内蒙古北山造山带白云山蛇绿混杂岩的厘定及其对北山洋俯冲消减的指示

The recognition of Baiyunshan ophiolitic mélanges belt in Beishan orogenic belt,Inner Mongolia and its indication for the subduction of the Beishan ocean

白云山蛇绿混杂岩带位于内蒙古北山造山带中部,呈北西西向展布,向东延伸至月牙山-洗肠井蛇绿混杂岩带,向西延伸至牛圈子-红柳园蛇绿混杂岩带。白云山蛇绿混杂岩带内发育俯冲期的糜棱面理褶皱、拼贴期逆冲断层系及隆升期走滑断层系3期构造变形,由不同类型的岩块与基质组成,岩块主要包括纯橄岩、辉橄岩、橄辉岩、辉石岩、碳酸盐化超基性岩、辉长岩、玄武岩、斜长花岗岩蛇绿岩岩块及硅质岩、灰岩和砂岩岩块,基质主要为蛇纹岩、绿泥片岩及砂板岩。在蛇绿混杂岩带中部发现保存较完整的洋壳残片,由南向北依次出露堆晶超镁铁质岩、堆晶辉长岩及变质玄武岩。结合大洋中脊玄武岩、洋岛玄武岩及晚寒武世岛弧钙碱性辉长岩的识别,认为白云山蛇绿混杂岩带寒武纪发育MOR型、OIB型、SSZ型等不同构造环境的蛇绿岩岩块,俯冲作用持续到晚志留世。

The NWW-striking Baiyunshan ophiolitic mélanges belt is exposed in the central part of Beishan orogenic belt,which extends from Hongliuhe-Niujuanzi in the west to Xichangjing in the east.There are three stages of structural deformation in the Baiyunshan ophiolitic mélanges belt,i.e.,mylonite fold in subduction period,thrust fault system in collage period and strike-slip fault system in uplift period.The Baiyunshan ophiolitic mélanges are composed of different types of blocks and matrix.The ophiolitic blocks include rocks of chassignites,pyroxene peridotites,olivine pyroxenolites,pyroxenites,carbonated ultramafic rocks,gabbros,basalts,plagiogranites,and the blocks include rocks of siliceous rocks,limestones and sandstones.The matrix is composed of serpentine matrix,green-schist matrix and sand-slate matrix.In the center of the ophiolitic mélanges belt,the fragments of intact oceanic crust from south to north were found,which are composed of cumulate ultramafic rocks,cumulate gabbros,basalts.Based on the identification of MORBs,OIBs and Late Cambrian island arc-calc alkaline gabbros,the authors believe that the ophiolite blocks of MOR type,OIB type and SSZ type were developed in the Cambrian in the Baiyunshan ophiolitic mélanges belt,and the subduction of the Hongliuhe-Xichangjiang Ocean lasted until late Silurian.