兴蒙造山带中段南缘晚古生代演化过程:来自苏尼特右旗地区沉积地层及火山岩的证据

本文选取内蒙古苏尼特右旗地区石炭-二叠纪地层及火山岩为研究对象,通过典型地层剖面测制、锆石U-Pb年代学、古生物和地球化学等方法,查明其形成时代、物质来源及沉积环境,并进一步探讨兴蒙造山带中段南缘晚古生代的演化过程。本巴图组2个碎屑岩样品最小年龄为299Ma和296Ma,结合古生物化石研究,其形成时代应为晚石炭-早二叠世;三面井组2个碎屑岩样品的最小年龄为271Ma和272Ma,同样结合其古生物化石资料,判断其形成时代为早-中二叠世。锆石组成和岩相特征显示本巴图组和三面井组形成于不同的沉积背景之下。本巴图组的沉积相变化显示为一个海进序列,形成于弧后伸展背景之下;三面井组则与之相反,显示沉积环境由稳定变为活动,形成于汇聚背景之下。安山岩的喷出年龄为277±1.4Ma,有着高镁安山岩的属性,显示出俯冲背景下陆缘弧火山岩的特征。结合以往区域地质资料,我们认为晚古生代兴蒙造山带南缘的构造演化过程可分为五个阶段:(1)泥盆纪,碰撞后伸展阶段;(2)早石炭世,俯冲开始阶段;(3)晚石炭,俯冲不强烈、局部发生伸展作用阶段,以本巴图组和阿木山组的碎屑岩-碳酸盐组合为代表;(4)早中二叠世,俯冲加强阶段,发育三面井组和大量的陆缘弧火山岩,此时古亚洲洋仍然存在;(5)晚二叠世之后,碰撞结束、古亚洲洋消失阶段,区域开始出现大量A型花岗岩,整体进入造山后环境。

Evolution of central-southern margin of the Xing-Meng Orogenic Belt in the Late Paleozoic: Evidence from Carboniferous-Permian sedimentary formation and volcanic rock in Sonid Right Banner, Inner Mongolia

In this study, we present typical sections, zircon U-Pb dating, paleontological and geochemical data for the Carboniferous-Permian sedimentary formations and volcanic rocks in Sonid Right Banner area, Inner Mongolia, to confirm their sedimentary ages, environments and provenances which could provide strong evidence for the Late Paleozoic evolution of central-southern margin of the Xing-Meng Orogenic Belt (XMOB). The youngest zircon ages of the two samples in Benbatu Formation are 299Ma and 296Ma, respectively. Combining with the studying upon the fossils found in this strata, we suggest that the Benbatu Formation was formed during the Late Carboniferous-Early Permian. The youngest zircon ages of the two samples in Sanmianjing Formation are 271Ma and 272Ma, respectively. Combining with the studying upon the fossils found in this strata, we conclude that the Sanmianjing Formation was formed in the Early-Middle Permian. The zircon age components and sections show that the Benbatu and Sanmianjing formations were formed in different tectonic settings. The sedimentary facies of Benbatu Formation present a transgression sequence and was formed in a local extensional setting, while the sequence and facies of Sanmianjing Formation show a transformation from stable to active setting which was formed in a convergent background. The eruption age of the high Mg andesite is 277±1.4Ma, and they have the characteristics of continental arc. Taken together with the previously regional researches, we can conclude that the evolution of the central-southern margin of the XMOB during the Late Paleozoic can be divided into five stages: 1) Intracontinental extension tectonic process occurred in Devonian; 2) Subduction process starting in the Early Carboniferous; 3) Slow subduction process in the Late Carboniferous, with some regions in an extension setting that developed along the central-southern margin of the XMOB when clastic and carbonate rocsk in Benbatu and Amushan formations were formed; 4) Conversion from extension to subduction in the Early-Middle Permian, during which arc volcanic rocks and Sanmianjing Formation were formed, with the Paleo Asian Ocean (PAO) still existing; 5) After the Late Permian, i.e., the end of collision and the disappear of the PAO, during this period, A-type granites was formed, and the central-southern margin of the XMOB came into an extension setting.