High-pressure metamorphism of phengite gneisses in the Lower Austro-Alpinenappe unit, Eastern Alps: Mineral equilibria, P-T parameters, and age

Mineral assemblages and the composition of minerals were studied in ortho- and paragneisses of the Lower Austro-Alpine unit (Wechsel and Grobgneis com plexes) and the overlying Permian-Triassic metasandstone in the Eastern Alp s. The mineral assemblages of these rocks and the compositions of minerals coexisting in them (muscovite-phengite, chlorite, garnet, biotite, tourmali ne, albite, and potassium feldspar) are similar and point to metamorphic co nditions of the garnet subfacies of the greenschist facies with the Grt + C hl + Phn + Qtz critical assemblage, T = 470-500 degrees C (Grt-Chl and Grt- Bt thermometers), P similar to 10 kbar (Phn-Bt-Kfs-Qtz barometer). The fact that the zoning of the garnet and tourmaline is always prograde and the ro cks bear no higher temperature minerals points to the prograde character of metamorphism. This is at variance with preexisting concepts of the retrogr ade character of metamorphism in the Wechsel and Grobgneis complexes and th e occurrence of an older stage of amphibolite metamorphism in them. The U-P b zircon age of the orthogneisses (metagranites) from the Wechsel Complex, which are metamorphosed to the same grade as the host rocks, indicates that the magmatic crystallization (protolith) age of the parent granite is appr oximately 603 Ma (Cadomian cycle), and the age of metamorphism under garnet subfacies conditions is similar to 109 Ma (Early Alpine cycle). This expla ins the similar metamorphic parameters and mineral equilibria in rocks of t he Wechsel and Grobgneis complexes and those of the Permian-Triassic cover. Given the general prograde character of the Early Alpine metamorphism, the temperature of the possible pre-alpine (603-109 Ma) metamorphic events in the Wechsel and Grobgneis complexes could not be higher than that of the ga rnet subfacies.