内蒙古巴林左旗九井子蛇绿岩锆石U-Pb定年:对西拉木伦河缝合带形成演化的约束
Zircon U-Pb dating of the Jiujingzi ophiolite in Bairin Left Banner, Inner Mongolia: Constraints on the formation and evolution of the Xar Moron River suture zone
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摘要: 本文对位于西拉木伦河蛇绿岩带东段的九井子蛇绿岩中辉长岩脉以及蛇绿岩的围岩开展了锆石U-Pb定年。结果表明辉长岩的形成时代为（274.7±1.7）Ma，MSWD=0.079，属于早二叠世晚期；结合前人地层、古生物、岩浆岩等方面的资料，表明内蒙古东南部早二叠世晚期还可能存在大洋盆地。与九井子蛇绿岩呈断层接触的粉砂岩碎屑锆石年龄大致构成4个峰值：2350~2700 Ma、1700~2100 Ma、370~470 Ma和250~290 Ma，通过与区域构造热事件的对比分析，表明其物源主要来自中朝古板块的北缘。粉砂岩中最小的锆石年龄为晚二叠世末-早三叠世初（（249±4.7）Ma），该年龄与内蒙古东南部海相地层消失的时代、安加拉植物群和华夏植物群出现混生的时代、西伯利亚和中朝古板块古纬度曲线收敛的时代以及区域上与碰撞相关的岩浆岩形成时代大致相同，据此本文认为九井子蛇绿岩的构造侵位时代应为晚二叠世末-早三叠世初，同时也可能代表古亚洲洋的最终闭合时代。Abstract: The Jiujingzi ophiolite belongs to the eastern segment of the Xar Monron River ophiolite belt in southeastern Inner Mongolia. In this paper, the authors carried out zircon U-Pb dating of a gabbro dike of the Jiujingzi ophiolite and the surrounding siltstone that is in fault contact with the ophiolite. The results indicate that the gabbro dike was formed in late Early Permian (274.7± 1.7 Ma, MSWD=0.079). Considering previous geological data on strata, paleontology and magmatic rocks, the authors hold that there was still an ocean basin in late Early Permian in southeastern Inner Mongolia. The detrital zircons of the adjacent siltstone constitute four age peaks:2350-2700 Ma, 1700-2100 Ma, 370-470 Ma and 250-290 Ma. A comparison with regional tectonothermal events shows that the provenance of the siltstone was mainly from the north margin of the Sino-Korean paleoplate. The age of the youngest zircon is late Early Permian-early Early Triassic (249±4.7 Ma), which is similar to the ages of the disappearance of the marine strata in southeastern Inner Mongolia, the mixing between the Cathaysian and Angaran floras, the convergence of the palaeolatitude curves of the Siberian and Sino-Korean paleoplates and the formation of the collision-related magmatic rocks. It is thus suggested that the tectonic emplacement of the Jiujingzi ophiolite might have occurred in late Early Permian-early Early Triassic, which might represent the final closure time of the Paleo-Asian Ocean.
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