雅鲁藏布缝合带中侏罗—早白垩世放射虫年代学研究及其对新特提斯洋演化的意义

发布时间：2018-01-29 19:41

本文关键词： 雅鲁藏布缝合带日喀则弧前盆地混杂岩放射虫侏罗纪-白垩纪　出处：《中国地质大学(北京)》2016年博士论文　论文类型：学位论文

【摘要】：雅鲁藏布缝合带位于青藏高原南部,代表了新特提斯洋消减闭合的部位。一直以来,该缝合带内的蛇绿岩带被公认是新特提斯洋岩石圈的残留。它们与雅鲁藏布缝合带中相关的海相地层共同记录了新特提斯洋演化及印度与欧亚板块碰撞过程。近年来,国内外学者在雅鲁藏布缝合带进行了大量卓有成效的研究,为认识新特提斯洋的演化提供了重要证据。但是,目前关于雅鲁藏布缝合带的研究重点集中在该蛇绿岩的同位素年代格架和地球化学属性等方面,对该缝合带内海相地层的古生物学和沉积学等方面的研究相对薄弱,使得对新特提斯洋的演化缺乏系统认识。本文以雅鲁藏布缝合带西段仲巴地区的侏罗-白垩纪含放射虫远洋硅质(泥)岩以及中段日喀则弧前盆地白垩纪冲堆组底部硅质岩为研究对象,开展了古生物地层学、沉积学和锆石U-Pb年代学等研究,取得了如下主要认识。在雅鲁藏布缝合带西段仲巴地区混杂岩带中实测了7条海相地层剖面。从其中放射虫硅质岩中,鉴定出了中生代放射虫31个属,42个种;识别出了4个侏罗纪放射虫组合和5个白垩纪放射虫组合。结合野外地质观察和构造分析,本文将该地区传统上被认为是正常沉积地层的桑单林组、郭雅拉组和盐多组理解为混杂岩,并将其分解为5个构造岩组:马泉河岩组、塔瓦岩组、尔捌岩组、衣桑岩组和踏弄岩组。根据江木那混杂岩中放射虫的年代序列,重建了大洋板块地层格架,揭示了新特提斯洋在中侏罗世-早白垩世的演化过程,即在早阿普特期,从远洋放射虫硅质岩转变成陆源碎屑为主的泥岩,暗示了新特提斯洋开始俯冲,与蛇绿岩年代学和地球化学研究成果吻合。而侏罗纪-白垩纪的放射虫年代,也否定了前人将该套地层定为始新世的认识。在雅鲁藏布缝合带中段纳虾、群让、则嘎普等地日喀则弧前盆地冲堆组海相地层实测了6条剖面。在其底部硅质岩中鉴定出早白垩世放射虫18个属,26个种,并识别出5个放射虫组合,显示其时代为晚巴列姆-晚阿普特期,与仲巴地区硅质泥岩时代一致。冲堆组上部碎屑岩在QFL图解中,均落入未切割岩浆弧-过渡岩浆弧区域。而碎屑锆石U-Pb年龄表现为显著的110Ma的单峰分布,与冈底斯岩浆岩年龄峰值吻合,表明冈底斯可能是冲堆组碎屑物的主要源区。通过上述研究,并结合冲堆组与蛇绿岩沉积不整合的接触关系,说明新特提斯洋在早白垩世晚期开始俯冲,日喀则弧前盆地开始发育,并接受来自冈底斯岛弧的碎屑物质。
[Abstract]:The Yalu Zangbo suture zone, located in the southern part of the Qinghai-Tibet Plateau, represents the subduction and closure of the Neo-Tethys Ocean. The ophiolite belt in the suture zone is recognized as a remnant of the Neo-Tethys lithosphere. They are associated with the marine strata in the Yarlung Zangbo suture belt to record the evolution of the Neo-Tethys Ocean and the collision process between India and the Eurasian plate. Years ago. Scholars at home and abroad have done a great deal of fruitful research in the Yalu Zangbu suture zone, which provides important evidence for understanding the evolution of the New Tethys Ocean. At present, the study of Yalu Zangbo suture zone focuses on the isotopic chronological framework and geochemical properties of the ophiolite. The paleontology and sedimentology of marine strata in the suture zone are relatively weak. In this paper, the Jurassic-Cretaceous ocean-going siliceous (mud) containing radiolarian in the Zhongba area of the western section of the Yalu Zangbo suture belt is used to make a lack of systematic understanding of the evolution of the Neo-Tethys Ocean. Rocks and siliceous rocks at the bottom of the Cretaceous Chong heap formation in the Xigaze forearc basin in the middle segment are the objects of study. Paleontological stratigraphy, sedimentology and zircon U-Pb geochronology have been carried out. Seven marine stratigraphic sections have been measured in the melange belt in the western section of the Yalu Zangbo suture belt. 31 genera of radiolarian have been identified from the radiolarian siliceous rocks. 42 species; Four Jurassic radiolarian assemblages and five Cretaceous radiolarian assemblages have been identified. Combined with field geological observation and structural analysis, this paper considers this area to be a normal sedimentary formation of Sundanlin formation. The Guoyala formation and the Yanduo formation are understood as melange and decomposed into five tectonic formations: maquanhe formation, Tahuyan formation and lxxiyan formation. According to the age series of radiolarian in Jiangmona melange, the stratigraphic framework of oceanic plate was reconstructed, and the evolution of the NeoTethys ocean during the Middle Jurassic to early Cretaceous was revealed. That is, in the early Apt period, it changed from distant-water radiolarian siliceous rock to land-based clastic mudstone, which indicated that the new Tethys ocean began to subduction. The age of radiolarian from Jurassic to Cretaceous also negates the previous understanding that the strata were defined as Eocene. Six sections were measured in the marine stratigraphy of the chongdui formation in Xigaze forearc basin, etc. 18 genera and 26 species of early Cretaceous radiolarian were identified in the siliceous rocks at its bottom, and 5 radiolarian assemblages were identified. It shows that its age is the late Barreem-late Aputte epoch, which is consistent with the age of siliceous mudstone in Zhongba area. The upper clastic rocks of the Chongdui formation are in the QFL diagram. The U-Pb age of clastic zircon shows a significant single-peak distribution of 110 Ma, which coincides with the peak age of Gangdis magmatic rocks. It is suggested that Gangdis may be the main source of clastic material in the alluvial formation. Through the above study and the contact relationship between the alluvial formation and ophiolite deposits, it is concluded that the Neo-Tethys Ocean began to subduction in the late early Cretaceous. The Xigaze forearc basin developed and received debris from the Gangdis Island Arc.
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P534.5;Q915

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