Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian western Alps)

Metabasites from the Austroalpine Mt Emilius Klippe preserve different stages of the evolution from pre-Alpine granulite- to Alpine eclogite-facies assemblages. The pre-Alpine protolith includes layered garnet I—clinopyroxene I—plagioclase—hornblende granulites and gabbros. The eclogitic paragenesis comprises garnet II—clinopyroxene II—epidote—Ca-amphibole ± chlorite. Eclogitic deformation is composite and involves two main episodes (D1A, D1B). D1A produces a layer-parallel foliation and incomplete transformation of granulites and is concluded by an episode of brittle deformation leading to the formation of eclogitic veins. D1B gives rise to widespread S—L-tectonites and mylonites. The eclogitic veins and surrounding haloes, enriched in fluids during veining, control the nucleation of D1B shear zones. Deformation is accomplished through ductile flow of reaction-weakened plagioclase and hornblende sites and cataclasis of garnet I and clinopyroxene I. Both meso- and microscale observations indicate that fluids play a primary role during eclogitization in both enhancing deformation and the kinetics of metamorphic reactions. Fluids influence plastic flow rates in matrix aggregates and promote fracturing of hard minerals. The extent of metamorphic re-equilibration is closely related to the density of the fluid pathways.