LOW DIFFERENTIAL STRESS DURING HIGH-PRESSURE METAMORPHISM - THE MICROSTRUCTURAL RECORD OF A METAPELITE FROM THE ECLOGITE ZONE, TAUERN WINDOW, EASTERN ALPS

Constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high-pre ssure metamorphic rocks, provided that this record has remained unchan ged during exhumation. The inclusion patterns in poikiloblastic garnet of a metapelite from the Eclogite Zone of the Tauern Window, Eastern Alps, reveal a foam microstructure of quartz controlled by grain bound ary free energy. The garnet blasts show concentric chemical zonation. Thermobarometry based on correlation between compositional domains in garnet, chloritoid and phengite coexisting with kyanite, rutile and qu artz shows that garnet growth commenced before peak P-T conditions clo se to 25 kbar and 600 degrees C had been reached and continued during the early stage of decompression. This implies that the microstructure of quartz, controlled by grain boundary free energy, persisted contin uously during the latest stage of burial to ca. 80 km depth and the ea rly stage of exhumation, involving detachment from the downgoing slab and release into the subduction channel. The foam structure of the ove rgrown quartz indicates that the differential stress was too low to dr ive dislocation creep during that period, when the rock crossed the in terface between the plates, with a high differential stress resulting in 'tectonic overpressure' or significant shear heating being ruled ou t. Outside the protective garnet the interfacial free energy controlle d quartz microstructure has been destroyed completely due to dynamic r ecrystallization during deformation by dislocation creep. This happene d while the crustal slice was exhumed to a depth of about 45 km. The p reservation of this synkinematic quartz microstructure indicates a tec tonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle-plastic transition. (C) 1997 Else vier Science B.V.