Stratigraphy and geochronology of the volcanic rocks in the Xiangshan uranium orefield, Jiangxi Province and its geological implications

The Xiangshan uranium orefield is located in Le'an-Chongren border area, northeast Jiangxi Province. Its tectonic position lies on the northern margin of Luoxiao folding belt, southern side of the Qingzhou-Hangzhou suture zone between the Yangtze paleo-plate and Cathaysia paleo-plate, belonging to the southwest section of the Ganhang uranium ore belt. According to unconformity surface and features of rock association, the volcanic rocks in the Xiangshan uranium orefield should belong to the Wuyi Group. According to features of lithology and lithofacies and volcanic activity regularity, Wuyi Group can be subdivided into the Shuangfengling Formation which includes lower purple conglomerate, sandstone, siltstone, mudstone and tuff that constitute eruption-sedimentary facies and the Ehuling Formation which is composed of upper porphyroclastic lava intercalated with ignimbrite combination of outpouring-flushout facies. These two formations respectively represent the initial stage of volcanic activity and the eruption period. The SHRIMP zircon ^206*Pb/²³⁸U age estimated for the welded tuff in the bottom part of the Shuangfeng Formation is (137.5±1.4)Ma (N=13, MSWD=0.73), the dacites in middle part of the Shuangfeng Formation have the ages of (137.4±1.1)Ma (N=14, MSWD=1.3) and (137.4±1.4)Ma (N=12, MSWD=2.5), the crystal tuff in the upper part of the Shuangfeng Formation has the age of 136.4±1.1Ma (N=14, MSWD=0.49), the porphyroclastic lava of the Ehuling Formation has the ages of (135.4±1.5)Ma (N=13, MSWD=1.3) and (135.2±1.2)Ma (N=12, MSWD=0.25), which not only indicate an early Early Cretaceous age but also are in accord with stratigraphic sequence. The boundary between the Shuangfeng Formation and the Ehuling Formation is not only an interface of lithology but also an interface of lithofacies The boundary can be recognized easily in the field, and the problem concerning the division of the original Daguding Formation or the original Ruyiting Formation is thus solved. In combination with existing zircon U-Pb age of the volcanic-intrusive complex in the Xiangshan uranium orefield, the authors hold that zircon U-Pb age of the volcanic-intrusive complex cannot provide the evidence for determination of the time sequence of the volcanic-intrusive complex in the Xiangshan uranium orefield.