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

Abstract

Mineral equilibria calculations in the system K 2 O–FeO–MgO–Al 2 O 3–SiO 2–H 2 O–TiO 2–Fe 2 O 3 (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO 2 and Fe 2 O 3 on greenschist and amphibolite facies mineral equilibria in metapelites. The end‐member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO 2 and Fe 2 O 3 on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO 2 and Fe 2 O 3 between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO 2 and Fe 2 O 3 in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO 2 to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe 2 O 3 develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe 2 O 3‐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.