Metamorphic peak conditions of eclogites in the Tauern Window, Eastern Alps, Austria: Thermobarometry of the assemblage garnet + omphacite + phengite + kyanite + quartz

Metamorphic peak P-T conditions of five kyanite eclogites from the Tauern Window, Austria, are evaluated on the basis of recent calibrations of the assemblage garnet + omphacite + phengite + kyanite + quartz. Results are about 25 kbar, 630 °C according to the dataset of Holland and Powell [Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology 16, 309-343 (updated 2002)]. Mostly higher P-T values are calculated with the calibrations of Krogh Ravna and Terry [Krogh Ravna, E.J., Terry, M.P., 2004. Geothermobarometry of UHP and HP eclogites and schists-an evaluation of equilibria among garnet-clinopyroxene-kyanite-phengitecoesite/quartz. Journal of Metamorphic Geology 22, 579-592] and Brandelik and Massonne [Brandelik, A., Massonne, H.-J., 2004. PTGIBBS-an EXCEL Visual Basic program for computing and visualizing thermodynamic functions and equilibria of rock forming minerals. Computers and Geosciences 30, 909-923], in part already in the stability field of coesite. However, no indications for this phase are evident from the Tauern samples. The presence of talc is consistent with these P-T values and high H2O activities. In contrast, the stability limits of paragonite and zoisite are situated at lower pressure and suggest a later formation during the decompression stage. THERMOCALC pseudosections in the NCFMASH system are constructed with the incorporation of fractional crystallization of garnet. Calculated garnet zonations are in better agreement with the observed compositions at peak pressures of about 25 kbar than results at lower pressures. This is also consistent with values from thermobarometry obtained with the same program.