Strain localization at eclogite-facies conditions induced by syn-metamorphic strength inversion

In convergent settings, continental and/or oceanic mafic rocks are subducted to great depths where they experience high pressures and temperatures and transform to eclogite. Accompanied mineral transformations subsequently result in mechanical changes and in density variations. In the last decades, numerous studies targeted eclogite and aimed at quantifying its mechanical behavior as well as characterizing strain weakening processes, yet the mechanisms driving strain localization remain unclear. We combine field, microstructural, petrographic and geochemical data to investigate the origin of strain localization at eclogite-facies conditions. Our study examines a shear zone containing two distinct eclogite-facies assemblages. The host-rock eclogite is composed of clinopyroxene, garnet, zoisite, amphibole, quartz, kyanite, and rutile, within which sigmoidal enclaves are enriched in zoisite, hornblende, and garnet. Protolith assemblage calculations suggests these enclaves originated as plagioclase-rich gabbro cumulates and were initially weaker than the surrounding gabbro. However, their sigmoidal geometry within ultramylonitic eclogite implies they were stronger during shear zone evolution. The ultramylonitic eclogite shows a fabric dominated by euhedral clinopyroxene, with grain boundary melt traces and junctions indicating deformation via melt-assisted grain boundary sliding. In contrast, enclave microstructures are dominated by elongated zoisite and amphibole aggregates that deform through dislocation creep and diffusion-mediated grain growth. These findings indicate a strength inversion during high-pressure metamorphism where initially weaker enclaves became stronger, concentrating stress at lithological boundaries and thus promoting strain localization in the weaker eclogitic matrix. Our results underscore the significance of compositional heterogeneities on strain localization at high pressures.