庐山汉阳峰组变流纹岩锆石U-Pb同位素定年及其地质意义

Zircon U-Pb isotopes dating of Hanyangfeng Formation in Lushan area and its geological significance

江西庐山地区新元古代地层序列从老至新出露较为齐全,该区对于分析整个"江南造山带"的构造演化至关重要。汉阳峰组仅发育(或残存)于庐山汉阳峰,由陆相喷发的变斑流纹岩或少斑流纹岩为主。本研究组获得庐山地区汉阳峰组变流纹岩SHRIMP锆石U-Pb加权平均年龄为(838±4)Ma,MSWD=1.12;汉阳峰组LA-ICP-MS锆石UPb加权平均年龄为(852±4)Ma,MSWD=0.037。新获得的精确锆石年龄表明汉阳峰组不属于板溪期沉积地层,据此,笔者首次提出将汉阳峰组明确定位于武陵运动(820±Ma)构造转换面之下的地层。新的研究结果证实汉阳峰组与筲箕洼组属同期火山-沉积地层,对构造背景有重要的指示意义。汉阳峰组和筲箕洼组与星子岩群的层位关系已经发生倒置,可能预示庐山地区青白口纪后期有重要的构造运动。

The outcropped Neoproterozoic strata are relatively complete from old to young in Lushan area,and hence this area is significant for analyzing the tectonic evolution of the whole ＂Jiangnan Orogen＂.The Hanyangfeng Formation,which exists only in Hanyang peak of the Lushan Mountain,is mainly composed of terrestrial eruptional metacrystal or phenocryst-rare rhyolite.The authors obtained the high-precision SHRIMP zircon U-Pb weighted mean age （838±4 Ma,corresponding MSWD=1.12）of the metamorphic rhyolite of the Hanyangfeng Formation in the Lushan area,and also obtained LA-ICP-MS zircon U-Pb weighted mean age （852±4 Ma,corresponding MSWD=0.037）of this Formation.These new data indicate that the Hanyangfeng Formation does not belong to the Banxi period.The authors initially hold that the Hanyangfeng Formation ought to be located under the tectonic transformation of the Wuling movement （820± Ma）.The new research result confirms that the Hanyangfeng Formation is of the same volcanic-sedimentary strata as the Shaojiwa Formation,and therefore it may be a significant indicator to the tectonic setting.Actually,the sequence relationship of the Hangyangfeng Formation and the Shaojiwa Formation to the Xingzi Complex group shows upside-down phenomenon,suggesting that there probably existed a significant geological movement in the Lushan area during the late Qingbaikou period.