How to creep and when? Deformation mechanisms at the eclogite type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria).

The presence of large volumes of eclogite in collision and subduction zones makes their formation and deformation highly relevant for the dynamics of convergent zones. There is however no consensus on the deformation behavior of eclogite. On the one hand, mylonitic eclogite shear zones showing evidence of dominant deformation by dislocation creep have frequently been reported. On the other hand, fluid supported formation and deformation has been recently suggested as a potential mechanism in eclogite whereby the main accommodating mechanism is dissolution-precipitation creep. This raises the question of the factors controlling the deformation behavior of eclogite.

In this contribution, we present microstructural, petrographical and chemical data from a series of eclogite samples derived from low Mg – high Ti gabbro collected at the eclogite type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria). The rocks are characterized by a pronounced foliation defined by the shape preferred orientation of the major minerals (omphacite, amphibole, epidote and garnet). Minor quartz is observed at dilation sites. Overall, grains show rather straight grain boundaries and a uniform extinction. These features are interpreted as evidence of diffusion and dissolution-precipitation dominated formation and strain accommodation. Thermodynamic forward modelling indicates that eclogitization occurred at around 2 GPa and 640–680°C and was supported by fluid. Locally, the eclogite fabric is crosscut by veins showing a similar paragenesis as the host eclogite. However, they are enriched in quartz and epidote, depleted in garnet and show overall a coarser grain size. Depending on their initial orientation, the veins were either reactivated as flanking structures or foliation sub-parallel shear zones. The reactivated veins are characterized by undulatory extinction, twinning and subgrain formation, all being indicative of dislocation creep. The identical paragenesis and similar mineral chemistry indicates that reactivation occurred at conditions close to those of eclogitization. The investigated samples therefore testify that eclogite can deform by two different mechanisms at similar pressure-temperature conditions. Our investigations document that dissolution-reprecipitation is bound to the process of eclogitization and low strain rate whereas post-eclogitization strain localization is accommodated by dislocation creep.