Revisiting Ultrahigh Temperature Granulites of Sri Lanka: New Prograde and Retrograde Mineral Textures from the Highland Complex

Abstract

In order to investigate probable multiple metamorphic events including Ultrahigh Temperature (UHT) metamorphism experienced by Sri Lankan metamorphic rocks, we undertook extensive sampling of granulites covering the whole Highland Complex (HC) for comprehensive studies on petrology, geochemistry and geochronology. The studied UHT metamorphic rocks are sapphirine, spinel and sillimanite bearing garnet-orthopyroxene gneiss, sapphirine and spinel bearing garnet-orthopyroxene-sillimanite gneiss, sapphirine and spinel bearing garnet-sillimanite gneiss, spinel bearing garnetiferrous quartzo-feldspathic gneiss, corundum and spinel bearing garnet-sillimanite-graphite gneiss and spinel bearing garnet-sillimanite-cordierite-graphite gneiss. UHT minerals and mineral assemblages occurring in these rocks are sapphirine, orthopyroxene+sillimanite+quartz, spinel + quartz and garnet+orthopyroxene+cordierite. The presence of orthopyroxene+sillimanite+quartz, spinel +quartz assemblages could indicated the metamorphic temperatures >900°C. The occurrence of kyanite inside the garnet, and sillimanite as an abundant mineral in the matrix indicate the clockwise P-T trajectory. Occurrence of orthopyroxene + sillimanite + quartz, imply the peak metamorphic history at UHT conditions under relatively higher pressures. Decompression histories are inferred mainly by cordierite+orthopyroxene symplectites around garnet. Above mineral textures demonstrate clear evidence for UHT metamorphism of the studied rocks. Further studies on thermobarometry, geochemistry and geochronology are underway to elaborate spatial and temporal evolution of the UHT pelitic granulites of the Highland Complex, Sri Lanka.