Multistage graphite precipitation through protracted fluid flow in sheared metagranitoid, Digana, Sri Lanka: evidence from stable isotopes

Abstract

The Digana metagranitoid, situated within the Digana shear zone, a critical junction between the Highland and Wanni complexes of Sri Lanka, hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain. Here we discuss their occurrence, association and stable isotopes. The carbon isotopes of graphite reported in this study from the Sri Lankan high-grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other Precambrian terrains. The stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range −1.6‰ to −8‰, suggesting precipitation by reduction from CO 2-rich fluids. Intracrystalline microscale traverses made on graphite crystals indicate the δ 13C composition of the rims becoming progressively lighter in 13C, in metagranitoid hosted samples, with the variation reaching up to 1.5‰, consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth, and shows the preservation of core at higher temperatures. A highly enriched carbon isotopic composition (−0.6‰) with a rimward enrichment of 13C observed in migmatites, contrary to the values in metagranitoid, indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution/reprecipitation of graphite. The oxygen isotopic composition of a quartz–calcite vein show values in the range 18.5‰ to 20‰, typical for recycled crustal source. The tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits. Prolonged fluid flow in this region is envisaged in this study, which is related to the multistage deformation of the crustal scale shear zone.