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 epsilon(Nd)-values (-12.4 and -11.1 at 780 Ma), favouring a crustal origin. Flat heavy rare earth elements (REE) patterns (Gd-N/Yb-N similar to 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. (C) 2002 Elsevier Science B.V. All rights reserved.