扬子北缘西乡群大石沟组和三郎铺组凝灰岩锆石U-Pb年龄及其地质意义  

Zircon U-Pb Ages for Tuffs from the Dashigou and Sanlangpu Formations of the Xixiang Group in the Northern Margin of Yangtze Block and Their Geological Significance

作　　者：邓奇[1,2] 王剑[1] 汪正江[1] 江新胜[1] 杜秋定[1] 伍皓[1,2] 杨菲[1] 崔晓庄[1] 

机构地区：[1]中国地质调查局成都地质调查中心,成都610081 [2]中国地质科学院研究生院,北京100037

出　　处：《吉林大学学报（地球科学版）》2013年第3期797-808,819,共13页Journal of Jilin University：Earth Science Edition

基　　金：国家自然科学基金重点项目(41030315);国家自然科学基金面上项目(41072088);中国地质调查局计划项目(1212011121111;1212011121109)

摘　　要：对采自扬子北缘西乡群大石沟组和三郎铺组的凝灰岩进行了LA-ICP-MS锆石U-Pb定年。结果表明,大石沟组和三郎铺组的凝灰岩年龄分别为(789.0±4.4)Ma(MSWD=0.66)和(760.4±4.5)Ma(MSWD=0.43)。根据已有的资料分析,西乡群的底界年龄约为820 Ma,区域上大体可与板溪群及其相当地层进行对比,为华南新元古代裂谷盆地的早期充填,其与下伏三花石群分别代表了上下2个不同的构造-沉积旋回。结合前人研究结果,华南新元古代的岩浆活动初步显示有三幕的特征,它们的峰值年龄分别约为825Ma、795Ma和760Ma,其中以前两期最为明显,分别标志着裂谷盆地的开启和全面打开。笔者报道的大石沟组和三郎铺组凝灰岩分别代表了795Ma和760Ma两期幕式岩浆活动,它们不仅是华南新元古代幕式岩浆活动的一部分,也与华南新元古代裂谷盆地的形成密切相关,具有不可忽视的地质意义。We present the U-Pb ages of the tuff zircons from the Dashigou and Sanlangpu Formations of the Xixiang Group at the northern margin of the Yangtze block,using the LA-ICP-MS dating analysis.Weighted mean ages are（789.0±4.4） Ma（MSWD=0.66） and（760.4±4.5） Ma（MSWD=0.43） respectively.According to existing data,the basal boundary age of the Xixiang Group should be ~820 Ma.Moreover,the Xixiang Group can be roughly compared with the Banxi Group and relevant strata in the regional scale,representing the early filling of the Neoproterozoic rift basin in the South China.Together with the underlying Sanhuashi Group,they represent the distinctive upper and lower tectonic-sedimentary circles,respectively.Combined with previous study results,it is indicated that there are three episodic magmatic activities during the Neoproterozoic epoch,with their peak ages being ~825 Ma,~795 Ma,and ~760 Ma,respectively.The first two episodes,representing the commencing age and fully extensional age of the rift basin respectively,are more apparent than the third one,The volcaniclastic rocks from the Dashigou and Sanlangpu Formations reported represent the latter two episodic magmatic activities（~795 Ma and ~760 Ma） respectively,their geological significances should not be neglected,either because they are parts of the Neoproterozoic episodic magmatic activities or because they are closely related to the formation of the Neoproterozoic rift basin in the South China.

关 键 词：新元古代 西乡群 凝灰岩 锆石U-PB年龄 地层学 幕式岩浆活动 扬子北缘 

分 类 号：P597.1[天文地球—地球化学]