地学前缘 ›› 2018, Vol. 25 ›› Issue (4): 50-64.DOI: 10.13745/j.esf.sf.2018.5.25

• 岩（矿）石、地层特征及地质意义 • 上一篇    下一篇

北羌塘东部晚三叠世同裂谷作用新证据：来自甲丕拉组火山岩和砾岩时代的重新厘定

李学仁，王剑   

  1. 1. 中国地质科学院, 北京 100037
    2. 中国地质大学(北京) 地球科学与资源学院, 北京 100083
    3. 国土资源部 沉积盆地与油气资源重点实验室, 四川 成都 610081
    4. 中国地质调查局 成都地质调查中心, 四川 成都 610081
  • 收稿日期:2018-02-09 修回日期:2018-03-02 出版日期:2018-07-15 发布日期:2018-07-15
  • 作者简介:李学仁(1986—),男,博士研究生,古生物学与地层学专业。E-mail：xueren.li@foxmail.com
  • 基金资助:
    国家自然科学基金项目(41502112,41702119)；中国地质调查局项目“羌塘盆地金星湖—隆鄂尼地区油气资源战略调查”(DD20160159)

New evidence for the Late Triassic synrifting in the eastern North Qiangtang depression: redefination of the stratigraphic age of volcanic rocks and conglomerates from the Jiapila Formation.

LI Xueren, WANG Jian   

  1. 1. Chinese Academy of Geological Sciences, Beijing 100037, China
    2. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
    3. Key Laboratory for Sedimentary Basin and Oil and Gas Resources, Ministry of Land and Resources, Chengdu 610081, China
    4. Chengdu Center of China Geological Survey, Chengdu 610081, China
  • Received:2018-02-09 Revised:2018-03-02 Online:2018-07-15 Published:2018-07-15

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摘要： 北羌塘坳陷东部沱沱河地区甲丕拉组主要为一套火山岩夹砾岩地层，与区域上以紫红色碎屑岩沉积地层为主的甲丕拉组，无论是在沉积特征上还是地层时代上均相差甚远。为了准确限定该套岩石组合的时代，本文对囊极和开心岭地区甲丕拉组火山岩、砾岩和雀莫错地区巴贡组砂岩开展了锆石UPb年龄测定。结果表明甲丕拉组玄武粗安玢岩的锆石谐和年龄为(216.4±1.1) Ma(n=28，MSWD=0.39)，在复成分砾岩和巴贡组砂岩中获得的最年轻碎屑锆石UPb年龄加权平均值分别为(222.2±3.7) Ma(n=2)和(222.8±1.9) Ma(n=9)，将火山喷发和碎屑岩最大沉积时限均限定在晚三叠世诺利期。结合区域上已发表数据，与晚三叠世连续地层剖面和最新钻井岩性柱做对比，我们将甲丕拉组火山岩重新厘定为那底岗日组，砾岩为晚三叠世北羌塘隆升剥蚀的产物。晚三叠世中晚期，北羌塘大部分地区隆升暴露，遭受风化和剥蚀，发生短暂沉积间断，在西部肖茶卡组顶部形成一层岩溶风化壳，东部结扎群顶部存在暴露面，砾岩的成分也不相同，表明东西部风化程度强弱不同。由于羌塘南部班公湖—怒江特提斯洋的开启，北羌塘进入同裂谷阶段并爆发大规模火山喷发，在西部和东部广泛发育那底岗日组火山沉积地层。而且，那底岗日组在地表的露头与上三叠统顶部裂陷基底匹配良好，表明这是一次区域性与裂陷作用相关的火山沉积事件。

关键词: 关键词：锆石U-Pb年龄, 甲丕拉组, 同裂谷作用, 火山沉积, 北羌塘, 晚三叠世

Abstract: The Jiapila Formation in the Tuotuo River region, North Qiangtang depression is mainly composed of volcanic rocks intercalated with conglomerates. It is very different from the regional Jiapila Formation of primarily purplishred clastic rocks in either sedimentary characteristics or stratigraphic age. In this paper, we performed magmatic and detrital zircon UPb dating of the Jiapila Formation, involving volcanic rock and conglomerate in the Nangji and Kaixin Ridge, as well as sandstone of the Bagong Formation in the Quem Co area, to accurately determine the stratigraphic age of these rocks. Zircon UPb dating from the basaltic trachyandesite in the Jiapila Formation yielded a concordant age (weighted mean 206Pb/238U age) of 216.4±1.1 Ma (n=28, MSWD=0.39). The youngest UPb age of detrital zircons from polymictic conglomerate and sandstone samples gave weight mean ages of 222.2±3.7 Ma (n=2) for the Jiapila Formation and 222.8±1.9 Ma (n=9) for the Bagong Formation, respectively. Therefore, we placed the eruption age and the maximum depositional ages for these rocks at the Late Triassic Norian. Combined with regional historical data and lithological column of the latest well with the continuous sequences, we redefined volcanic rock of the Jiapila Formation as Nadigangri Formation, and confirmed that the conglomerate was formed by the tectonic uplift in the Late Triassic. During the mid to late stage of the Late Triassic, most areas of the North Qiangtang was subjected to the uplift and erosion. In the hiatus, a layer of weathering crust on limestones was formed at the top of Xiaochaka Formation in the west, along with the formation of an exposure surface on top of the Jiezha Group in the east, as well as formation of the distinct components of conglomerates, indicating varying degree of weathering between west and east. Due to expansion of the BangongNujiang Tethys Ocean, the North Qiangtang rifted with massive volcanic eruptions, causing widescale deposit of the volcanicsedimentary sequences of the Nadigangri Formation in the east and west. Furthermore, exposure of the Nadigangri Formation matched well with the rifting basement of the Upper Triassic strata, which indicates the volcanicsedimentary event was a localized event related to rifting in the region.

Key words: zircon U-Pb age, Jiapila Formation, synrifting, volcanicsedimentary, North Qiangtang, Late Triassic

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