Petrology of Mn carbonate-silicate rocks from the Gangpur Group, India

Abstract

Metamorphosed Mn carbonate-silicate rocks with or without oxides (assemblage 1) and Mn silicate-oxide rocks with minor Mn carbonate (assemblage II) occur as conformable lenses within metapelites and metacherts of the Precambrian Gangpur Group, India. The petrology of the carbonate minerals: rhodochrosite, kutnahorite, and calcite that occur in these two assemblages is reported. Early stabilisation of spessartine, aegirine, quartz, and carbonates (in a wide solid solution range) was followed by pyroxmangite, tephroite, rhodonite through decarbonation reactions. Subsequently, jacobsite, hematite, braunite, hollandite and hausmannite have formed by decarbonation-oxidation processes during prograde metamorphism. Textural characteristics and chemical composition of constituent phases suggest that the mineral assemblages reflect a complex relationship between protolithic composition, variation of X-CO2 (< 0.2 to similar to 0.3) and oxygen fugacity. A variation of X-CO2 and f(O2) would imply internal buffering of pore fluids through mineral reactions that produced diverse assemblages in the carbonate bearing manganiferous rocks. A minor change in temperature (from around 400 to 450 degrees C) does not appear to have had any major influence on the formation of different mineral associations. (c) 2004 Elsevier Ltd. All rights reserved.