High-temperature metasomatism and retrogression of granulite facies ultramafics from the Highland Complex of Sri Lanka: Field relationships, phase equilibria and fluid fluxes

Abstract

Concordant ultramafic rocks are exposed in the lower crustal granulites of Sri Lanka. The ultramafic rocks at Rupaha had emplaced early. They had been subjected to deformation and granulite facies metamorphism at 850oC at 9 kbar during the PanAfrican tectono-thermal episode. The results of thermometry of ultramafic rocks are consistent with those of geo-thermobarometry obtained from surrounding granulites. Fluids circulating in the deep crust had caused the formation of phlogopite blackwalls. Structure, texture and mineralogy of the blackwalls suggest that the K-metasomatism had taken place contemporaneous to the granulite facies metamorphism. The metasomatic reactions had started due to infiltration of K2O and SiO2 between ultramafic rocks and surrounding gneisses and diffusion of these elements between two wall rocks respectively. Carbonation and hydration had occurred in the blackwall rocks at the upper level of the crust on cooling together with surrounding rocks as indicated by textures of a partial retrogression to tremolite and dolomite, which formed through introduction of CaO and CO2. The stability of tremolite + dolomite at 730–640oC and 9 kbar condition records a 0.40 ≥ XCO2 ≥ 0.15 for the fluid phase.