Abstract
We report the occurrence of coexisting garnet+corundum in spinel- and corundum-bearing, garnet–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 (Grt1 and Grt2), sillimanite, plagioclase, alkali-feldspar, corundum, spinel and biotite. Grt1 encloses rare Ti-rich biotite and numerous rutile needles and apatite rods. Grt2 contains rare sillimanite and/or spinel inclusions. Corundum occurs in mutual contact with Grt2, partially embedded at the rim or as an inclusions in Grt2.Thermobarometry on inclusion phases in Grt1 indicates that during the prograde history pelitic granulites attained a P of 10.5–11kbar at T of ~850°C. Textural observations coupled with both pseudosections calculated in the NCKFMASHTMnO system and Ti-in-Garnet geothermobarometry suggest that peak metamorphism occurred at ultrahigh temperature (UHT) conditions of 950–975°C and pressures of 9–9.5kbar. Peak T was followed by a period of isobaric cooling that formed corundum and Grt2 at approx. 930°C along with exsolution of rutile needles and apatite rods in Grt1. Thermodynamic modelling confirms that corundum appears along an isobaric cooling path at about 920–930°C and 9–9.5kbar. 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.
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