Abstract
Abstract Piemontite-garnet bearing assemblages with or without braunite, occur in calc-silicate rocks in Sausar Group, India. The studied assemblages equilibrated under similar physical condition of metamorphism, at a high fO2 regime. Chemically both piernontite and garnet are enriched in Mn in braunite-bearing assemblages, as compared to the braunite-free ones. petrographic features show that garnet+calcite±hematite are the late overprints in these assemblages, derived through a devolatilization reaction involving piemontite+precursor carbonate+quartz±braunite. The study shows that in the braunite-bearing assemblages, the 'maximum' Mn3+ content of the piemontite was not dictated by the mere presence of braunite, but it is rather correlatable with the modal abundance of braunite. Further, the XMn3+ in the piemontite and the modal abundance of braunite have a direct relationship with Al/Mn, Mn/Al+Si and Si/Al) ratios in the bulk composition of the assemblages. In the present situation the Al-Mn3+-Fe3+ distribution coefficients amongst equilibrating piemontite-garnet pairs show strong dependence on the bulk chemical composition.
Published Version
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