Petrology of Mg-Mn Amphibole-Bearing Assemblages in Manganese Silicate Rocks of the Sausar Group, India

Somnath Dasgupta, P. K. Bhattacharya, G. Chattopadhyay, H. Banerjee, N. Majumdar, M. Fukuoka* and Supriya Roy
Department of Geological Sciences, Jadavpur University, Calcutta-700 032, India
*Dept. of Geology, Kyushu University, Fukuoka, Japan.

Abstract: Mg-Mn amphibole (tirodite), with or without pyroxmangite in the total absence of pyroxenes and high-calcic pyroxenoids, occurs in the Mn silicate rocks of the Sausar Group, India. The rocks were metamorphosed to amphibolite facies condition (T ∼ 650°C, P ∼ 6 kbar). Tirodite-pyroxmangite pairs developed in both carbonate-free and rhodochrosite-bearing assemblages. Also tirodite coexists with either kutnahorite or manganoan calcite in the absence of pyroxmangite. Mineral reactions inferred from modal abundances and compositions of the phases indicate stabilization of the amphibole alone from a bivalent cation-bearing residual unbuffered XCO2 system with XMn < 0.3. On the other hand, tirodite-pyroxmangite pairs appeared in unbuffered low to intermediate XCO2 assemblages with XMn > 0.35. Pyroxenes and high-calcic pyroxenoids did not appear in the present situation, though they occur elsewhere in rocks with broadly similar contents of immobile components. Closely associated assemblages of diverse mineralogy suggest that the XMn and XCO2, rather than the physical conditions of metamorphism, are the decisive factors in promoting the observed phase assemblages.

Keywords: magnesium • manganese • amphibole • tirodite • Sausar Group • India

Mineralogical Magazine; March 1988 v. 52; no. 364; p. 105-111; DOI: 10.1180/minmag.1988.052.364.09
© 1988, The Mineralogical Society
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