Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni, Sausar Group, India and their interpretation

Abstract

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500-550° C and 3-4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite (MnO 1%), magnetite (MnO 15%) and magnetite (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low X_CO2 (<0.2), and the latter equilibrated with a CO₂-rich fluid. Rhodochrosite+magnetite+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower f_O2 through complex mineral-fluid interactions.