长江中下游怀宁盆地火山杂岩锆石定年及其地质意义

LA-ICP-MS Zircon U-Pb Ages of the Volcanic Complex in the Huaining Basin,Middle—Lower Yangtze Region,and Geological Significance

怀宁盆地是长江中下游地区晚中生代发育的一系列断陷型火山岩盆地中的一个,盆地内的火山岩主体属橄榄玄粗岩(shoshonite)系列,但晚期出现高钾钙碱性系列的玄武质—流纹质双峰式岩石组合。本文测得怀宁盆地早期形成的彭家口组粗面岩和潜火山岩相的霏细斑岩以及晚期形成的江镇组流纹岩的LA-ICP-MS锆石U-Pb年龄分别为128.7±0.8Ma、126.3±0.7 Ma和123.5±0.7 Ma,进一步证实怀宁盆地是长江中下游地区中生代火山岩盆地带中火山作用结束最晚的一个。区域对比表明,长江中下游地区火山活动的峰期时间基本相当,均处于约130 Ma前后,但怀宁盆地内火山活动结束的时间明显晚于其他火山岩盆地约6 Ma。结合溧阳盆地火山活动的起始时间早达140Ma,显示整个长江中下游地区晚中生代的火山活动随时间具有从南东侧向北西方向迁移的趋势,并隐约显示火山活动由火山岩带中心区域向周边扩展的趋势。这种双向迁移性可能是太平洋板块与亚洲大陆板块相对运动和晚中生代发生在长江中下游地区的岩石圈减薄事件两种动力学机制共同作用的结果。

The volcanic rocks in the Huaining basin, which is one of a series late Mesozoic volcanic basins developed the Middle--Lower Yangtze region, are main belong to shoshonitic series, but appear bimodal volcanic association of high-K calc-alkaline series in the late stage of volcanism. In this work, we used LA-ICP-MS zircon U-Pb technique to date the trachyte and felsophyre samples from the Pengjiakou formation, early volcanism in the basin, and rhyolite from the Jiangzhen formation, late volcanism in the basin, yield emplacement ages of 128.7 ± 0.8Ma, 126. 3 ± 0.7 Ma and 123.5 ± 0.7 Ma respectively, which further confirmed that the late Mesozoic volcanism in the Middle--Lower Yangtze region was ended latest in the Huaining basin. Based on comparison with other volcanic basins in the Middle--Lower Yangtze region, the rush time of volcanism in whole volcanic basins are similar （ 130 Ma）, but the finished time of volcanism in the Huaining basin was late about 6 Ma than other volcanic basins. This combined with the earliest volcanism developed in the Liyang basin（140Ma） ,show that the volcanism migrating currently northwestward with time, and migrating from center area of the volcanic belt outward subtilly. The bidirectional migration of the volcanism may be the result of relative movement of the Pacific plate and the Asia continental plate and late Mosozoic lithospheric thinning happened in the Middle--lower Yangtze region.