【作者】 李楠；

【导师】 朱利东；

【作者基本信息】 成都理工大学 ， 沉积学（含：古地理学）， 2021， 硕士

【摘要】 沉积盆地记录了造山过程的重要信息,可为造山带精细结构研究、造山作用历史恢复和古地理重建提供沉积学依据。本文聚焦冈底斯东段特提斯洋俯冲-增生过程在昌果地区的盆地记录,选取西藏昌果地区上三叠统-下侏罗统刘琼组为研究对象,以野外地质调查为基础,开展岩石学、沉积学、锆石U-Pb年代学、微量元素以及Hf同位素分析,对刘琼组地层特征、地层沉积时代和物源进行系统研究,进而综合讨论盆地属性及其大地构造意义,为探讨冈底斯东段特提斯洋俯冲-增生过程提供沉积学约束。研究表明,刘琼组以碎屑岩为主体,包括复成分砾岩、含砾砂岩、岩屑杂砂岩、粉砂岩和泥岩,向上可见微晶灰岩和泥灰岩,局部见火山岩夹层,总体上以深海-半深海沉积为主,底部为斜坡相,以重力流沉积为主体,碎屑岩中见包卷层理、鲍玛序列和滑塌构造等沉积构造;上部逐渐向混积陆棚相过渡;局部可见火山喷发相。获得刘琼组3件砂岩样品最年轻碎屑锆石U-Pb年龄分别为206.8Ma、204.5Ma和206.2Ma,侵入刘琼组中石英闪长玢岩和闪长岩的年龄分别为137.2～141.2 Ma(加权平均年龄为139.2±0.5 Ma)、137.7～141.6 Ma(加权平均年龄为139.4±0.6 Ma),结合火山岩年代数据将刘琼组地层时代厘定为晚三叠世-早侏罗世。砂岩碎屑组分统计结果和碎屑锆石微量元素特征显示刘琼组碎屑沉积岩物源区具有火山弧构造背景,结合砂岩的锆石U-Pb年龄和Hf同位素组成,反映刘琼组物源来自冈底斯南缘同期岩浆弧。综合刘琼组基底属性、岩石组合、沉积环境及其物源特征,论文提出刘琼组为冈底斯南缘岩浆弧前缘弧前盆地沉积,为雅鲁藏布江洋向北俯冲、向南增生过程的盆地响应。更多还原

【Abstract】 Sedimentary basins record important information about the orogenic process,and can provide sedimentological evidence for the study of the fine structure of orogenic belts,historical restoration of orogeny,and paleogeographic reconstruction.This article focuses on the basin records of the Tethys ocean subduction-accretion process in the Changguo area of the eastern part of the Gangdese,and selects the Upper Triassic-Lower Jurassic Liuqiong Formation in the Changguo area of Tibet as the research object,and carried out petrology,sedimentology,zircon U-Pb geochronology,zircon trace elements and zircon Hf isotopic analysis based on detailed field geological surveys.The Liuqiong Formation’s stratigraphic characteristics,stratigraphic sedimentary age and provenance were systematically studied,and then the basin attributes and its tectonic significance were comprehensively discussed,providing sedimentary constraints for discussing the subduction-accretion process of the Tethys Ocean in the eastern section of Gangdese.Studies have shown that the Liuqiong Formation is dominated by clastic rocks,including complex conglomerate,conglomerate sandstone,lithic complex sandstone,siltstone,and mudstone.Microcrystalline limestone and marl can be seen upwards,and can be seen locally volcanic rock interlayer.The Liuqiong Formation is mainly composed of deep-semi-deep sea deposits.The bottom of the Liuqiong Formation is slope facies,with gravity flow deposition as the main body.In the clastic rocks,there are sedimentary structures such as convolute bedding,Bauma sequence,and slump structure;the upper part gradually transitions to mixed shelf facies;volcanic eruptions can be seen locally phase.The youngest detrital zircon U-Pb ages of the three sandstone samples from the Liuqiong Formation are 206.8Ma,204.5Ma and 206.2Ma,respectively.The ages of the quartz diorite porphyrite and diorite intruding into the Liuqiong Formation are 137.2～141.2Ma(weighted average age 139.2±0.5Ma)and137.7～141.6Ma(weighted average age 139.4±0.6Ma),respectively.Combined with the volcanic rock age data,we infer that the Liuqiong Formation’s stratigraphic age is Late Triassic-Early Jurassic.The statistical results of detrital composition of sandstones and the trace element characteristics of detrital zircons show that the source rocks of the clastic sedimentary rocks of the Liuqiong Formation were formed in the volcanic arc tectonic setting.Combining the zircon U-Pb age and Hf isotopic composition of the sandstone,it can be determined that the source of the Liuqiong Formation comes from the magmatic arc of the southern margin of Gangdese.Based on the basement attributes,rock assemblages,sedimentary environment and provenance characteristics of the Liuqiong Formation,we infer that the Liuqiong Formation was a forearc basin deposit on the southern margin of the Gangdese Plate,which was a basin response to the northward subduction and southward accretion of the Yarlung Zangbo Ocean.更多还原