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Melt/fluid evolution processes are important in determining the rheological behavior of lithosphere subducted to and exhumed from ultrahigh-pressure (UHP), controlling the flow of lithospheric material, the generation of melts from the upper mantle, and potentially contributing to magmatism and growth of the continental crust. In these circumstances, determining the controls on deformation mechanisms of deeply subducted eclogite is crucial to understanding exhumation dynamics of UHP terranes. However, few studies are reported on the multi-stage evolution of microstructures in natural eclogites exhumed along the same retrograde P-T-t path, particularly in the presence of supercritical fluid or melt. Recently, evidence of melt-fluid events within UHP eclogite has been reported from the Sulu belt, China. The eclogite was deformed by multiple stages of folding associated with two phases of melt/fluid evolution during exhumation: Dehydroxylation of nominally anhydrous minerals produced a supercritical fluid in the early stage of exhumation from UHP conditions, and then, omphacite-breakdown melting during the late stage of exhumation from HP conditions. Here, we present a microstructural analysis of UHP eclogite from different structural zones in single outcrops from Yangkou and Taohang, in the Sulu belt. These eclogites preserve assemblages formed at different metamorphic stages controlled by strain partitioning or located in same structural position but affected by different degrees of melt/fluid modification. Results show that deformation mechanisms switch from overall brittle to ductile with increased melt/fluid mobility. Omphacite fabrics change from S to L type, corresponding to progressive enhancement of strain associated with melt/fluid migration from early fold hinges, to fold limbs and then to strain-localized shear zones. We conclude that strain partitioning and inherited HO content of UHP rocks control fluid distribution, and influence subsequent microfabrics, deformation mechanisms, partial melting, and retrogression at different structural locations during exhumation. These variations can play critical roles in reducing strength of eclogite and thus contribute to rheological heterogeneities in convergent settings at UHP conditions, potentially enhancing exhumation.
Chen et al. (Mon,) studied this question.