Low-grade metamorphic rocks are distributed continuously along the northern margin of the northeast margin of Yangtze block, and they are thought to be scraped off from the Yangtze plate during the Triassic continental subduction. Their protoliths were dominated by Mid-Neoproterozoic igneous and sedimentary rocks, which are important to understand the Neoproterozoic evolution of the northeast margin of Yangtze plate. The low-grade metamorphic rocks, namely Zhangbaling Group, were also identified in the northern Zhangbaling Tan-Lu fault zone located between the Dabie and Sulu Orogens. The Zhangbaling Group can be divided into the Xileng and Beijiangjun Formations, which were predominately composed of meta-volcanic and meta-clastic rocks, respectively. However, the formation age of the Zhangbaling Group and the relationship between the Xileng and Beijiangjun Formations are still controversial. To address this, a combined study of field investigation and MC-LA-ICP-MS dating were performed on the low-grade metamorphic rocks in the Zhangbaling Group. Zircon U-Pb datings gave a protolith age of 723±7 Ma for the quartz keratophyre from the top layer of the Xileng Formation, and yielded detrital zircon ages of 916～614 Ma with a cluster peaking at ca. 730 Ma for the meta-siltstone from the bottom of Beijiangjun Formation. The age distribution of the detrital zircons was identical with those of Neoproterozoic igneous rocks in Xileng Formation and Feidong complex, implying that the provenance of the Beijiangjun Formation was derived from the Xileng Formation and Feidong complex. The youngest detrital zircon age of 677±12 Ma provides a lower limit for the deposition age, and the geological occurrence of overlying Sinian system provides an upper limit of 635 Ma for the deposition age. This paper considers that the sedimentary age limit of the Beijiangjun Formation is 677～635 Ma, which was inferred that the Beijiangjun Formation covered on the Xileng Formation during late-stage of the Mid-Neoproterozoic. The comparative study shows that the Zhangbaling Group may be an important part of the volcanic-sedimentary rocks of the rift valley in the northeast margin of the Neoproterozoic.
Cai Qianru Niu Manlan Wu Qi Yuan Xiaoyu Wang Ting Li Xiucai Sun Yi. Revisiting the formation age of the Zhangbaling Group in the southern segment of the Tan-Lu fault zone[J]. Chinese Journal of Geology, 2019, 54(3): 781-795.