江西相山铀矿田火山岩系地层学、年代学特征及地质意义

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

江西相山铀矿田位于赣东北乐安县和崇仁县边境,处于扬子古板块与华夏古板块之间的钦(州)—杭(州)结合带南侧罗霄褶皱带的北缘,属赣杭铀成矿带西南段。根据不整合面和岩石组合特征可将相山铀矿田赋矿火山岩系归入武夷群,根据岩性岩相特征和火山活动产物的规律性,可将武夷群下部紫红色砾岩、砂岩、粉砂岩、泥岩夹熔结凝灰岩、凝灰岩构成的喷发-沉积相组合归入双峰岭组、上部碎斑熔岩夹熔结凝灰岩构成的喷溢-侵出相组合归入鹅湖岭组,分别代表火山活动初始期和喷发期的产物。SHRIMP锆石U-Pb年龄表明,双峰岭组下部熔结凝灰岩年龄为(137.5±1.4)Ma(N=13,MSWD=0.73)、中部2个英安岩样品的年龄分别为(137.4±1.1)Ma(N=14,MSWD=1.3)和(137.4±1.4)Ma(N=12,MSWD=2.5)、上部晶屑凝灰岩的年龄为(136.4±1.1)Ma(N=14,MSWD=0.49),鹅湖岭组2个碎斑熔岩样品的年龄分别为(135.4±1.5)Ma(N=13,MSWD=1.3)和(135.2±1.2)Ma(N=12,MSWD=0.25),不仅说明相山铀矿田武夷群地质时代属早白垩世早期,而且年龄与地层的叠置关系吻合。划分的双峰岭组与鹅湖岭组界线不仅是岩性界面,而且是岩相界面,野外容易识别,解决了原打鼓顶组或原如意亭组划分存在的问题。结合相山铀矿田已有的高精度锆石U-Pb年龄数据,提出锆石U-Pb测年结果不能为相山铀矿田火山-侵入杂岩的时序厘定的提供依据。

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 zircon206*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 of136.4±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.