Neoproterozoic Metamorphism of the Douling Complex at the Northern margin of the Yangtze Craton: Petrology, Zircon U-Pb-Hf-O Isotopes and Tectonic Implications

Neoproterozoic evolution of the Yangtze Craton has long been debated for its relationship with the assembly and breakup of the Rodinia supercontinent. The Douling complex, which represents one of the oldest crystalline basements, records Neoproterozoic high grade metamorphism and may provide vital clues into a reliable Neoproterozoic configuration of the Yangtze Craton. We conducted an integrated study of petrography, zircon U-Pb age and Hf-O isotopic compositions for garnet amphibolite and gneiss in the Douling complex. Zircon grains from the gneiss mostly reveal core-rim structures. The cores display oscillatory zoning, high Th/U and 176Lu/177Hf ratios, slightly high 18O values (5.87 0.94), high trace element contents, as well as HREE enriched patterns, consistent with a magmatic origin. They yielded a weighted mean 206Pb/238U age of 868 9 Ma, which is taken as the protolith age of the gneiss. The cores have strongly positive Hf(t) values of 7.0-15.6 and young single-stage Hf model ages of ca. 1150-800 Ma, indicating that gneiss protolith was dominantly sourced from prompt reworking of Meso-Neoproterozoic juvenile crust. The zircon rims in gneiss and zircon crystals from two amphibolites exhibit weakly zoned patterns, relatively low Th/U and 176Lu/177Hf ratios, high 18O values, pronounced negative Eu anomalies and flat HREE profiles, suggesting they might form coeval with garnet and plagioclase under amphibolite-facies conditions. Metamorphic zircon in three samples yield a weighted mean age of 817 4 Ma, which is taken as the best estimated age for the peak stage of amphibolite-facies metamorphism. Phase equilibrium modeling and conventional geothermobarometry calculation define peak P-T conditions of 9-10 kbar and 670-690, and shape a clockwise P-T path. Considering the widespread contemporaneous low 18O magmatism in the northern Yangtze Craton, we propose that the Douling complex experienced tectonic transformation related to southwards oceanic subduction under the periphery of the Yangtze craton, and then rapidly uplifted in response to extensional collapse of the thickened orogen.