江西南部余田群长英质火山岩SHRIMP锆石U-Pb年龄及其地质意义  

The SHRIMP Zircon U-Pb Dating of Felsic Volcanic Rocks and Its Geological Significance from Yutian Group in Southern Jiangxi

作　　者：冀春雨[1] 巫建华[1] 

机构地区：[1]东华理工大学地球科学与测绘工程学院,江西抚州344000

出　　处：《东华理工大学学报（自然科学版）》2010年第2期131-138,共8页Journal of East China University of Technology(Natural Science)

基　　金：中国核工业地质项目"中国中新生代铀成矿作用"

摘　　要：江西南部龙南、菖蒲盆地余田群"流纹岩-玄武岩"双峰式火山岩酸性端元的Rb-Sr等时线年龄为175～148Ma,不仅变化幅度较大,而且多与基性端元的Rb-Sr等时线年龄(179～173Ma)不匹配。笔者采用精度较高的SHIRMP锆石U-Pb定年法,获得菖蒲双峰式火山岩组合最下部的流纹岩和龙南盆地双峰式火山岩最上部的英安岩SHIRMP锆石U-Pb年龄分别为(195.2±2.8)Ma和(191±1.7)Ma,两者在误差范围内基本一致。这不仅说明两盆地的双峰式火山岩是同期岩浆活动的产物,而且它们是在喷发间隔很短的时间内形成的。根据国际地层表,菖蒲盆地和龙南盆地双峰式火山岩的地质时代属早侏罗世早期。SHRIMP锆石U-Pb年龄明显大于全岩Rb-Sr等时线年龄可能是全岩Rb-Sr等时线定年法存在误差造成的。江西南部双峰式火山岩195～191Ma的SHIRMP锆石年龄,说明江西南部在早侏罗世早期就进入了拉张构造环境;结合双峰式火山活动之后,长英质火山岩145～130Ma的SHIRMP锆石年龄的获得,说明江西南部在190～145Ma期间为长达45Ma的火山活动宁静期,这个现象难以用太平洋板块向欧亚板块俯冲的单一模式进行合理地解释。Past researches have showed that the Rb-Sr isochron ages of felsic end member for ＂rhyolite-basah＂ bimodal volcanic rocks of Yutian Group in the Changpu and Longnan Basin in Southern Jiangxi Province are 175 - 148 Ma, not only does its amplitude change more significantly, but it does not match with the Rb-Sr isochron ages （ 179 - 173 Ma） of basic end member. As a result, I choose a method of zircon U-Pb dating with a higher accuracy, to obtain the rhyolite in the bottom of bimodal volcanic rocks in the Changpu Basin and the dacite in the top of of bimodal volcanic rocks in the Longnan Basin, whose zircon SHIRMP U-Pb age are respectively （ 195.2 ± 2.8） Ma and （191 ± 1.7）Ma. What＇s more, they are both almost the same in the error limit. It shows that the bimodal volcanic rocks in these both two basins are the product of the same session of magma movement. Simultaneously, it explains they form in a flash during the eruption intervals. According to the the newest InternationalStratigraphic Chart （Gradsrein et al. ,2004）, in terms of geological age, the bimodal volcanic rocks in Changpu Basin and Longnan Basin, belonging to the early Early Jurassic. The zircon SHIRMP U-Pb age are distinctly older than the whole-rock Rb-Sr isochron age, it is probably because of the deviation of the dating method for the whole-rock Rb-Sr isochron age. The zircon SHIRMP U-Pb age of bimodal volcanic rocks are 191 - 195 Ma in Southern Jiangxi Province, which indicates that there had been an extensional environment. And after the bimodal volcanic activity, The zircon SHIRMP U-Pb age of felsic volcanic rocks are 145 - 130 Ma. Both of the ages shows a as long as 45 Ma quiet period between 190 Ma and 145 Ma. It is unreasonable possible to interpreted by the single pattern of pacific plate subducting to eurasian plate.

关 键 词：长英质火山岩 SHRIMP 锆石U-PB年龄 早侏罗世 江西南部 

分 类 号：P588.14[天文地球—岩石学]