赣南龙南地区余田群玄武岩SHRIMP锆石U-Pb年龄及其地质意义

SHRIMP zircon U-Pb age of Yutian Group basalts in Longnan area of southern Jiangxi Province and its geological significance

江西南部余田群由双峰式火山岩组成,分布在近EW向展布的三南-寻乌断裂带的临江、东坑、白面石、菖蒲等火山盆地之中。SHRIMP锆石U-Pb定年结果显示：东坑盆地余田群最下部玄武岩的年龄为194.4Ma±2.0Ma,菖蒲盆地余田群最上部玄武岩的年龄为191.9Ma±2.2Ma。这2个年龄结果,一方面指示余田群的地质时代属早侏罗世早期,另一方面说明玄武岩和长英质火山岩的年龄一致,且与剖面层序吻合,属典型的双峰式火山岩组合。余田群双峰式火山岩地质时代的厘定,为江西南部早侏罗世早期处于拉张构造环境提供了年代学证据。同时,菖蒲盆地玄武岩中存在2571~2225Ma残留锆石或捕获锆石,为江西南部可能存在新太古代晚期—古元古代的基底提供了旁证。

The bimodal volcanic rocks of Yutian Group are located in Longnan,Baimianshi and Changpu basins along the EW-trending Sannan-Xunwu fault belt in southern Jiangxi Province.The zircon U-Pb dating yields the following results： the age of the basalt at the bottom of Yutian Group in Dongkeng basin is 194.4Ma±2.0Ma,whereas the age of the basalt at the top of Yutian Group in Changpu basin is 191.9Ma±2.2Ma.These dating results not only indicate that the age of Yutian Group belongs to the early Early Jurassic but also show that the basalt and the rhyolite have the same age,coincident with the stratigraphic sequence of the section and belonging to typical bimodal volcanic rocks.Age determinations of the bimodal volcanic rocks in Yutian Group provide the age evidence for the extension tectonic environment during the early Early Jurassic in southern Jiangxi Province.Meanwhile,the residual zircon（2571-2225Ma） exists in the basalt of Changpu basin,which also supplies the collateral evidence to the existence of the late Neo-Archean to Palaeo-Proterozoic basement in southern Jiangxi.