湘东北岳阳地区冷家溪群和板溪群凝灰岩SHRIMP 锆石U-Pb年龄——对武陵运动的制约

对冷家溪群及其上覆板溪群斑脱岩中的锆石进行研究,测得冷家溪群小木平组斑脱岩SHRIMP锆石U-Pb年龄(822Ma±10Ma)和上覆板溪群张家湾组斑脱岩锆石U-Pb年龄(802.6Ma±7.6Ma),结合“江南造山带”东部变质基底双桥山群和西南地区四堡群、下江群的SHRIMP锆石U-Pb年龄,将冷家溪群与其相应的江南古陆变质地层明确定位于新元古界。该年龄对重新界定“武陵运动”的时限和进行同期地层的区域对比、构造演化研究都有重要意义。冷家溪群与双桥山群、梵净山群、四堡群、双溪坞群一样,均为低变质绿片岩系,构成了江南古陆地区的变质基底。多年来冷家溪群一直划归中元古界,并且视为“武陵运动”的主体,其时代的定位将影响整个江南古陆变质基底的地层划分和对比,也将制约江南造山带的地质背景和成矿条件解疑。上述锆石U-Pb年龄不仅标示了湘东地区新元古代地层的时代,也为江南古陆中部变质基底提供了新的、精确的年代学数据。

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.