-absent fields, the Si contents of phengite mainly indicate variance of temperature. For a given Si value, the phengite in kyanite-bearing assemblages shows higher pressures than in the K-feldspar-bearing assemblages. Addition of Na_2O into the KFMASH leads to reduction of stability of the AFM phases. The Si isopleths of phengite in the mineral assemblages containing only one Na-phase are similar to those in the corresponding KFMASH assemblages. But in the assemblages with two or more Na-phases, the phengite Si isopleths differ remarkably from those in the KFMASH.