中亚造山带东南部晚古生代-早中生代地壳增生和古亚洲洋演化:来自内蒙古东南部林西-东乌旗地区岩浆岩的证据

Crustal accretion and Paleo-Asian Ocean evolution during Late Paleozoic-Early Mesozoic in southeastern Central Asian Orogenic Belt:Evidence from magmatism in Linxi-Dongwuqi area,southeastern Inner Mongolia

本文在系统收集内蒙古林西-东乌旗地区晚古生代-早中生代岩浆岩的年代学、岩石地球化学以及锆石Hf同位素资料基础上,通过分析岩浆岩岩石组合随时空的变化规律,并结合区域地质资料,探讨了中亚造山带东南部洋盆演化和地壳增生等重要地质问题。研究结果表明,二连浩特-贺根山蛇绿岩带南、北两侧晚古生代-早中生代岩浆岩在年代学上显示不同的活动期次,具有不同岩石组合和地球化学特征,指示它们分属于不同的构造岩浆岩带。蛇绿岩带以北晚泥盆世-中二叠世岩浆活动在时间上呈连续分布的特征,并在晚石炭-早二叠世时期达到活动峰值。火成岩构造组合分析表明,晚泥盆世-石炭纪和早-中二叠世岩浆活动分别与二连浩特-贺根山洋盆向乌里雅斯太大陆边缘之下的俯冲和洋盆闭合后俯冲板片断离引起的软流圈上涌造成的区域伸展背景有关。蛇绿岩带以南岩浆活动时间上呈现石炭纪、早-中二叠世、晚二叠世-三叠纪幕式分布特征,各期岩浆活动前锋有随时间向南迁移的趋势。这三期岩浆活动分别与古亚洲洋板片向宝力道岛弧之下的俯冲、板片后撤以及洋盆消失之后古板块的碰撞造山作用有关。锆石Hf同位素分析表明,中亚造山带东南部晚古生代至早中生代时期存在显著的地壳增生;其中二连浩特-贺根山蛇绿岩带以北表现为地壳的垂向增生,以南表现为地壳的侧向增生。

In this paper,the geochronology,geochemistry and zircon Hf isotopic data of Late Paleozoic-Early Mesozoic magmatic rocks in Linxi-Dongwuqi area,Inner Mongolia were systematically collected.Based on the analysis of the temporal-spatial variation of the magmatic rocks and the regional geological data,the evolution of the paleo-Asian Ocean and the crustal accretion in the southeast of the Central Asia Orogenic Belt(CAOB)are discussed.The results show that the Late Paleozoic-Early Mesozoic magmatic rocks in the north and south of Erenhot-Hegenshan ophiolite belt display different activity stages,rock assemblages and geochemical characteristics,indicating that they belong to different tectono-magmatic belts.The magmatic activity in the north of Erenhot-Hegenshan ophiolite belt lasted from Late Devonian to Middle Permian continuously,and reached its activity peak during Late Carboniferous-Early Permian.The analysis of igneous petrotectonic assemblages shows that the Late Devonian-Carboniferous and Early-Middle Permian magmatism were related to the subduction of Erenhot-Hegenshan oceanic plate under the Uliastai continental margin and the regional extension induced by the upwelling of asthenosphere caused by the break-off of the subducted oceanic plate after the closure of the Erenhot-Hegenshan ocean basin,respectively.The magmatic activity in the south of Erenhot-Hegenshan ophiolite belt is characterized by episodic distribution and can be divided into three periods of Carboniferous,Early-Middle Permian and Late Permian-Triassic generally.And the magmatic front of each period tended to migrate southward with time.Regional geological analysis shows that these three periods of magmatic activity were related to the subduction of the paleo-Asian Ocean plate under the Baolidao island arc,the roll-back of the oceanic plate and the collision orogeny between the Siberian and the Sino-Korean paleoplates after the closure of the paleo-Asian Ocean.Zircon Hf isotopic analysis shows that there was significant crustal accretion during Late Paleozoic to Early Mesozoic in the southeast of the CAOB.The north area of Erenhot-Hegenshan ophiolite belt is characterized by vertical crustal accretion,while the south area of the ophiolite belt is characterized by lateral crustal accretion.