北羌塘盆地东部晚三叠世-早侏罗世沉积转换研究—来自地质浅钻岩芯记录的证据

The Late Triassic to Early Jurassic sedimentary succession was well developed in the east of Northern Qiangtang Basin. This succession records a depositional transition history from foreland basin to rift basin. More importantly, the Late Triassic depositional transition is also linked to mass extinction events in southern Tibet, and appears to coincide with large-scale volcanic-eruption and volcanic-sedimentary events in the eastern Tethyan domain. In 2016, the QZ-16 well was carried out in the east of Northern Qiangtang Basin, which drilled the most complete and continuous Late Triassic to Early Jurassic cores for the first time in the Qiangtang Basin and provide a good material to the depositional transformation study. In this work, we choose the core samples of QZ-16 well as well as the typical sections investigation as the main target. Depositional transition event will be identified by basal conglomerate, sedimentary sequence and provenance analysis. The onset age of depositional transition will be carried out by in-situ zircon U-Pb dating. Furthermore, the sedimentary environment across the depositional transition is studied by using mineral analysis and geochemistry. Additionally, the tectonic setting of the depositional transformation surface is determined by petro geochemistry and isotope geochemistry. And at last, a correlation of the regional depositional transition will be studied. This study may have important implications for the Qiangtang Basin evolution and nature, as well as for the Late Triassic biotic-extinction, volcanic-eruption, and regressive and transgressive events.

北羌塘盆地东部具有保存完好的晚三叠世-早侏罗世沉积，这些地层记录了北羌塘盆地从前陆盆地到裂谷盆地的沉积转换。更重要的是，该沉积转换与区域上发生的晚三叠世生物灭绝事件、火山喷发-火山沉积事件有紧密的联系。2016年，在羌塘盆地东部完成的QZ-16井首次钻遇了连续、完整的晚三叠世-早侏罗世地层，这为研究此次沉积转换事件提供了很好的研究素材。本项目以QZ-16井岩芯为主要研究对象，结合典型的剖面地质调查，通过底砾岩、物源区差异分析、沉积演化序列等确定沉积转换；通过同位素年代学确定沉积转换的时间；并通过矿物学、沉积地球化学确定沉积转换界面上下沉积环境变化；通过岩石地球化学和同位素地球化学确定沉积转换界面的性质；最后，开展区域地层的沉积转换对比研究。该研究不仅对于羌塘中生代盆地沉积演化史，确立沉积盆地性质，而且，对于沉积转换与区域生物绝灭事件、火山喷发事件的对比都具有重要意义。

按照项目研究计划，本项目在2018-2020年期间，对北羌塘盆地东部和中部的上三叠统-中下侏罗统连续的钻井岩心和野外剖面开展了系统的调查工作。通过详细的沉积学、矿物学、地球化学和同位素年代学研究，恢复了北羌塘盆地晚三叠世-早中侏罗世沉积充填序列，确定了该时期沉积转换的过程和时间，进一步明确了盆地性质；识别出晚三叠世火山喷发幕事件和火山喷发期次；并揭示了沉积转换前后的沉积古环境、古气候和物源区差异。. 晚三叠世时期，羌塘盆地在前陆盆地萎缩阶段沉积了土门格拉组（或巴贡组），主要形成于三角洲沉积环境，为海相沉积逐渐消亡的海退序列；那底岗日组为同裂谷沉积，代表了羌塘中生代裂谷盆地的开启，为火山爆发-火山溢流沉积环境。中下侏罗统雀莫错组与下伏那底岗日组为不整合接触关系，为裂谷盆地开启后最早的沉积地层，形成于辫状河、局限台地和潮坪环境，经历了从陆相向海相的沉积超覆，与典型的裂谷盆地充填样式一致。在雀莫错南首次识别出完整的晚三叠世火山喷发旋回，自底到顶，获得凝灰岩的锆石U-Pb年龄分别为220.0±1.1Ma、210.9±0.9Ma、208.3±0.8Ma和202.9±1.1Ma，一共包括四个主要喷发期次，火山喷发时间持续了近18Ma。在盆地中部的方湖剖面也识别出了221.7±1.3Ma、217.0±1.5Ma和207.1±1.3Ma三期火山喷发旋回。通过晚三叠世火山喷发的最早年龄，将晚三叠世-早中侏罗世沉积转换的时间限定在221~220Ma左右。地球化学特征显示，晚三叠世火山岩形成于大陆板内拉张的构造背景。在该沉积转换期间，沉积环境从缺氧的海相环境转变为富氧的陆相环境，古气候条件从温暖湿润转变为炎热干旱的气候环境，物源区也从单一的中央隆起转变为中央隆起和松潘-甘孜复理石带的双向物源。晚三叠世火山喷发产生大量的CO2等气体可能导致了区域性的炎热干旱气候和生物的绝灭。. 在项目的执行过程中，培养了2名博士和1名硕士，至今发表论文15篇，其中SCI论文9篇。

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