Osumilite and spinel plus quartz from Sri lanka:: implications for UHT conditions and retrograde P-T path

We report here the occurrence of osumilite as fine isolated inclusions in garnet porphyroblasts from a granulite facies rock in the southwestern part of the Highland Complex in Sri Lanka. Garnet porphyroblasts in the matrix are partly rimmed by sillimanite-biotite-K-feldspar assemblage as well as cordierite-K-feldspar-quartz assemblage. Spinel coexisting with quartz is also observed in the same sample. The pressure-temperature condition must be above 950 degrees C at around 7 kbar pressure, on the basis of osumilite stability in KFMASH system. The cooling and decompression continued down to 850 degrees C, into the biotite-sillimanite-K-feldspar-quartz stability field. During decompression f(O2) must have increased leading to the stability of Zn-rich-spinel with quartz after cordierite. Our findings provide direct evidence for ultrahigh-temperature (UHT) metamorphic conditions in southwestern Highland Complex, and also suggest the existence of a possible UHT zone within the high-grade terrane of Sri Lanka.