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Manganocummingtonite from the mesoproterozoic,Sausar fold belt,central India |
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| Authors: | D R Kanungo D B Malpe B E Leake |
| Institution: | 1. Geological Survey of India, Central Region, Seminary Hills, Nagpur, 440 006, India 2. Department of Geology, R.T.M. Nagpur University, Law College Square, Nagpur, 440 001, India 3. School of Earth & Ocean Sciences, Cardiff University, Park Place, Cardiff, CF10 3AT, UK |
| 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)(Si7.931–7.997Aliv 0.003–0.069)O22(OH1.5–2.0F0.0–0.5)] 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 $X_{CO_2 } $ system, stabilized with $X_{Mn/(Mn + Fe^{2 + } + Mg)} $ = 0.25 to 0.28 in the amphibole and 0.85 in the garnet and formed under unusually low fO 2 conditions for the Sausar region, near channelized fluids which deposited quartz may have controlled the fO 2 . |
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