0016-7622/2014-83-1-93/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.84, January 2014, pp.93-99 Manganocummingtonite from the Mesoproterozoic, Sausar Fold Belt, Central India D. R. KANUNGO 1 , D. B. MALPE 2 and B. E. LEAKE 3 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 Email: dbmalpe@yahoo.com Abstract: Manganocummingtonite occurs with spessartine, quartz and pyrolusite in the Chikmara area, Sausar fold belt, central India. Its composition is [Ca 0.3–0.35 (Mg 3.3–3.5 Mn 1.6–1.8 Fe 2+ 1.4–1.5 )(Si 7.931-7.997 Al iv 0.003-0.069 )O 22 (OH 1.5-2.0 F 0.0-0.5 )] 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 Fe 3+ 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 . Keywords: Manganocummingtonite, Spessartine, Oxygen fugacity, Sausar fold belt, Central India. In central India Mn amphiboles have been reported from the Chikla, Tirodi, Gowari Wadhona, and Mansar-Kandri manganese deposits from the Mesoproterozoic Sausar fold belt (SFB), by many workers (e.g., Dunn and Roy, 1939; Kilpady and Dave, 1954; Bilgrami, 1955; Basu, 1958; Babu and Nayak, 1961; Roy and Mitra, 1964; Roy and Purkait, 1965, 1968; Roy, 1966, 1974; Dasgupta et al. 1988, 1993). Although the petrology of Mg-Mn amphibole- and garnet- bearing assemblages in the SFB were excellently described by Dasgupta et al. (1988), all their assemblages were either in silicate–carbonate rocks with minerals such as rhodochrosite, pyroxmangite, kutnahorite, calcite and hematite, or in silicate-oxide rocks with pyroxmangite and hematite, none of which occur in the present samples. The present paper describes the manganocummingtonite and its coexisting phases in Mn-silicate±oxide assemblages from Chikmara area, SFB and the deduced stability and prevailing metamorphic conditions of formation. GEOLOGICAL BACKGROUND The Mesoproterozoic rocks of the Sausar fold belt (SFB) are metasediments with stratified syngenetic manganese ores and form a linear arcuate belt in central India. They include INTRODUCTION Manganese bearing silicates and oxides in different lithologies can give an indication of the oxygen fugacity (fO 2 ) during geological processes. They are used to decipher the conditions under which Mn ore deposits were metamorphosed (Brusnitsyn, 2007) and may indicate the sites of old fluid emanations (Melcher, 1995). Mg–Mn amphibole–bearing metamorphosed manganese silicate– carbonate rocks have been widely reported from several localities including Nsuta, Ghana (Jaffe et al. 1961), Wabush iron-formation, Labrador (Klein, 1964, 1966), Buritirama, Brazil (Peters et al. 1977), Tatehira, Japan (Kobayashi, 1977), Balmat, U.S.A (Peterson et al. 1984) and eastern Taiwan (Yui et al. 1989). In all these deposits except at Nsuta, Mg–Mn amphibole is associated with carbonate and pyroxene with or without rhodonite, (Mn 2+ , Fe 2+ , Mg, Ca 2+ ) SiO 3 , or pyroxmangite, (Mn 2+ , Fe 2+ )SiO 3 , whereas, in the Sausar belt of central India Mn-rich rocks lack pyroxenes. The phase compatibilities of Mn silicates involving Mn amphiboles are still not clearly understood due to inadequate petrographic characterization, incomplete chemical data of the coexisting phases and scarcity of experimental studies on systems with Mn (Dasgupta et al. 1988; Brusnitsyn, 2007).