江西南部蔡坊盆地火山岩系层序与地质时代

Stratigraphical Sequence and Geochronology of the Volcanic Rock Series in Caifang Basin,South Jiangxi

江西中生代晚期火山岩系发育,形成了北部的峡江-广丰火山岩带和南部的三南-寻乌火山岩带。峡江-广丰火山岩带的火山岩系为一个火山旋回的产物,称为武夷群,并进一步划分为双峰岭、鹅湖岭和石溪组三个组。三南-寻乌火山岩带蔡坊盆地火山岩系也为一个火山旋回的产物,包括上、下两个岩性岩相单元:下单元以沉积岩为主夹火山岩,属火山喷发-沉积相,代表火山活动初始期的产物;上单元以火山岩为主,属火山喷发相,代表火山喷发期的产物。上单元顶部流纹岩SHRIMP锆石U-Pb年龄为(130.79±0.73)Ma,根据新的国际地层表,晚侏罗世与早白垩世之间的界线限定为(145.5±4)Ma,蔡坊盆地火山岩系的地质时代属早白垩世早期。通过岩性岩相对比,结合地层层位,蔡坊盆地火山岩系可归入峡江-广丰火山岩带的武夷群,其中下单元可归入双峰岭组、上单元可归入鹅湖岭组。蔡坊盆地火山岩系层序和地质时代的确定,可为三南-寻乌火山岩带火山系的研究提供参照标准。

The late Mesozoic volcanic rocks in Jiangxi constitute two volcanic belts：the northern is Xiajiang-Guangfeng volcanic belt,the volcanic rocks series belong to one volcano cycle and named Wuyi group which is subdivided into three formations（Shuangfengling formation,Ehuling formation and Shixi formation）; the southern is Sannan-Xunwu volcanic belt,the volcanic rocks series in Caifang basin which locates on Sannan-Xunwu volcanic belt also belong to only one volcano cycle.It can be subdivided into two lithology and lithofacies units（upper and lower）：the lower unit consists of sedimentary rocks and associated with a subordinate amount of volcanic rocks,it belongs to erupt-deposit facies which is the product of early volcanic stage; the upper unit is mostly composed of volcanic rocks,it belongs to erupt facies that is the volcanic eruption product. SHRIMP zircon U-Pb age of rhyolite？which locates at the top of the upper unit is （130.79±0.73） Ma. According to the new International Stratigraphic Chart,the boundary of Jurassic and Cretaceous is （145.4±4.0） Ma,so the age shows that the geologic period of Caifang volcanic rocks series is early Early Cretaceous epoch. On the basis of lithological correlation,lithofacies and stratigraphic horizon analysis,the volcanic rock series in Caifang basin fall under Wuyi group,and the lower unit could be incorporated into Shuangfengling formation,the upper unit could be incorporated into Ehuling formation.The subdivision of sequence and the determination of geochronology of the volcanic rock series in Caifang basin provide some references for the study of the late Mesozoic volcanic rocks series of the Sannan-Xunwu volcanic belt.