Late Proterozoic magmatism and metamorphism recorded in gneisses from the Dabie high-pressure metamorphic zone, eastern China: evidence from zircon U–Pb geochronology

This study presents zircon ages and geochemical and Nd-isotopic data for metagranites and quartzites from the Susong high-pressure metamorphic zone in the Dabie–Sulu ultrahigh-pressure collisional belt. This belt belongs to the eastern part of the Qinling–Dabie orogenic belt that formed during Early Mesozoic collision of the North and South China Blocks. Two metagranites give zircon U–Pb model ages of 785±7 and 205±12 Ma, likely representing a Late Proterozoic magmatic event and an Early Mesozoic overprint. They have low initial εNd-values (−12.4 and −11.1 at 780 Ma), favouring a crustal origin. Flat heavy rare earth elements (REE) patterns (GdN/YbN∼1.2) probably reflect that melting took place at a shallow crustal section where heavy-REE-bearing mineral phases are unstable. All zircons of three quartzites yield young discordant U–Pb ages and define a discordia with U–Pb model ages of 784±6 and 213±3 Ma, identical to those of the metagranites. We assume that all detrital zircons had lost radiogenic Pb prior to the Early Mesozoic overprint, probably facilitated by fluid participation during a metamorphic event contemporaneous with the intrusion of the metagranites. This simultaneous metamorphic and magmatic event was probably related to a rift setting along the periphery of the Yangtze (South China) Block during Late Proterozoic.