兴蒙造山带中部晚古生代构造格局:来自晚泥盆-早石炭世色日巴彦敖包组碎屑锆石和火山岩岩浆锆石年代学的制约

Late Paleozoic tectonic framework of the central Xing-Meng Orogenic Belt:Constrains from detrital and igneous zircon U-Pb geochronology of Late Devonian-Early Carboniferous Seribayanaobao Formation in southern Sunid Zuoqi

内蒙古苏尼特左旗地区位于兴蒙造山带中段,是研究古生代俯冲-增生造山作用和地壳生长的关键地区。在苏尼特左旗南部,晚泥盆-早石炭世色日巴彦敖包组角度不整合在早古生代增生楔之上,已有研究对于其沉积环境、盆地属性及区域构造意义一直存在较大分歧。本文对色日巴彦敖包组敖木根呼都格剖面和阿拉塔特剖面碎屑岩和火山岩夹层开展锆石U-Pb测年及Hf同位素分析,并进行了沉积相分析,进一步查明了地层层序、时代及物源特征,并对沉积环境和盆地属性进行了探讨。研究表明,敖木根呼都格剖面色日巴彦敖包组底部含砾粗砂岩最小谐和碎屑锆石年龄为391±5Ma,中下部凝灰质砂岩最小谐和碎屑锆石年龄为338.2±4.3Ma,凝灰岩锆石年龄为336±2Ma、333±3.5Ma,结合古生物化石,表明底部形成于晚泥盆世,中上部地层形成于早石炭世晚期。阿拉塔特剖面中上部安山质火山碎屑岩最小谐和碎屑锆石年龄加权平均值为320.4±6.2Ma,表明其形成于晚石炭世。碎屑锆石年龄谱及Hf同位素组成表明,底部及中部砂岩碎屑物主要来自近源的早古生代白音宝力道和二连东乌旗地区弧岩浆岩,少量来自北侧南蒙古弧。结合沉积相分析,推测苏尼特左旗南部色日巴彦敖包组下部为一套形成于弧陆碰撞后被动陆缘滨浅海相沉积建造,中上部为形成于弧前盆地的一套杂砂岩夹火山岩建造。以上资料表明,该区从早石炭世开始转为活动陆缘背景,暗示古亚洲洋中东段在晚古生代仍未封闭。

The Xing-Meng Orogenic Belt is a key area to study the accretionary history of the Central Asian Orogenic Belt,therefore,its tectonic framework and evolution have been studied for decades.Seribayanaobao Formation,distributed in southern Sunid Zuoqi,covers the Ordovician accretionary complex with an angular unconformity,whose sedimentary environment and tectonic setting are still controversial.In this study,we carried out detrital and igneous zircon U-Pb geochronology and Hf isotopic analysis from sandstone in the bottom and lower,tuff in the middle and volcaniclastic rocks in the upper part of Seribayanaobao Formation to discuss its sedimentary facies,ages and clastic provenance for the rebuilding of its sedimentary environment and basin type.Zircon U-Pb dating show that the minimum concordial age of the gravel-bearing coarse sandstones in the bottom is 391±5Ma,the lower part tuffaceous sandstones have a minimum weighted average age of 338.2±4.3Ma,and the ages of two tuff interlayers in the middle part of the Aomugenhuduge section are 336±2Ma and 333±3.5Ma,respectively.In the Alatate section we obtained minimum weighted average age of 320.4±6.2Ma in the volcaniclastic rocks from the upper part,indicating a Late Carboniferous age.The detrital zircon age spectrum suggests that the main sediments were mainly from the proximal Early Paleozoic Baiyinbaolidao and Erlian-Dongwuqi arc magmatic rocks together with minor sediments from ancient recycled crustal material in South Mongolian arc.The age spectrum of detrital zircons suggests that the clastic rocks from the bottom of Seribayanaobao Formation should be deposited in passive marginal basin after arc-microcontinent collision,while the clastics in middle part(Early Carboniferous)in a fore-arc basin setting,indicating the initiation of Late Paleozoic plate subduction.Combining the above results of detrital zircon U-Pb data,sedimentary setting,clastic provenance and basin analyses,it can be concluded that the central-eastern segment of the Paleo-Asia Ocean was still not closed during the Late Paleozoic.