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

Abstract

Abstract We have investigated the effects of different Fe 2 O 3 bulk contents on the calculated phase equilibria of low‐ T /intermediate‐ P metasedimentary rocks. Thermodynamic modelling within the MnO–Na 2 O–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O (Mn NKFMASHTO ) 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 Fe 2 O 3 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–X Fe 2 O 3 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 rock‐forming silicates that often yield important thermobaric information. In Fe 2 O 3 ‐rich compositions, garnet‐ and carpholite‐in curves shift towards higher temperatures with respect to the Fe 2 O 3 ‐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–X Fe 2 O 3 phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.