Located between the Yangtze massif and the Cathaysian block, the Jiangnan orogenic belt is composed of a series of Pre-Sinian low-grade meta-volcaniclastic rocks. Between the Sinian and the Pre-Sinian strata, there is a regional angular unconformity. Within the Pre-Sinian strata, there are several local unconformities, whose precise ages and tectonic affinities play a significant role in understanding the tectonic evolution of the Jiangnan orogen. Unfortunately, the tectonic models of the Jiangnan orogen based on data available fail to consider these unconformities and regard the Neoproterozoic period as a continuous tectonic evolution episode. In the Guizhou-Guangxi-Hunan border area, the unconformity within the Neo-Proterozoic Qingbaikou System defines the Wuling or Sibao Movement. A growing body of high-quality in situ zircon geochronologic data has demonstrated that the Jiangnan orogen is mainly composed of Neoproterozoic geological bodies rather than Middle Proterozoic ones, and this rules out the possibility that the Wuling/Sibao Movement is of Grenville episode. Thus, precise dating of the strata below this unconformity plays a fundamental role in tectonic reconstruction of the Jiangnan orogenic belt. The Fanjingshan area in eastern Guizhou Province lies in the southwest segment of the Jiangnan orogen, where strata with variable ages are well exposed with clear structural relationships, and hence this area is an ideal area for studying tectonic evolution of the Jiangnan orogen. The Pre-Nanhua strata in the Fanjingshan area are assigned to the Fanjingshan Group consisting of seven formations. From the bottom upwards, the grain size of the sediments decreases, suggesting a complete basin sedimentary sequence. The middle segment of the Fanjingshan Group consists of numerous volcanic rocks comprising basalt, tuff, and volcaniclastic rocks. This paper reports four in-situ zircon LA-ICP-MS U-Pb dating results to precisely constrain the sedimentation time of the Fanjingshan Group. CL images reveal that the fine-grained zircons from these samples have similar internal structures. Most zircons have well-preserved sector zoning, like those derived from a mafic rock, while the zoning of some zircons has been partially destroyed probably due to late stage fluid activity. In some zircon grains, zoning is totally absent. LA-ICP-MS U-Pb analyses indicate that all the zircons without zoning have ²⁰⁶Pb/²³⁸U ages older than 1.0 Ga; on the other hand, the zircons whose zoning has been destroyed yielded unusually low ²⁰⁶Pb/²³⁸U age value down to 50 Ma. Analyses of other grains yielded accordant ²⁰⁶Pb/²³⁸U ages, showing slightly different weighted mean ages of various samples. Two volcanic samples yielded weighted mean zircon U-Pb ages of 832 Ma (MSWD=6.5, the Tongchang Formation) and 851 Ma (MSWD=1.5, the Yujiagou Formation) respectively, whereas analyses of two fine volcaniclastic samples from the Yujiagou Formation yielded weighted mean detrital zircon U-Pb ages of 849 Ma(MSWD=8.9)and 845 Ma (MSWD=1.18), respectively. These new data, along with previously published data, demonstrate that the Fanjingshan Group was deposited during the period of 850～815 Ma, which precisely constrains the timing of the Wuling/Sibao Movement, provides critical data for geological correlation, and helps understand the possible tectonic evolution of the Jiangnan orogenic belt.