南祁连拉脊山口增生楔的结构与组成特征

Texture and composition of the Lajishankou accretionary wedge of the South Qilian belt,NW China

造山带内增生楔/增生杂岩结构与组成的精细研究可为古洋盆演化和古板块构造格局重建提供最直接证据。北祁连构造带发育多条增生杂岩带,记录了阿拉善和中祁连地块之间原特提斯洋的俯冲和闭合过程,然而南祁连构造带大地构造演化长期存在争议。地质填图结果表明,南祁连构造带拉脊山口地区存在一套强烈片理化的玄武岩、灰黑色和红色硅质岩、砂岩和泥岩组合,它们与一套呈现"块体裹夹于基质"结构特征的混杂岩共同构成了增生杂岩,发育双重逆冲构造、逆冲断层、无根褶皱、紧闭褶皱和透入性面理。该增生杂岩与蛇绿岩之间为断层接触,并位于断层下盘。混杂岩是由斜长花岗岩(561Ma)、斜长岩(507Ma)、辉绿岩、玄武岩、硅质岩和砂岩等外来或原地岩块与浊流成因的细碎屑岩基质共同组成;基质和砂岩块体均发育同沉积构造,呈现出滑塌堆积典型特征。空间上,拉脊山口增生杂岩与上覆蛇绿岩被断层所分割且共同仰冲于中祁连南缘青石坡组浊积岩之上,具有与东侧昂思多地区增生杂岩和蛇绿岩相似的岩石组成、构造变形和时空结构特征。它们与南侧的岛弧带共同构成了南祁连构造带寒武纪-早奥陶世沟-弧体系,指示了寒武纪-早奥陶世时期南祁连洋盆向南俯冲。

Detailed studies on the texture and composition of accretionary wedge/complex can provide important information on the evolution of ancient ocean basins and the reconstruction of ancient plate tectonics. The North Qilian belt contains multiple belts of accretionary complex and record Early Paleozoic subduction and closure processes of the Proto-Tethyan Ocean between the Alxa and Central Qilian blocks. However,the tectonic evolution of the South Qilian belt is still hotly debated. Recently,our geological mapping demonstrates that the Lajishankou accretionary complex of the South Qilian belt,which consists of highly foliated basalt,black and red chert,sandstone,mudstone,and mélanges showing typical block-in-matrix fabric and is characterized by intense deformation recorded by duplex,thrust faults,rootless intrafolial folds,tight fold,and penetrative foliations,crops out structurally beneath the ophiolite complex. The mélange is composed of various exotic or native blocks,including plagiogranite（ 561 Ma）,anorthosite（ 507 Ma）,dolerite,basalt,chert and sandstone,within a foliated fine-grained turbiditic matrix. The synsedimentary structures in matrix and sandstone blocks suggest an olistostromal origin for this mélange. Spatially,the Lajishankou accretionary complex and the overlying ophiolite complex were thrust over turbidite of Qingshipo Formation and share the similar composition,deformation,spatial and temporal distribution with those in Angsiduo area. The accretionary complex,ophiolite,and the southern island arc rocks exposed in both Lajishankou and Angsiduo areas constitute a Cambrian-Early Ordovician trench-arc system within South Qilian belt during the Cambrian-Early Ordovician southward subduction of the South Qilian Ocean.