祁连山蛇绿岩带和原特提斯洋演化

Ophiolite belts and evolution of the Proto-Tethys Ocean in the Qilian Orogen

位于阿拉善地块和柴达木地块之间的祁连造山带记录原特提斯洋扩张、俯冲、闭合、大陆边缘增生和碰撞造山的完整过程。从南向北,祁连造山带发育有三条平行排列、不同类型的蛇绿岩带:(1)南部南祁连洋底高原-洋中脊-弧后蛇绿岩混杂带;(2)中部托勒山洋中脊型蛇绿岩带;(3)北部走廊南山SSZ型蛇绿岩带。南部南祁连蛇绿混杂岩带以拉脊山-永靖蛇绿岩为代表,为典型的洋底高原型蛇绿岩,是大洋板内地幔柱活动的产物,形成年龄为525~500Ma;中部托勒山蛇绿岩带沿熬油沟-玉石沟-冰沟-永登一线分布,为大洋中脊型蛇绿岩,蛇绿岩形成年龄为550~495Ma;北部蛇绿岩带包括弧前和弧后两种类型,弧前蛇绿岩以大岔大阪蛇绿岩为代表,形成时代为517~487Ma,反映初始俯冲/弧前扩张到弧后盆地的过程;弧后蛇绿岩以九个泉-老虎山蛇绿岩为代表,为典型的SSZ型蛇绿岩,是弧后扩张的产物,形成时代为奥陶纪(490~445Ma)。三个蛇绿岩带分别代表了新元古代-早古生代祁连洋演化历史不同环境的产物,对了解秦祁昆构造带原特提斯洋的构造演化过程有重要意义。蛇绿岩及弧火山岩的时空分布特征限定了原特提斯洋的俯冲极性为向北消减俯冲。

The Qilian Orogen,which lies between the Alashan Block in the north and the Qaidam Block in the south,records the entire tectonic evolution of Proto-Tethys Ocean from its spreading,subduction,closing,marginal accretion to continental collision.There are three different types of parallel ophiolite belts extending in the Qilian Orogen,from south to north,including:( 1) the South Qilian ophiolitic complex belt with oceanic plateau-type,MOR-type and backarc-type ophiolites in the south;( 2) the Tuoleshan MORtype ophiolite belt in the middle;and( 3) the Zoulangnanshan back-arc-basin( BAB)-type ophiolite belt in the north. The southern oceanic plateau-type ophiolite belt is represented by Lajishan-Yongjing ophiolite,which is a typical ophiolite fragment of oceanic plateau originated from the Cambrian mantle plume activity during ca. 525 ~ 500 Ma. The middle ophiolite belt extends along Aoyougou,Yushigou,Binggou and Yongdeng,representing the ophiolite complex of the MOR-type oceanic crust with the ages of 550 ~495 Ma. The northern ophiolite belt contains forarc-type and BAB-type ophiolites. The forarc-type ophiolite,represented by the Dachadaban ophiloite,formed in response to the processes from subduction initiation,forearc extension to back-arc basin development during 517 ~ 487 Ma;the BAB-type ophiolite,represented by the Jiugequan and Laohushan ophiolites,are the SSZ-type ophiolite and the products of back arc basin extension during Ordovician( 490 ~ 445 Ma). The three ophiolite belts are the products formed in different tectonic settings during the history of Qilian Ocean from Neoproterozoic to Early Paleozoic,and thus is of great significance in understanding the tectonic evolution of Proto-Tethys Ocean in Qin( ling)-Qi( lian)-Kun( lun) Orogenic Belt. The timing and distribution of the three ophiolite belts and arc volcanics constrain a northward subduction polarity.