Phyllonite Formation and Alteration of Gneisses in Shear Zones (Gleinalmkristallin, Eastern Alps/Austria)

The leucophyllite (quartz-muscovite-chlorite-phyllonites) deposit Weißkirchen is located within the Core-Complex of the "Gleinalmkristallin" which is part of the Austroalpine Crystalline Unit. Retrograde metamorphism and shearing responsible for the formation of the phyllonites took place during the Alpine (Upper Cretaceous) event. The age of the micas from the leucophyllites and from the unaltered host rocks is 79.6 ± 0.8 Ma. The positive correlation of the HREE, A1203, TiO₂, Zr and Nb versus Mg show that these elements are enriched together with Mg. On the other hand SiO₂, CaO, Sr and alkali elements decrease with advanced alteration. This source for the formation of the phyllonites may be of minor importance. During the alteration and the decomposition of the plagioclase Eu is removed from the system. Oxygen isotope mineral equilibria for the host rock paragenesis (quartz-muscovitegarnet) yielded temperatures in the range from 623 to 642 °C which is interpreted as the temperature of the Variscan metamorphism. The biotite of these gneisses has evidently been reset by later metamorphic events. The investigated phyllonites in the shear zone represent totally reequilibrated rocks under Early Alpine metamorphic conditions. Oxygen isotope analyses yielded concordant temperatures of 480°C for the formation of the peak metamorphic paragenesis muscovite, chlorite and rutile. Slightly lower temperatures (approx. 450°C) were obtained by the measured fractionations between phlogopite, plagioclase and quartz (late stage metamorphic assemblage). Calculation of the oxygen isotope composition of the coexisting fluid for Alpine peak metamorphism yielded a mean 0180-value of + 7.5‰.