Plastic flow of coesite eclogite in a deep continent subduction regime: Microstructures, deformation mechanisms and rheologic implications

Abstract

Ultrahigh pressure (UHP) metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts. Despite the long distance transport of the UHP rocks in a deep subduction channel and the fast strain rate required for rapid exhumation, little evidence for UHP deformation is found in the exhumed UHP terrains so far identified. Some UHP rocks have even preserved primary igneous and volcanic structures. These observations have led to the postulation that the stress level is low (a few MPa) in the deep subduction zones. Deformation microstructures of some eclogite mylonites from a ductile shear complex in the Sulu UHP metamorphic belt, eastern China, provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the UHP rocks in a highly strain-localized shear zone at the UHP conditions. Omphacite, phengite and kyanite are the main strain-accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration. Coesite is either fractured, with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure. Garnet does not show optically discernable plastic deformation. P–T conditions for the deformation are estimated to be P≈3.3 GPa and T≈624 °C using garnet–omphacite–kyanite–phengite–coesite/quartz thermobarometry, suggesting that deeply subducted continental crust remains “cold” at the upper mantle depths. Large part of the deeply subducted crust remains relatively undeformed. Strain is strongly localized into narrow shear zones. Stress level in the deep subduction channel is extrapolated on the order of tens of MPa, much higher than it was previously thought.