Petrography and stable isotope geochemistry of graphite bearing coarse grained calcitic marble: a case study from parts of eastern ghats province

Abstract

The calcitic marble rock in Araku vally of Eastern Ghats province is poly-metamorphosed and poly-deformed associated with metapelites have suffered granulite facies metamorphism. The stable assemblage of forsterite + spinel + calcite represent peak metamorphic condition. Graphite are present mainly on white coloured band of cakcitic marble rock. Graphite is present at the boundary of calcite, apatite, biotite and along the fracture of olivine and spinel. Textural study indictes formation of graphite after the formation of calcite and silicates. Graphite grains show deformational structure. Carbon isotope thermometry has been applied to coexisting calcite and graphite in calcitic marble rock. Δ13C (Cc-Gr) is useful to estimate the temperature of formation of graphite from mathematical formula. Obtained temperature from the formula ranges between 855⁰C to 952⁰C. Carbon isotope ratio of a fluid deposited graphite is an important tool for identify the source of carbon (organic matter, mantle, or carbonates). δ13C value for our sample ranges betwenn(-0.7—1.2) which indicate an inorganic carbon source. δ13C and δ18O value of calcite is also an important tool to indicate the source of carbon. Precipitation on graphite can occure from a CO2 bearing fluid form due to decarbonation reaction at the prodrade path. From this fluid graphite can be precipited by cooling in retrograde path under low fO2 condition.