Manganocummingtonite from the Mesoproterozoic, Sausar Fold Belt, Central India

Abstract: Manganocummingtonite occurs with spessartine, quartz and pyrolusite in the Chikmara area, Sausar fold belt, central India. Its composition is [Ca0.3–0.35(Mg3.3–3.5Mn1.6–1.8Fe2+1.4–1.5)(Si7.931-7.997Aliv0.003-0.069)O22(OH1.5-2.0 F0.0-0.5)] being fairly rich in Ca, which is indicative of metamorphic temperature in the amphibolite facies. The garnet contains 77.5% spessartine, 13% almandine and minor andradite, grossular and pyrope components. Unusually, there is no carbonate, pyroxene, pyroxmangite, rhodonite, magnetite or hematite. The available Al in the rock stabilized garnet and this mineral incorporated minor Fe3+ present in the rock as andradite component. The manganocummingtonite–garnet pairs developed at ~600°C during amphibolite facies metamorphism in low XCO2 system, stabilized with XMn/(Mn+Fe2++Mg)= 0.25 to 0.28 in the amphibole and 0.85 in the garnet and formed under unusually low fO2 conditions for the Sausar region, near channelized fluids which deposited quartz may have controlled the fO2.