内蒙古苏尼特左旗早古生代乌华敖包弧前盆地的识别及其古地理意义

Recognition of the Early Paleozoic Wuhuaaobao Forearc Basin and Its Paleogeographic Significance in Sunid Zuoqi,Inner Mongolia

通过野外观察和剖面测量、锆石年代学分析及沉积相分析等方法,对新发现的苏尼特左旗乌华敖包地区浅变质岩系进行研究,并探讨其古地理意义。结果表明,该套浅变质岩系以变质火山岩、变质砂岩与大理岩互层为特征,构成3个沉积旋回,代表弧前盆地中‒晚期向上变浅的海相火山喷发‒沉积环境;多层火山岩的出现揭示持续的岩浆活动;火山岩锆石U-Pb定年结果为454.4±4.1 Ma,与北部早古生代宝力道岛弧带的岩浆岩年龄相近,表明该套浅变质岩系与早古生代岛弧岩浆活动有关。由于该岩系位于北部宝力道岛弧岩浆岩带与南部混杂岩带之间,因此其古地理位置应属早古生代弧前盆地。乌华敖包浅变质岩系的层序、成分、时代和位置特征揭示研究区曾存在弧前盆地。根据早古生代地层、岩浆岩和变质特征,认为弧前盆地经历了俯冲及碰撞两个阶段。

Analysis of sedimentary sequence,material composition and geochronology are used to study a newly discovered epimetarmorphic rock series in Wuhuaaobao area of Sunid Zuoqi,Inner Mongolia,and its paleogeographic significance is also discussed.The epimetarmorphic rock serie is characterized by the interbedded sequence of metavolcanic rock,metasandstone and marble,which contains three cycles and represents an upward-shallowing marine volcanic-sedimentary environment.The presence of multilayer volcanic rocks indicates that they formed during continuous magmatic activity.Zircon U-Pb age of the metavolcanic rock is 454.4±4.1 Ma,which is coincided with the magmatic rock ages of early Paleozoic Bolidao island arc belt in the north.Because the serie is located between the Baolidao arc magmatic belt in the north and the mélange belt in the south,its paleogeographic position should be an early Paleozoic forearc basin.Sequence,composition,age and position characteristics of the Wuhuaaobao epimetarmorphic rock serie reveal the presence of the forearc basin in the study area.Two evolutionary stages of the forearc basin have been suggested during subduction and collision,respectively,according to the early Paleozoic strata,magmatic rocks and metamorphic events.