相山流纹英安斑岩锆石SHRIMP U-Pb年龄及地质意义

相山火山杂岩位于北东向深断裂(永丰-抚州断裂带)和近南北向断裂带(宜黄-宁都断裂带)的交汇处。前人对其年代学已有过较为详尽的报道,但其定年方法多数精度不高。本文在前人的基础上利用SHRIMP测定了相山火山杂岩第一亚旋回中的流纹英安斑岩的锆石U-Pb年龄,并获得了一组很好的206Pb/238U加权平均年龄为136.6±2.7Ma(MSWD=1.8)。结合前人报道的相山火山杂岩年龄、野外岩石组合以及对定年方法精度的对比,我们确认:相山火山杂岩中的流纹英安斑岩的形成时间应该为早白垩世。

SHRIMP Zicon U-Pb Dating and Its Geological Implication for the Xiangshan Porphyric Dacite-Rhyolitic

The Xiangshan volcanic complexes is located at the intersection of two fault systems:the first one,with northeast strike,is named the Yongfeng-Fuzhou fault and the other,with nearly north-south strike,is called the Yihuang-Ningdu fault.The volcanic eruption can be divided into two events.The lower part of the first event was dominated by volcanic sediments,with a 70 m thick interlayer of tuff lava,while the major component of the upper part was porphyritic dacite-rhyolite.Distinct from the first event,the second one was produced a rhyolite-tuff with volcanic sedimentary laminas at bottom and porphyroclastic rhyolite lavas on top.Previous reports have provided many dates for these strata,such as 140.3 Ma for the porphyroclastic rhyolite lavas in the second event and the 136.0 Ma for the porphyritic dacite-rhyolite in the first event.However there is some conflict due to the low accuracy of the data,for the porphyroclastic rhyolite lavas should be younger than the porphyritic dacite-rhyolite.In this paper,the SHRIMP zircon U-Pb dating technique has been applied to the porphyritic dacite-rhyolite in the first event of Xiangshan volcanic complexes,and we obtain an 206Pb/238U age 136.6±2.7 Ma(MSWD=1.8).Then we can confirm that the eruption time of the porphyritic dacite-rhyolite should be about 136.6 Ma.Combined with the former reported ages,it seems more reasonable that the actual ages of the porphyroclastic rhyolite lavas should be a little younger than that formerly reported.