Zircon U-Pb isotopes dating of Hanyangfeng Formation in Lushan area and its geological significance

The outcropped Neoproterozoic strata are relatively complete from old to young in Lushan area, and hence this area is significant for analyzing the tectonic evolution of the whole "Jiangnan Orogen". The Hanyangfeng Formation, which exists only in Hanyang peak of the Lushan Mountain, is mainly composed of terrestrial eruptional metacrystal or phenocryst- rare rhyolite. The authors obtained the high- precision SHRIMP zircon U- Pb weighted mean age（838 ± 4 Ma, corresponding MSWD=1.12）of the metamorphic rhyolite of the Hanyangfeng Formation in the Lushan area, and also obtained LA- ICP- MS zircon U- Pb weighted mean age（852 ± 4 Ma, corresponding MSWD=0.037）of this Formation. These new data indicate that the Hanyangfeng Formation does not belong to the Banxi period. The authors initially hold that the Hanyangfeng Formation ought to be located under the tectonic transformation of the Wuling movement (820± Ma). The new research result confirms that the Hanyangfeng Formation is of the same volcanic- sedimentary strata as the Shaojiwa Formation, and therefore it may be a significant indicator to the tectonic setting. Actually, the sequence relationship of the Hangyangfeng Formation and the Shaojiwa Formation to the Xingzi Complex group shows upside- down phenomenon, suggesting that there probably existed a significant geological movement in the Lushan area during the late Qingbaikou period.