Rare evidence for formation of garnet plus corundum during isobaric cooling of ultrahigh temperature metapelites: New insights for retrograde P-T trajectory of the Highland Complex, Sri Lanka

We report the occurrence of coexisting garnet + corundum in spinel- and corundum-bearing, gamet-sillimanite-biotite-graphite gneiss (pelitic granulites) from the Highland Complex (HC), Sri Lanka. In the investigated pelitic granulites, two domains such as quartz-saturated and quartz-undersaturated are distinguishable. The quartz-saturated domains consist of porphyroblastic garnet, quartz, plagioclase, alkali-feldspar and biotite flakes rimming garnet The quartz-undersaturated domains are constituted of two generations of garnet (Grt(1) and Grt(2)), sillimanite, plagioclase, alkali-feldspar, corundum, spinet and biotite. Grt(1) encloses rare Ti-rich biotite and numerous rutile needles and apatite rods. Grt(2) contains rare sillimanite and/or spinel inclusions. Corundum occurs in mutual contact with Grt(2), partially embedded at the rim or as an inclusions in Grt(2). Thermobarometry on inclusion phases in Grt(1) indicates that during the prograde history pelitic granulites attained a P of 10.5-11 kbar at T of similar to 850 degrees C. Textural observations coupled with both pseudosections calculated in the NCKFMASHTMn0 system and Ti-in-Garnet geothermobarometry suggest that peak metamorphism occurred at ultrahigh temperature (UHT) conditions of 950-975 degrees C and pressures of 9-9.5 kbar. Peak T was followed by a period of isobaric cooling that formed corundum and Grt(2) at approx. 930 degrees C along with exsolution of rutile needles and apatite rods in Grt(1). Thermodynamic modelling confirms that corundum appears along an isobaric cooling path at about 920-930 degrees C and 9-9.5 kbar. Therefore, the investigated granulites provide a rare example of post-peak crystallization of garnet + corundum along a retrograde metamorphic trajectory under UHT conditions. Thus, isobaric cooling at the base of the crust could be regarded as an alternative process to form coexisting garnet + corundum. (C) 2015 Elsevier B.V. All rights reserved.