Genesis of UHP Eclogite-Vein System and Metamorphic Fluid Evolution in Subduction Zones

The system of ultrahigh-pressure (UHP) metamorphic rocks and veins is a natural laboratory to understand the nature and behavior of metamorphic fluids in subduction zones. This paper presents a review of the studies on three suits of eclogite (amphibolite)-vein system from the Dabie UHP terrane in order to discuss the dissolution and crystallization processes of subduction-zone metamorphic fluids, variation in fluid oxygen fugacity (fO2), and fluid-assisted boron (B) transfer. The study of UHP eclogites and enclosed multiple veins indicates that UHP fluid transferred materials by the dissolution of various components. This solute-rich fluid then experienced a three-stage crystallization process, which produced omphacite-epidote vein, epidote-quartz vein, and kyanite-epidote-quartz vein. La and Cr contents and δEu values of vein epidote are critical geochemical indicators for assessing the precipitating sequence of veins. The investigation on an eclogite-amphibolite-vein system indicates that low-pressure fluids have much higher fO2 conditions than high-pressure (HP) and UHP fluids in continental subduction zones. Such high fO2 conditions also lead to the growth of some unusual minerals (e.g., low pressure retrograde rutile). The investigation on tourmaline-bearing eclogite-vein system indicates that metacarbonate is an important reservoir of isotopically heavy boron (B) in subducted continental crust, and the release of B of metacarbonate at convergent boundary exerts a significant influence on deep B cycling. The studies above provide important insights into the fluid evolution and material cycling in subduction zones. © 2019, Editorial Department of Earth Science. All right reserved.