The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist andamphibolite facies conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3

Mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-Ti O2-Fe2O3 (KFMASHTO) using THERMOCALC and its internally consistent thermody namic dataset constrain the effect of TiO2 and Fe2O3 on greenschist and amp hibolite facies mineral equilibria in metapelites. The end-member data and activity-composition relationships for biotite and chloritoid, calibrated w ith natural rock data, and activity-composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO2 an d Fe2O3 on the stability of these minerals. Thermodynamic models for ilmeni te-hematite and magnetite-ulvospinel solid solutions accounting for order-d isorder in these phases allow the distribution of TiO2 and Fe2O3 between ox ide minerals and silicate minerals to be calculated. The calculations indic ate that small to moderate amounts of TiO2 and Fe2O3 in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in met apelites at greenschist to amphibolite facies, compared with those calculat ed in KFMASH. The addition of large amounts of TiO2 to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; i n contrast, rocks rich in Fe2O3 develop a markedly different metamorphic su ccession from that of common Barrovian sequences. In particular, Fe2O3-rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.