Evolution of fluid pathways during eclogitization and their impact on formation and deformation of eclogite: A microstructural and petrological investigation at the type locality (Koralpe, Eastern Alps, Austria)

Abstract

Eclogites representing three strain stages from the type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria) have been analyzed with regard to their microstructural, petrological and mechanical evolution during the gabbro-eclogite transformation and subsequent deformation. Thermodynamic forward models suggest the presence of a H2O dominated fluid phase during eclogitization and following deformation. While all three eclogite types show the same mineral paragenesis composed of garnet, omphacitic clinopyroxene, quartz and a fine grained polyphase aggregate of kyanite, clinozoisite, quartz and retrograde plagioclase, we do observe a microstructural and mechanical evolution with strain.Eclogitization is controlled by dissolution of metastable gabbro and precipitation of the stable finer grained eclogitic mineral paragenesis. This induces a micro-porosity which allows fluids to migrate in an unchannelized way. Distributed eclogitization results in strain weakening, where both transformational and volumetric strain are accommodated by dissolution-precipitation creep. Subsequent deformation of eclogite results in the development of pronounced eclogitic foliation accompanied by grain coarsening and reduction of porosity. The latter results in a reduced efficiency of dissolution-reprecipitation possibly causing apparent strain hardening. Based on our observations we suggest a model for the rheological evolution during the transformation of a dry mafic rock into an eclogite where the combination of volumetric and transformational strain at the onset of eclogitization assisted by fluid induces rapid weakening followed by apparent hardening once eclogitization is completed and steady state deformation of the eclogite mineral paragenesis sets in.