Metamorphic evolution of high-pressure and ultrahigh-temperature granulites from the Highland Complex, Sri Lanka

Sapphirine+quartz-bearing pelitic granulites, garnet-clinopyroxene-quartz-bearing mafic granulites and quartzo-feldspathic granulites with corundum-garnet-quartz constitute rare but important members of the Highland Complex in Sri Lanka. Peak metamorphic conditions from the Highland Complex generally have been considered to be up to ∼850-900 °C and ∼8 10 kbar. However, this study on the above mentioned rocks indicates that ultrahigh-temperature and high-pressure conditions (>1100 °C and ∼12 kbar) were attained during peak conditions. A metamorphic evolution of the ultrahigh-temperature metamorphic rocks was determined from careful analyses of shifts in divariant assemblages and reaction textures. This shows a clockwise P-T path from more high-pressure conditions (∼1000 °C and ∼17 kbar; stage 0 as part of the prograde metamorphic path) to lower-pressure and -temperature conditions (∼950 °C and ∼9 kbar; stage 4 as part of the retrograde path) through the peak metamorphic conditions (stage 1). Widespread lower pressure and temperature granulite-facies metamorphic rocks surround the ultrahigh-temperature granulites and are interpreted to have formed by the strong effect of the retrograde metamorphism and deformation. The widely reported Pan-African metamorphic ages derived from similar granulite-facies metamorphic rocks in the Gondwana fragments (Highland Complex in Sri Lanka, Lützow-Holm Complex in east Antarctica, etc.) may also be the result of retrograde metamorphism of ultrahigh-temperature metamorphic rocks. There remains a possibility that this early ultrahigh-temperature/high-pressure granulite-facies metamorphism in the Highland Complex, as well as that in the Lützow-Holm Complex, might pre-date Pan-African metamorphism.