Shear-zone hosted graphite in southern Kerala, India: implications for CO2 infiltration

Highly crystalline, coarse graphite flakes fill mesosopic shear zones and pegmatitic veins which cut across the regional penetrative fabric of granulite-facies supracrustals in the Kerala Khondalite Belt (KKB) of southern India. Carbon stable isotope analysis yield δ¹³C values of −8.2 to −12.4% for the shear-hosted graphites and −10.1 to −15.1% for the pegmatite-hosted graphites. These values are distinctly from the lighted carbon enriched values obtained from the disseminated graphite in the host gneisses (up to −32.1%). From the mode of occurrence and carbon isotopic composition, the graphite disseminations in the KKB supracrustals can be linked to the process of graphitisation of organic material trapped within sediments during regional high-grade metamorphism. A mechanism for the formation of shear- and vein-hosted graphites is discussed. It involves the influx of CO₂-rich fluids from a possible juvenile igneous source along structural pathways and precipitation of graphite under low oxygen fugacity conditions. Where the fluid pathways intersect impervious lithologies, fluid ponding and copious precipitation of graphite can be noticed at the lithologic contact. The graphite precipitation is considered to be synchronous with, and complementary to, the formation of arrested “incipient charnockites” elsewhere in the KKB, where upper amphibolite-facies gneisses transform to anhydrous granulite-facies assemblages through the influx of CO₂-rich fluids.