Deformation Microstructures and Seismic Properties of UHP Eclogites: Response to Strain Localization, P–T Path and Fluid/Melt Interaction Within a Paleo-Subduction Channel

Abstract

Providing spatio-temporal constraints on what influences the rheology of deeply subducted continental crust during subduction–exhumation remains elusive but crucial for understanding the exhumation dynamics of ultrahigh pressure (UHP) terranes. Here, we report results of a systematic study of microstructures, crystallographic preferred orientations (CPOs) and seismic properties of four UHP–HP eclogites formed along a common P–T path from Yangkou Bay, Sulu belt, China. The eclogites have different bulk compositions and record heterogeneous strain patterns. Peak metamorphic conditions (800°C–900°C and >5.5 GPa) were retrieved from early F1 isoclinal fold hinges. Subsequent overprinting by F2 tight folds occurred during the transition to quartz-eclogite facies. Localized shear zones exhibit amphibolite-facies retrogression, indicative of enhanced fluid activity. Omphacite exhibits crystal plasticity, while garnet displays a brittle–plastic transition during exhumation. A change from S- to L-type CPO in omphacite was controlled by folding geometry during subduction–exhumation. Strain localization controlled intergranular fluid connectivity and redistribution, correlating with increasing strain from F1 folds to localized shear zones. This process led to progressive dynamic recrystallization, and changes in deformation mechanisms and seismic properties. Dynamic recrystallization resulted in significant grain refinement, thereby triggering diffusion creep assisted grain boundary sliding in the presence of fluid. Seismic anisotropy is linked to the omphacite fabric and the presence of phengite, with modal phengite as the primary determinant in UHP–HP eclogites. Fluid migration controlled by strain localization led to heterogeneous weakening of eclogite, which enabled exhumation of tectonic slices of UHP crustal rocks from mantle depths.