High-temperature metasomatism in ultramafic Granulite of Highland complex, Sri Lanka

Abstract

Concordant ultramafic rocks are exposed in lower crustal granulites in Sri Lanka. The ultramafic rocks at Rupaha, Sri Lanka were emplaced early and subjected to deformation and granulite facies metamorphism at 850oC at 9 kbar during the Pan-African tectono-thermal episode. The results thermometry of ultramafic rocks is consistent with geothermobarometric results obtained from the surrounding granulites. Fluid circulating in the deep crust resulted in the formation of phlogopite blackwalls. Structures, textures and the mineralogy of the blackwall suggest that the K-metasomatism took place contemporaneous to the granulite facies metamorphism. The metasomatic reactions were started by infiltration of K2O and SiO2 along the lithological contacts between ultramafic rocks and surrounding gneisses and diffusion of these elements between two wall rocks. Carbonation and hydration had occurred on these blackwall rocks at the upper level of the crust up on cooling together with surrounding rocks as evidenced 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.