贵州漾头陡山沱组与留茶坡组界线凝灰岩的锆石U-Pb年龄及其地质意义

Zircon U-Pb Ages and Its Geological Significance of Tuffs between Doushantuo and Liuchapo Formations at Yangtou Section,Guizhou Province

前寒武纪-寒武纪过渡时期是地质历史上最重要的转折期之一,对这一时期不同沉积相地层建立精确的对比标志,是理解寒武纪动物大爆发等一系列重大地质问题的关键.对贵州漾头剖面陡山沱组顶部和留茶坡组底部的两层凝灰岩中的锆石进行了LA-ICP-MS U-Pb年代学研究,分别获得了554.2±3.3 Ma和550.6±3.3 Ma的加权平均年龄,证明华南扬子地台深水相区留茶坡组开始沉积的时间约为550 Ma.扬子地台在埃迪卡拉纪-寒武纪过渡时期存在明显的台-盆沉积相分异,表明扬子地台在这一时期进入拉张的构造背景;因此,留茶坡组底部550.6±3.3 Ma的绝对年龄代表了扬子地台进入伸展构造背景和强烈海底热液活动开始的时间.另外,凝灰岩锆石微量元素特征显示,两层凝灰岩的源岩均属于花岗岩类,形成的大地构造背景属于岛弧环境或造山环境.

The Precambrian-Cambrian transition is one of the most important periods in the geological history.Establishing precise stratigraphic correlations among different sedimentary facies lays the foundation for deciphering important geological events including the Cambrian Explosion.LA-ICP-MS U-Pb zircon geochronology was carried out for the two tuffs in the upmost Doushantuo Formation and the basal Liuchapo Formation at the Yangtou section,Guizhou Province in this study.The weighted mean ages of the two tuffs are 554.2±3.3 Ma and 550.6±3.3 Ma,respectively.The new data suggests that the deposition time of the Liuchapo Formation in the deep-water facies of the Yangtze Platform is about 550 Ma.During the Ediacaran-Cambrian transition,the distinct differentiation of deposite facies across the Yangtze Platform indicates that the Yangtze Platform turned into extensional setting during this period.Therefore,the age of 550.6±3.3 Ma at the basal Liuchapo Formation also represents the time when the Yangtze Platform turned into the extensional setting and intense hydrothermal activities.In addition,trace elements of zircons indicate that the two tuffs derived from granite which formed in the magmatic arc or the orogenic environment.