Qinling Mountain, as a very complex continental collision orogeny, was formed by multistage subduction-accretion processes, which resulted in the closure of different oceans and the collision of different continental blocks. The Mianlüe Ocean occurred in the southern Qinling Mountain and its closure time is still poorly constrained. In this paper, internal structure analysis, in situ trace element analysis and U-Pb dating were carried out for zircons of a hornblende biotite schist collected from the Foping area, South Qinling. Most of these zircons display section zoning in CL images, whereas others do not show section zoning or any clear oscillatory zoning, indicating typical internal structure of metamorphic zircon. The studied zircon grains are depleted in LREE, but enriched in HREE with positive Ce anomalies and negative Eu anomalies. This kind of REE patterns is similar to that of zircons formed by metamorphic recrystallization. In addition, they have uniform Lu-Hf isotopic compositions. Their 176Hf/177Hf and 176Lu/177Hf ratios vary from 0.282 436 to 0.282 570 and from 0.000 262 to 0.001 138, respectively. The zircons have εHf（t）values between -7 and -2 and two stage model ages（TDM2）at 1 707～1 409 Ma. The TDM2 ages are younger than the deposition age of the Changjiaoba Group, which is likely because the addition of fluids during metamorphism increased the 176Hf/177Hf ratios of zircons. Our LA-ICP-MS U-Pb dating of the metamorphic zircons yielded a weighted 206Pb/238U mean age of 220±1 Ma, which is interpreted as the metamorphic age of the hornblende biotite schist. This metamorphic age and the calculated zircon Ti temperatures（mean = ～716 ℃）are in a good agreement with the metamorphic age and temperature of the Foping thermal dome, which suggest that the hornblende biotite schist was formed in an extension environment with high temperatures. Because typical Thermal Dome always developed in an extension environment, our constraints on the metamorphic age of the Foping thermal dome indicate that the closure of the Mianlüe oceanic basin likely happened prior to the Late Triassic.
Yang Xiaoxiao Wang Yujing Li Zhenhua Wang Jingyu Zhu Xi Long Xiaoping. Zircon U-Pb dating and Lu-Hf isotopic study of hornblende biotite schist from the Foping area in South Qinling[J]. Chinese Journal of Geology, 2018, 53(3): 1100-1118.