Lithos, 19 (1986) 219-234 219 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria F.S. SPEAR 1 and G. FRANZ 2 Department of Geology, Rensselaer Polytechnic Institute, Troy, NY 12180 (U.S.A.) 2 Faehgebiet Petrologie, Technische Universitiit Berlin, Strasse des 17. Juni 135, D-I O00 Berlin 12 (Federal Republic of Germany) LITHOS Spear, F.S. and Franz, G., 1986. P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria. In: W.L. Griffin (Editor), Second International Eclogite Conference. Lithos, 19; 219-234. Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote -+ dolomite -+ Zn-staurolite. The estimated peak meta- morphic conditions are 19 -+ 2 kbar, 590 -+ 20°C. Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage re- suited in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage re- suited in the formation of sudoite (a di/trioctahedral Mg-A1layer silicate) and kaolinite. The early altera- tion is bracketed at conditions between 3 and 10 kbar, 450-550°C and the later alteration between 200 and 350°C, P <~ 3 kbar. The P-T path is characterized by maximum burial to approximately 19 kbar (60-70km) (at-~590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H20-rich fluids, with the composition of the fluid locally buffered by the mineral assem- blage. (Received November 20, 1985 ; accepted after revision March 11, 1986) 1. Introduction The Eclogite Zone (EZ) in the south-central Tau- ern Window, Austria, consists of a lithologically diverse sequence of calc-mica schists, mica schists, metaquartzites, siliceous dolomites, and diverse types of eclogites including amphibole-, epidote- and mica-bearing varieties. The stratigraphy within the EZ is continuous on a scale of tens of meters to kilometers (Raith et al., 1980). The interpretation of the tectonic evolution of the EZ is that it was sub- ducted as a coherent unit during an early phase of the Alpine orogeny, and was then thrust up into a shal- lower structural level (see, e.g., England and Holland, 1979; Holland, 1979; Franz and Spear, 1983; and ref- erences therein for a discussion of the general setting of the EZ and vicinity). The petrology of several different lithologies from the EZ has been described in the literature, with the ~mphasis on garpet-omphacite eclogites by Abra- ham et al. (1974), Miller (1974, 1977) and Raith et al. (1977), on vein assemblages in eclogites by Holland (1979), and on siliceous dolomites by Franz and Spear (1983). All authors conclude that the peak metamorphic temperatures experienced by the EZ were on the order of 500 ° to 600°C. Pressure estimates vary be- tween approximately 10 and 20 kbar, and there is 0024-4937/86/$03.50 © 1986 Elsevier Science Publishers B.V.