兴蒙造山带南缘早—中二叠世砂岩碎屑锆石U-Pb年龄、Hf同位素组成及其构造意义

摘要

利用LA-MC-ICP-MS分析技术,对兴蒙造山带南缘下二叠统三面井组和中二叠统额里图组砂岩中的碎屑锆石进行了U-Pb年龄和Hf同位素研究.结果显示,三面井组砂岩记录了3组碎屑锆石年龄:280~369Ma(4颗)、1800~2271Ma(43颗,峰值1877Ma)和2507~2711Ma(9颗).额里图组砂岩碎屑锆石年龄集中分布在271~371Ma(9颗)、1258~1395Ma(18颗,峰值1326Ma)和1695~2454Ma(34颗,峰值1864Ma)3个年龄区间.对比研究发现,样品中的元古宙和太古宙碎屑锆石是华北克拉通古老基底的反映;晚古生代锆石具有较低的εHf(t)值(-5.3~-23.7)和较老的Hf模式年龄(1124~3039Ma),与华北克拉通北缘晚古生代岩浆岩具有一致性,而明显区别于兴蒙造山带内岩浆岩Hf同位素特征,说明前者是其物源供给区.结合区域资料认为,华北克拉通北缘在早―中二叠世期间不存在类似安第斯型的俯冲大陆边缘,三面井组和额里图组砂岩是在伸展背景下沉积形成的.%In this paper, the authors used the LA-MC-ICP-MS analytical techniques to determine U-Pb ages and Hf isotopic com-position of detrital zircons from the Lower Permian Sanmianjing Formation and Middle Permian Elitu Formation in Zhengxiangbai Banner area of Inner Mongolia on the southern margin of the Xing-Meng orogenic belt. Detrital zircon dating of the sandstone of the Sanmianjing Formation yielded three main age populations respectively at 280~369Ma (4 grains), 1800~2271Ma (43 grains, peaked at 1877Ma), and 2507~2711Ma (9 grains), whereas the Elitu Formation displays prominent age groups at 271~371Ma (9 grains), 1258~1395Ma (18 grains, peaked at 1326Ma) and 1695~2454Ma (34 grains, peaked at 1864Ma), respectively. The detrital zir-con geochronological and Hf isotopic studies indicate that the Meso-Proterozoic to Neoarchean grains were probably derived from the basement of the North China Craton. The Late Paleozoic zircons exhibit negative εHf(t) values of -5.3~-23.7 and old Hf model ages of 1124~3039Ma. These characteristics show a strong resemblance to the Late Paleozoic igneous zircons from the north margin of the North China Craton, suggesting that the source of the Sanmianjing Formation and Elitu Formation partly came from the North China Craton. Integration of provenance data with regional geological information and magmatic records suggests that no An-dean-type active continental margin was generated on the northern margin of the North China Craton during Early to Middle Perm-ian, and the Sanmianjing Formation and Elitu Formation were deposited in an extensional tectonic environment.