Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite-rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

We have investigated the effects of different Fe2O3 bulk contents on the calculated phase equilibria of low-T/intermediate-P metasedimentary rocks. Thermodynamic modelling within the MnO–Na2O– K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system of chloritoid-bearing hematite-rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe2O3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe2O3 diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post-compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rockforming silicates that often yield important thermobaric information. In Fe2O3-rich compositions, garnet- and carpholite-in curves shift towards higher temperatures with respect to the Fe2O3-free systems. The presence of a ferric-iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe2O3 phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.