Comparison of calculated P-T pseudosections for a kyanite eclogite from the Tauern Window, Eastern Alps, Austria

Abstract

P-T pseudosections were calculated with the programs THERMOCALC, DOMINO and PERPLEX in the simplified NCFMASH system in order to constrain the metamorphic path of a kyanite eclogite. Variation of rock composition, thermodynamic data set and solid solution models results in partly distinct different P-T stability fields of mineral assemblages. The rock composition was determined by XRF analyses and by combining mineral modes from thin section measurements with microprobe analyses. With other parameters kept constant, compositional variations resulting from different parts of the specimen and from zonation of garnet affect the results. THERMOCALC pseudosections suggest that the assemblage omphacite + glaucophane + chlorite ± lawsonite/zoisite/clinozoisite ± paragonite is stable at relatively low temperatures. The garnet-in isograd is placed around 500°C and garnet growth continues up to the kyanite-in isograd at about 600°C. An extended temperature interval for garnet growth is also confirmed by Fe-Mg exchange temperatures of omphacite inclusions in garnet. A tentative metamorphic path is given with a maximum P-T position at about 635°C, 21.5 kbar in the assemblage garnet + omphacite + kyanite according to THERMOCALC and additional paragonite or zoisite is calculated by DOMINO or PERPLEX. The replacement of kyanite by paragonite during the retrograde stage is predicted by THERMOCALC but the calculated formation of glaucophane is not observed, probably due to unsufficient H 2 O in the rock. Comparison of calculated and observed modal amounts and of mineral compositions is only in part satisfactory, mainly due to uncertainities in thermodynamic data and solid solution models, above all for amphiboles and to the problematic definition of equilibrium bulk composition.