A General Conceptual Petrological Model for the Subsolidus Transformation of Chromitite with Application and Implications

Abstract

A general petrologic model for the transformation of chromitite in the FeO–MgO–Al2O3–Cr2O3–SiO2–H2O (FMACrSH) system is presented based in mass-balance and thermodynamic constraints. In the model, the transformation of chromitite reaches the common Cr-spinel+chlorite assemblage of transformed chromitites upon reaction with external fluid. This metasomatic process takes place in two major sequential steps involving a net-transfer reaction of olivine consumption first ensued by Cr-spinel+chlorite dissolution–precipitation. The first step is completed early in the hydration/metasomatic process producing new Cr-spinel (+chlorite±brucite) with restricted composition close to the composition of reacting mantle Cr-spinel as a function of Cr-spinel/olivine ratio and the stoichiometric coefficients of olivine and Cr-spinel in the net-transfer reaction. The second transformation step, triggered upon exhaustion of olivine, is protracted and continuously produces increasing chlorite and decreasing Cr-spinel contents, the latter with continued more deviated composition from reacting mantle Cr-spinel, as a function of continued infiltration of external fluid. The mass-balance model does not prejudice transformation under isothermal-isobaric conditions, heating, or cooling, but thermodynamic calculations confirm that all these thermal scenarios are possible for the generation of the predicted mineral assemblages and compositions. These calculations demonstrate that extreme Cr-spinel compositions are a strong function of decreasing spinel volume upon reaction progress at reaction sites under strongly overstepped conditions. The application of the model to mantle chromitites of the Cadomian Calzadilla metaophiolite (Ossa-Morena Complex, SW Iberia) allows reinterpreting the thermal scenario for chromitite transformation in a context of prograde metamorphism at near-isothermal-isobaric conditions. Proposals of cooling during transformation of regionally metamorphosed chromitites should be revisited in light of the petrologic model offered.