Characterization of oxide assemblages of a suite of granulites from Eastern Ghats Belt, India: implication to the evolution of C-O-H-F fluids during retrogression

Abstract

We document complex intergrowth involving spinel-ilmenite-magnetite-hematite-corundum-rutile in various combinations from a suite of granulite facies rocks of Eastern Ghats Belt, India. Individual oxide phase shows considerable compositional variation among textures and samples. These textures arguably developed by oxidation reaction of early spinel and ilmenite solid solution from near-peak to subsequent retrogressive stages. Oxygen fugacity is measured at a constant pressure of 8 kbar and different temperatures estimated from geothermometric analyses involving oxide and silicate-bearing equilibria in different samples. The calculated fO₂ values are 2-3 log units higher than the QFM buffer except for one sample. Uncertainties in fO₂ calculation in some samples arise presumably due to extensive compositional readjustment of different oxide systematics at lower temperatures. The persistence of high fO₂ in mineral assemblages could be inherited from an oxidized precursor, but field evidence and presence of H₂O-CO₂-rich fluid inclusions in the studied samples imply possible involvement of externally-derived oxidizing fluid in the later part of the retrograde history. Recent experimental and natural data suggest CO₂ charged brine solution could be suitable for oxidation of mineral assemblages. Presence of brine inclusions are not detected, but fluorine enriched nature of biotite grains could possibly provide some indication for its presence. The mixed fluid possibly has its source in the crystallizing mafic magma emplaced at the lower crust that is exposed in the adjacent area.