New constraints on the P–T path of HT/UHT metapelites from the Highland Complex of Sri Lanka

We report here rare evidence for the early prograde P–T evolution of garnet–sillimanite–graphite gneiss (khondalite) from the central Highland Complex, Sri Lanka. Four types of garnet porphyroblasts (Grt1, Grt2, Grt3 and Grt4) are observed in the rock with specific types of inclusion features. Only Grt3 shows evidence for non-coaxial strain. Combining the information shows a sequence of main inclusion phases, from old to young: oriented quartz inclusions at core, staurolite and prismatic sillimanite at mantle, kyanite and kyanite pseudomorph, and biotite at rim in Grt1; fibrolitic sillimanite pseudomorphing kyanite ± corundum, kyanite, and spinel + sillimanite after garnet + corundum in Grt2; biotite, sillimanite, quartz ± spinel in Grt3; and ilmenite, rulite, quartz and sillimanite in Grt4.;
The pre-melting, original rock composition was calculated through stepwise re-integration of melt into the residual, XRF based composition, allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections. The earliest recognizable stage occurred in the sillimanite field at around 575 °C at 4.5 kbar. Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P–T trajectory with an average dP/dT of ∼30 bar/°C in the kyanite field, up to ∼660 °C at 6.5 kbar, before crossing the wet-solidus at around 675 °C at 7.5 kbar. The highest pressure occurred at P > 10 kbar and T around 780 °C before prograde decompression associated with further heating. At 825 °C and 10.5 kbar, the rock re-entered into the sillimanite field. The temperature peaked at 900 °C at ca. 9–9.5 kbar. Subsequent near-isobaric cooling led to the growth of Grt4 and rutile at T ∼880 °C. Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T < 400 °C, which probably occurred after uplift to upper crustal levels. U–Pb dating of zircons by LA-ICPMS of the khondalite yielded two concordant 206Pb/238U age groups with mean values of 542 ± 2 Ma (MSWD = 0.24, Th/U = 0.01–0.03) and 514 ± 3 Ma (MSWD = 0.50, Th/U = 0.01–0.05) interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling.