西藏南部绒布地区上白垩统宗卓组沉积相与沉积环境演化

Sedimentary facies and environment evolution of the Upper Cretaceous Zongzhuo Formation in the Rongbu area,southern

宗卓组地层作为中生代海相沉积的重要组成部分,记载了西藏南部地区白垩纪古海洋、古气候等重要地质历史信息。通过对砂岩进行镜下观察、物源分析、粒度分析等方面的研究,笔者等对绒布地区上白垩统宗卓组的沉积相类型和沉积环境演化有了新的认识。粒度分析指示沉积时期水体搬运动力较弱、存在浊流沉积;宗卓组内3种不同类型的砂岩镜下特征、地球化学特征和锆石U-Pb年龄,指示3类砂岩的物源不同。笔者等在宗卓组共识别出陆棚相、大陆斜坡相和深海盆地3类沉积相,并对其进行了更为详细的划分。晚白垩世新特提斯洋壳向北俯冲,俯冲结束后印度—亚洲大陆发生初始碰撞。结合研究区构造背景分析,笔者等推测这是导致研究区沉积环境由半深海环境变为深海环境的原因。对研究区宗卓组地层和砂岩来源进行分析后得出:在洋壳俯冲阶段,宗卓组为被动大陆边缘沉积;在之后印度—亚洲陆陆碰撞的初始碰撞阶段,特提斯喜马拉雅被动陆缘已不复存在,并完全过渡为前陆盆地。

Based on the study of sandstone observation,provenance analysis and grain size analysis,this paper has a new understanding of sedimentary facies types and sedimentary environment evolution of the Zongzhuo Formation in Rongbu area.Results:Grain size analysis indicates that the water transport power is weak and turbidity deposition exists in the sedimentary period.The microscopic characteristics,geochemical characteristics and zircon U-Pb ages of three different types of sandstones in the Zongzhuo Formation indicate that the provenance of the three types of sandstones is different.In this paper,three types of sedimentary facies are identified from the Zongzhuo Formation,namely shelf facies,continental slope facies and deep-sea basin facies,and they are classified in more detail.Conclusions:The Late Cretaceous new Tethys Ocean crust subducted northward,and the initial collision between the Indian and Asian continents occurred after the subduction.Combined with the structural background analysis,this paper supposes that this is the reason that the sedimentary environment in the study area changes from semi-deep-sea environment to deep-sea environment.Based on the analysis of the strata and sandstone sources of the Zongzhuo Formation in the study area,it is concluded that the Zongzhuo Formation was a passive continental margin deposit during the subduction stage of ocean crust.During the initial phase of the Indo—Asian land—continental collision,the passive continental margin of the Tethys Himalaya ceased to exist and completely transitioned into a foreland basin.