Mn-Pyroxenoids from Gangpur Group of rocks, Orissa, India.

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Abstract

Petrography and petrochemistry of Mn-pyroxenoids in the Mn silicate-oxide and Mn silicate-carbonate-oxide rock assemblages of Precambrian Gangpur Group, Orissa, India are described. The two pyroxenoids, rhodonite and pyroxmangite, appear mutually exclussive of each other. Rhodonite occurs associated with spessartine, Mn-aegirine and tirodite while pyroxmangite coexists with spessartine and tephroite. In the coexisting carbonate members, kutnahorite is associated with rhodonite while rhodochrosite is found with pyroxmangite. Jacobsite is the common oxide present in both the pyroxenoids. Braunite and hematite are the other oxides recorded in rhodonite association in contrast to magnetite with pyroxmangite. The pyroxmangites are richer in FeO and poor in CaO as compared to rhodonites. These two pyroxenoids are considered to have been formed due to decarbonation-oxidation reactions from different progenitors under XCO<sub>2</sub> variation in closed spaced area and strong internal buffering of fO<sub>2</sub> during metamorphism.

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