Low-T eclogite in the Dabie terrane of China: petrological and isotopic constraints on fluid activity and radiometric dating

摘要: While extensive studies have demonstrated fluid release during subduction of oceanic crust, little attention has been paid to activity and exhumation continental crust. Abundant occurrence quartz veins within eclogites in the Dabie-Sulu orogenic belt China provides us with an opportunity study origin role vein-forming fluids respect heat mass transfer ultrahigh pressure (UHP) metamorphism its relevant processes. This focuses on kyanite-quartz vein that occurs as polycrystalline aggregates low-T eclogite Dabie terrane, which are interpreted pseudomorphs after former porphyroblasts lawsonite. Coesite were found for first time garnet, resulting a revised estimate peak P–T conditions at 670°C 3.3 GPa thus upgrading high-P unit UHP unit. On basis relationship between calculated path metamorphic reactions well absence foliation texture, undulose extinction quartzes vein, we conclude lawsonite breakdown into kyanite–quartz–zoisite assemblage took place onset subsequent pressure. Retrograde caused O H isotope disequilibria some minerals, but retrograde was internally buffered stable compositions. Zircon U–Pb dating whole-rock Nd–Sr analyses indicate protolith is paleoceanic basalt derived from depleted mantle by magmatism about 1.8 1.9 Ga experienced hydrothermal alteration surface waters. The altered underwent Triassic zircon growth/overgrowth not only 242±3 Ma prior also 222±4 Ma decompression dehydration hydroxyl exsolution low-T/UHP eclogite. Consistent ages 236.1±4.2 Ma 230±7 Ma obtained mineral Sm–Nd Rb–Sr isochron dating, respectively, indicating attainment preservation Nd Sr equilibria eclogite-facies metamorphism. Ar–Ar paragonite gave consistent plateau 241.3±3.1 Ma 245.5±9.8 Ma, date crystallization prograde timing constrained sometime 236.1±4.2 Ma. Thus termination age may be different slices deep-subducted slab.