David Daniel McNamara , John Wheeler, Mark Pearce, David J. Prior

Eclogites are important components of subduction-collision zones, with the mineral omphacite acting as the supporting framework mineral that accommodates the majority of any accumulated strain. As such it is important to determine which deformation mechanisms operate in omphacite during and after its formation to understand the rheology of deforming subducted crust. Such information is key to determining important information on the conditions, dynamics, and kinematics of subduction-collision tectonic regions. Using a combination of microanalytical techniques on eclogitic LS-tectonites from the Zermatt-Saas unit of the Italian Alps, we explore the mechanisms which resulted in both an omphacite shape and lattice preferred orientation. Omphacite defines both foliation and lineation in these rocks and a strong S-type lattice preferred orientation. Scarcity of microstructures associated with dislocation creep and sharp asymmetrical chemical zonation in omphacite grains suggest lattice preferred orientation formation via predominantly diffusion creep. Modelling of the P-T conditions possible for observed mineralogy and mineral geochemistry, and textural relationships between omphacite and retrogressive minerals, place the action of diffusion creep, at the latest, by the onset of retrogression of these Zermatt-Saas eclogites. We propose a model of eclogite deformation that occurred initially via small amounts of dislocation creep which moved quickly into a dominant diffusion creep field, particularly as exhumation/retrogression of these eclogite rocks began. This result suggests that diffusion creep can dominate eclogite deformation at high P-T conditions in subduction zones.