Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka: Implications for granulite formation in Gondwanian deep crust.

Abstract

Patches, veins and oriented zones of “incipient charnockites” occurring in the Precambrian granulite terrane of Sri Lanka provide compelling evidences for fluid-controlled granulite genesis. Transformation from gneiss to granulite involves breakdown of biotite or amphibole to orthopyroxene, with the resultant coarse charnockitic assemblage testifying to increased reaction kinetics aided by the influx of fluids. Fluid inclusion studies across a typical gneiss-charnockite reaction front at Kurunegala reveal that carbon dioxide was the ambient fluid species during incipient charnockite formation, with a melting temperature close to that for pure CO2 and a density of 0.87g/cm3. Fluid evolution is traced from early pure carbonic through intermediate mixed carbonicaqueous to late aqueous regime. Visual decrepitation of carbonic inclusions in polished wafers indicate that the charnockite entrapped almost double the amount of CO2 as compared to the gneiss, indicating external addition of fluids which effected dehydration. Combined solid-fluid equilibria define a P-T path characterized by its convexity towards the temperature axis, suggesting an isothemal uplift history. The close similarity between the decompression-related metamorphic uplift paths for Sri Lanka, South India and Antarctica strenghthen the current discussions on the juxtaposition of these continents in the Gondwana reconstruction.