西藏尼玛盆地南部古近系物源分析及地质意义

Provenance of Paleogene Sediments in the South Depression of Nima Basin, Central Tibet and its Geological Implications

位于羌塘地体与拉萨地体之间的尼玛盆地包含有青藏高原演化过程中重要的沉积记录,恢复该地区的古地理特征及讨论其构造演化过程,具有重要地质意义。尼玛盆地南部古近系由古新世-始新世牛堡组与渐新世丁青湖组构成,主体为一套扇三角洲相与湖泊相碎屑沉积建造。采用沉积学、年代学方法,对该盆地南部古近系查昂巴剖面进行沉积特征、碎屑锆石U-Pb年代学及古地理研究,以确定其物源区,并分析构造作用对盆地的改造过程。从牛堡组到丁青湖组,稳定碎屑组分逐渐减少而不稳定组分增加,显示古近纪时盆地沉积过程中受到构造活动影响;碎屑锆石U-Pb年龄中存在36~70 Ma,100~130 Ma,500~550 Ma,750~900 Ma,1800~1900 Ma及~2500 Ma等年龄峰值。碎屑岩组分、锆石年龄特征及其他分析显示,拉萨地体、羌塘地体及缝合带内的逆冲带为其物源区,存在多向物源。在古近纪碰撞、挤压的构造条件下,伴随着地壳缩短、逆冲断层及造山系统的活动,尼玛盆地南部演化为一个受构造活动控制的独立沉积中心;挤压及逆冲变形决定了其古地理特征,沉积过程及物源与区域隆升、剥蚀活动联系密切。

The Nima Basin Paleogene detrital rocks,which has been recognized as the sedimentary records of the evolution of the Tibetan plateau,locates in the Bangonghu-Nujiang suture zone.The Paleogene paleogeography and tectonic evolution history of the Nima Basin is restored based on the study of petrology and provenance of the sediments in this contribution.Fan delta facies and lacustrine facies were identified in the Niubao Formation and Dingqinghu Formation of Paleogene in the south of the Nima Basin based on analyses of the rock assemblage and sedimentary features.The stable clastic composition significantly decreases and the unstable clastic component increases from the Niubao to the Dingqinghu Formation.This change indicated that the sedimentary process in the south depression of the Nima Basin was controlled by the tectonic movement during Paleogene.Detrital zircon U-Pb ages display six peaks of36–70Ma,100–130Ma,500–550Ma,750–900Ma,1800–1900Ma and ca.2500Ma.The geochronological features and sedimentary features indicated that the Lhasa terrane,the southern margin of the Qiangtang terrane and thrust belts in the Bangonghu-Nujiang suture zone are important sources for the Niubao and Dingqinghu Formations.With the shortening of the crust and the activation of tectonic movements,the south of the Nima Basin evolved into a depocenter controlled by the tectonic activity of collision and extrusion.Palaeogeographic characteristics were altered by the extrusion and thrusting deformation.The sedimentary processes and sources are closely related to the activities of uplift and erosion in the Nima Basin.