Melt-Supported Strain Localization at Eclogite-Facies Conditions Triggered by Syn-Metamorphic Strength Inversion

Abstract

In convergent high-pressure settings dominated by dry mafic rocks, strain has been proposed to be predominantly localized in eclogite. However, the processes initiating strain localization, accommodating strain and rheological consequences remain to be identified. We combine field, microstructural, petrographic and geochemical data to investigate the origin of strain localization under eclogite-facies conditions. The studied shear zone bears rocks with two eclogite-facies mineral assemblages: The host-rock eclogite contains clinopyroxene, garnet, zoisite, amphibole, quartz, kyanite and rutile, in which sigmoidal enclaves dominated by zoisite, hornblende and garnet occur. Protolith assemblage calculations suggest that enclaves have originally been plagioclase-rich cumulates within a gabbro. Strength estimates indicate that the enclaves were initially less competent than the gabbro. However, the sigmoidal shape of the enclaves surrounded by ultramylonitic eclogite suggests that the enclaves were stronger during shear zone development. Microstructural investigations of the ultramylonitic eclogite reveal a fabric dominated by euhedral clinopyroxene. Triple- and quadruple-junctions and melt traces at grain boundaries suggest that eclogite is dominantly deformed by melt-supported grain boundary sliding. On the other hand, the microstructure of the lenses is dominated by elongated zoisite and sigmoidal amphibole aggregates deforming by combined dislocation creep and diffusion-mediated grain growth. Our findings demonstrate that high-pressure metamorphism led to a strength inversion, with lenses initially weaker than their host becoming stronger during eclogite-facies metamorphism. Strength-inversion initiated stress concentration at the lithological contact and subsequent strain localization in the weaker eclogitic mineral assemblage. Our results highlight the critical role of compositional heterogeneities on strain localization at high pressures.