内蒙古苏尼特左旗晚泥盆世弧背前陆盆地的发现及构造意义

Discovery of a Late Devonian retroarc foreland basin in Sunid Zuoqi,Inner Mongolia and its tectonic implications

兴蒙造山带位于中亚造山带东段,是研究地壳生长与古亚洲洋演化的热点区域。内蒙古中部苏尼特左旗地区位于兴蒙造山带中段,区内分布的上泥盆统色日巴彦敖包组磨拉石建造是古亚洲洋闭合的重要证据之一,前人对其岩石组合、地层层序与沉积环境进行了报道,并指出其属于晚泥盆世的前陆盆地,但未就前陆盆地的属性、分类及其与造山带的空间关系展开详细研究。因此,本研究选择该区东北部的昌特敖包剖面,利用野外实测、锆石U-Pb同位素定年、沉积相分析等方法查明该碎屑沉积的地层层序、形成时代与物质来源,在此基础上补充完善了兴蒙造山带北造山带二级构造单元的划分方案。研究结果表明,昌特敖包剖面碎屑沉积属上泥盆统色日巴彦敖包组,而非前人认为的二叠纪大石寨组地层,其碎屑物质主要来自南部的宝力道岛弧与北部兴安-爱力格庙地块。沉积环境分析表明昌特敖包剖面色日巴彦敖包组主要为冲积扇相沉积,上部过渡到干旱气候火山活动背景下的滨浅海相沉积。综合昌特敖包剖面的沉积、年代、物源及大地构造位置特征,可以推断其代表晚泥盆世造山带的弧背前陆盆地。由此,苏尼特左旗地区北造山带的构造单元在前人划定的前陆变形带、混杂带、周缘前陆盆地、岛弧岩浆岩、同碰撞花岗岩以北,还存在弧背前陆盆地。本研究结果为恢复中亚造山带的构造演化过程提供了关键沉积学证据。

The tect0nic evolution of Xing＇an-Mongolia Orogenic Belt（ XMOB） has been extensively studied for the past few decades. It is proposed that Late Devonian Seribayanaobao Formation（ D3 s）,was accumulated in a foreland basin along the North Orogenic Belt（ NOB） in XMOB,representing the final closure of Paleo Asian Ocean（ PAO）. However,our knowledge about the classification,tectonic setting and the location in orogenic belt of the foreland basin is still limited. Therefore,this study focused on Changteaobao area,northeast of Sunid Zuoqi,central Inner Mongolia,where a foreland basin has been discovered. Geochronological and sedimentological analyses were performed to determine the depositional age,provenance,sedimentary environment and tectonic setting of the basin. On that basis,we further discuss the tectonic units and the formation of NOB during Middle Paleozoic. The results reveal that the youngest zircon U-Pb age at the bottom of D3 s is 439 ± 5 Ma,and the volcanic layer at the top of the formation was formed at 346. 5 ± 4. 2 Ma. Therefore,D3 s in the Changteaobao section was formed during Late Devonian to Early Carboniferous rather than Late Permian as previous work suggested. The detrital zircons from D3 s have one age peak of 439 - 493 Ma（ N = 41） and three Neoproterozoic ages,indicating that the provenance of the strata were Balidao arc-pluton on the south and the Xing＇an-Arigin Sum block（ XAB） on the north. Sedimentary facies analysis reveals that the D3 s mainly represent an alluvial fan deposit. It regressively transits to coastal deposit with volcanic activity in a warm climate. According to the sedimentary facies,age,provenance and location of the Changteaobao section,we completed the tectonic units of NOB from north to south into： retro-arc foreland basin,arc-pluton complex,peripheral foreland basin,melange and fold belt. From 490 - 420 Ma,the PAO subducted below XAB,forming Baolidao arc pluton,south of Sunidzuoqi and back-arc basin on XAB. During the Late Devonian,the collision between PAO and XAB gave rise to the peripheral and retro-arc foreland basins that accumulated thick bedded alluvial fan type deposit.