Abstract
Constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high-pressure metamorphic rocks, provided that this record has remained unchanged 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 boundary 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 quartz shows that garnet growth commenced before peak P-T conditions close to 25 kbar and 600°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 continuously during the latest stage of burial to ca. 80 km depth and the early stage of exhumation, involving detachment from the downgoing slab and release into the subduction channel. The foam structure of the overgrown quartz indicates that the differential stress was too low to drive dislocation creep during that period, when the rock crossed the interface between the plates, with a high differential stress resulting in ‘tectonic overpressure’ or significant shear heating being ruled out. Outside the protective garnet the interfacial free energy controlled quartz microstructure has been destroyed completely due to dynamic recrystallization during deformation by dislocation creep. This happened while the crustal slice was exhumed to a depth of about 45 km. The preservation of this synkinematic quartz microstructure indicates a tectonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle-plastic transition.
Published Version
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