内蒙古北山地区百合山蛇绿混杂岩带的厘定及其洋盆俯冲极性——基于1∶5万清河沟幅地质图的新认识

The identification and subduction polarity of the Baiheshan ophiolite mélanges belt in the Beishan area, Inner Mongolia——New understanding based on the geological map of Qinghegou Sheet(1:50000)

造山带内蛇绿混杂岩带结构与组成的精细研究可为古板块构造格局重建和古洋盆演化提供最直接证据。北山造山带内存在多条蛇绿混杂岩带,记录了古亚洲洋古生代以来的俯冲和闭合过程,然而其大地构造演化长期存在争议。红石山—百合山蛇绿混杂岩带位于北山造山带北部,主要由蛇绿(混杂)岩和增生杂岩组成,具典型的“块体裹夹于基质”的混杂岩结构特征,发育紧闭褶皱、无根褶皱、透入性面理和双重逆冲构造。蛇绿混杂岩带中岩块主要由超镁铁质?镁铁质岩(变质橄榄岩、辉石橄榄岩、异剥辉石岩、蛇纹岩)、辉长岩、玄武岩、斜长花岗岩、硅质岩等洋壳残块以及奥陶纪火山岩、灰岩等外来岩块组成,基质则主要为蛇纹岩、砂板岩及少量的绿帘绿泥片岩;在蛇绿混杂岩带北侧发育有台地相灰岩与深水浊积岩组成的沉积混杂块体,具滑塌堆积特征。蛇绿混杂岩带内发育三期构造变形,前两期为中深构造层次下形成的透入性变形,第三期为浅表层次的脆性变形,未形成区域性面理。空间上,由增生杂岩和蛇绿(混杂)岩组成的百合山蛇绿混杂岩带共同仰冲于绿条山组浊积岩之上,具有与红石山地区蛇绿混杂岩带相似的岩石组成、构造变形和时空结构特征。百合山蛇绿混杂岩带南侧发育同期的明水岩浆弧,由晚石炭世石英闪长岩?花岗闪长岩?二长花岗岩以及白山组岛弧火山岩组成,其与百合山蛇绿混杂岩带共同构成了北山造山带北部石炭—二叠纪的沟?弧体系,指示了红石山—百合山洋盆向南俯冲的极性。

Detailed studies of the texture and composition of ophiolite mélanges belt can provide important information for the reconstruction of ancient plate tectonics and the evolution of ancient ocean basins.The Beishan orogenic belt contains multiple belts of ophiolite mélanges and records Paleozoic subduction and closure processes of the Paleo-Asian Oceans.However,the tectonic evolution of the North Beishan orogenic belt(NBOB)is still hotly debated.Hongshishan-Baiheshan ophiolite mélanges belt which consists of ophiolite and accretionary complex is located in the north of the Beishan orogenic belt.The ophiolite mélanges show that typical block-in-matrix fabric is characterized by intense deformation recorded by tight fold,rootless intrafolial folds,penetrative foliations and duplex thrust faults.The mélange is composed of various exotic or native blocks,including ultramafic rock(metamorphic peridotite,pyroxene peridotite,peel pyroxenite and serpentinite),gabbro(344Ma),basalt,plagiogranite(297Ma),chert,limestone and Ordovician volcanic rocks.The matrix consists of serpentinite,sandstone and a small quantity of epidotechlorite schist.The sedimentary blocks composed of limestone and turbidite suggest a characteristic of slump deposit.The ophiolite mélanges belt developed three distinct foliations which showed middle and deep tectonic stratigraphy in the first two phase and nonpenetrative foliation in the third phase.Spatially,the Baiheshan ophiolite mélanges belt composed of ophiolite and accretionary complex was thrust over turbidite of Lvtiaoshan Formation and shared similar compositions,deformations,spatial and temporal distributions to that in Hongshishan area.The Mingshui magmatic arc consisting of quartz diorite-granodiorite-monzonitic granite and Baishan Formation arc volcanic rocks is located in the north of the Baiheshan ophiolite mélanges belt.Baiheshan ophiolite mélanges belt and the Mingshui magmatic arc constitute a Carboniferous-Permian trench-arc system within the North Beishan orogenic belt(NBOB)during the Carboniferous-Permian southward subduction of the Hongshishan-Baiheshan Ocean.