SHRIMP zircon U-Pb dating of tuff in Fanjingshan Group and Xiajiang Group from Guizhou and Hunan Provinces and its stratigraphic implications

Zircon dating (840Ma±5Ma) of tuff bedding is reported for the first time for the sixth member of the Huixiangping Formation in the Fanjingshan Group. The SHRIMP U-Pb dating of detrital zircon of the Furongba Formation is combined with tuff zircon dating (813.5Ma±9.6Ma) of the Xinzhai Formation in Xiajia Groups in Guizhou and Hunan Provinces, and also with the dating of the Sibao and Danzhou Groups in Guangxi. The tuff zircon age of the Huixiangping Formation in Fanjingshan Group is the same as the age of the Yuxi Formation of the Sibao Group, which intruded into early basic dykes and lava pillow in both groups, and developed late granite. It is very important to conduct the accurate dating of the events for the tectonic evolution in the southwest part of the Jiangnan orogenic belt. According to zircon U-Pb dating of lower metamorphosed rocks in the Jiangnan orogenic belt, there exist certain time differences for sedimentation between Fangjingshan Group, Sibao Group, Lengjiaxi Group, Shuangqiaoshan Group, Cangxi Rock Group, Yifeng Rock Group, Zhangcun Group and Shuangxiwu Group. Even the overlying strata have different ages, such as the Jialu Formation (814Ma) of the Xiajiang Group in Guizhou, the Hetong Formation (802Ma) of the Danzhou Group in Guahgxi, the Zhagnjiawan Formation (802Ma) of the Benxi Group in Hunan, the Dengjia Formation (766Ma) of the Dengshan Group in Jiangxi, and the Luojiamen Formation (791Ma) of the Heshangzheng Group in Zhejiang; there exists an angler unconformity between them, suggesting the existence of obvious orogenic event between the two tectonic horizons. The authors thus consider that the obtaining of more qualified chronostratigraphic U-Pb data will considerably influence the stratigraphic subdivision and correlation of the metamorphosed rocks in whole Jiangnan continent and restrict the geological background and metallogenic conditions of the Jiangnan orogen.