西藏雅鲁藏布江缝合带二叠纪灰岩体的动物群及其古地理意义

Faunas from the Permian Limestone Blocks at the Yarlung Zangbo Suture Zone in Southern Xizang(Tibet) and Its Paleogeographical Implications

西藏雅鲁藏布江缝合带含有从砾石级到几十平方千米大小的二叠纪灰岩体,它们没有完整的地层层序,与围岩形成混杂或滑杂堆积,长期以来对其来源的解释存在争论。本文通过对灰岩体的动物群类型、古生物地理区系及其岩石学特征等方面进行研究和比较,认为雅鲁藏布江缝合线一带的二叠纪灰岩体总体上都呈现出冈瓦纳冷水型与华夏暖水型动物群混生特点,应形成于相同或类似的沉积环境。根据珊瑚、竺蜓、腕足类等动物群大致分为与南方冈瓦纳大陆北缘内陆棚相动物群比较接近和与更靠近古赤道区的拉萨地块动物群较为接近两种类型,其时代从早二叠世晚期至长兴期不等。灰岩体主要由肉红色或灰色纯生物碎屑灰岩组成,不含或含有很少的陆源碎屑,均孤立地分散于中生代地层中,与围岩往往呈断层接触,断层带或灰岩夹层中经常有玄武岩或其他火山岩。因此,灰岩体可能为位于冈瓦纳大陆北缘外陆棚上小型碳酸盐台地或新特提斯洋最初裂解带上的海山型碳酸盐岩沉积,受后期印度—欧亚大陆板块碰撞作用而成为外来体夹于缝合带的其他海相沉积中。

One of the most spectacular features in the Yarlung Zangbo (Indus-Tsangpo) Suture Zone in southern Xizang (Tibet), China, is that it contains pebble-sized to kilometer-scale limestone blocks. Because these blocks are isolated in the surrounding matrix rocks and have no obvious stratigraphical sequences, the interpretations of their origins of the blocks have been the subject of considerable debate for a long time. Studies of the faunas in the limestone blocks suggest that they are all characterized by the mixed characters between the Cathaysian warm-water faunas and the Gondwanan cold-water faunas. The faunas in the limestone blocks in the Yarlung Zangbo Suture Zone, as a whole, are similar one another, and therefore may represent a generally similar sedimentary environment and paleogeographical setting. These faunas range from the late Early Permian to Changhsingian in the limestone blocks and can be largely divided into two types based on corals, fusulinids and brachiopods. One is characterized by the dominance of solitary corals, absence of fusulinids and more cold-water brachiopods, and probably represents a paleogeographical setting closer to the peri-Gondwanan margin. The other is characterized by the presence of compound corals, fusulinids and more warm-water brachiopods and therefore indicates a paleogeographical setting closer to the Lhasa Block. The limestone blocks are mostly composed of reddish or gray massive bioclastic limestone and commonly faulted with volcanic rocks or have a volcanic basement. Therefore, it is most likely that these blocks represent some carbonate deposits on the outer shelves of the peri-Gondwanan region or seamount deposits in the initial rifting stage of the Neo-Tethys, but subsequently sandwiched into the Mesozoic/Cenozoic deposits during the collision between the Indian and Eurasian Plates.