Zircon SHRIMP U-Pb dating of the tuff bed of Lengjiaxi and Banxi groups, northeastern Hunan: constraints on the Wuling Movement

Zircon dating of the tuff bed in Lengjiaxi Group of Hunan Province was conducted in this study. In combination with the SHRIMP U-Pb dating of the tuff bed in Sibao and Shuangqiaoshan groups, the Lengjiaxi Group is considered to be Neoproterozoic in age. The isotopic data obtained are very important in redefining the age of the Lengjiaxi Group and in regional correlation of strata of the corresponding period as well as the study of tectonic evolution. Rocks of the Lengjiaxi Group (as well as the Fanjingshan, Sibao and Shuangqiaoshan groups) comprising epimetamorphic rocks are well developed in the Jiangnan Orogen and have formed the metamorphosed basement of the Jiangnan old land. The Lengjiaxi Group was formerly assigned to Mesoproterozoic and was regarded as the main part of the Wuling Movement. Such dating has adversely affected the stratatigraphic subdivision and correlation of the metamorphosed rocks in the whole Jiangnan old land and restricted geologists in their study of geological background and ore-forming conditions of the Jiangnan Orogen. During the study of bentonite in the Lengjiaxi Group, the authors accurately dated the zircon at the top of the group at 822 ±10 Ma and that in the Zhangjiawan Formation of the Banxi Group at 802.6 ±7.6 Ma. These data have resolved the problem of the Neoproterozoic age and, what is more, provided a new marker for the Jiangnan Orogen.