Geological field investigation, facies analysis and detrital zircon U-Pb geochronology were used to study the so-called'Langxian Cretaceous mélange (KL), which outcrops on the southern side of the eastern Yarlung Zangbo suture zone in Shannan Pre-fecture, southern Tibet. The results obtained indicate that the southern'KL'is quite similar to the Upper Triassic flysch Langjiexue Group in lithofacies, provenance and relative probability of detrital zircons U-Pb age (distinctive ca. 230Ma and 530Ma in age popu-lation), but different from the narrow belt of the northern counterpart. It is suggested that most of the'KL'can be assigned to the Langjiexue Group, implying that the collision boundary between Indian and Eurasian continents is the Naidong-Jindong-Moluo fault (Zhanang-Jiacha-Nang-Milin zone), rather than the Zedang-Jiacha-Langlagangze fault in the north or Naidong-Qusong-Bailu fault in the south.%采用野外地质点观察、岩相分析及碎屑锆石U-Pb定年方法,对雅鲁藏布江缝合带东段南侧的朗县白垩纪混杂岩(KL)进行研究.分析结果显示,以乃东-金东-莫洛断裂为界,该断裂南侧具有以板岩/千枚岩为主夹砂岩、粉砂岩的特征,属于海底扇沉积,其碎屑锆石年龄谱有230Ma和530Ma 2个峰值,与南部的上三叠统复理石郎杰学群极其相似;而该断裂北侧狭长地带的KL则主要为陆棚相板岩与大理岩,属于拉萨南缘沉积.为此认为,南部主体属于上三叠统郎杰学群,暗示印度大陆与欧亚大陆在西藏东南部的缝合碰撞边界可能是乃东-金东-莫洛断裂,而不是北侧的泽当-加查-朗拉岗则断裂或南侧的乃东-曲松-白露断裂.
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