雅鲁藏布江缝合带残留古海山研究

Seamounts, the topographic highlands 1000 m above seabeds, document lots of important information on intraplate volcanic activity, oceanic crust accretion and subduction process. As the relict of the several thousands kilometers wide Neo-Tethys Ocean, where have ever been located in the eastern open ocean of the supercontinent Pangea during the Mesozoic era, the Yarlung Zangbo suture zone has preserved seamounts, which are crucial to resocnstruct the disappeared ocean. Based on the observation and measurement of cross sections in southern Tibet, we will analyze the lithofacies, geochemistry, and zircon U-Pb geochronology of the basement rocks to obtain the age and tectonic property of paleosemounts, and study the biostratigraphy and lithofacies of the sedimentary caps with zircon U-Pb geochronology of volcanic ash intercalation to explore the seamount age, paleodepth, depostional environment and their change after the formation of the basement. By the combination of the contact relationship of basement and cap as well as the section correlation along the suture, the total age interval, texture, sequence, and cycles will be determined to reveal and to precisely delineate the geodynamics and evolution history of the seamounts, providing newly geoscientific materials and data for the reconstruction of the Neo-Tethys Ocean in paleoceanography, paleotectonics, and paleogeography.

大洋中高出海底1000m以上的地貌/地质单元海山蕴含有丰富的板内火山活动、洋壳增生、俯冲过程、金属成矿等重要地质信息。雅鲁藏布江缝合带，作为中生代时期全球超级大陆（Pangea）在东部的宽阔喇叭口大洋——新特提斯洋的遗留，保存了较多的古海山。拟在野外地质调查和剖面观测基础上，通过基座火成岩岩相学、岩石地球化学及锆石U-Pb同位素年代学分析，查明古海山的形成时代及大地构造属性；通过沉积盖层的生物地层学、火山灰锆石U-Pb同位素年代学和沉积相分析，阐明基座形成之后的古海山延续时代、古水深、沉积环境及其变化；结合基座与盖层之间的接触关系及相关剖面的时序对比，确定古海山的发育时限、结构类型、层序与旋回；进而在年代格架下揭示并精确刻画古海山的驱动力及其发展历史，为重建新特提斯洋古海洋、古构造格局和古地理提供科学证据。

雅鲁藏布江缝合带的残留古海山是新特提斯洋演化的重要见证者，但因缺乏全面关键的证据至今知之甚少。本课题对雅鲁藏布江缝合带中、西段古海山残留体开展了年代学、岩相学、地球化学、海山结构-层序等多方面的研究，探讨了其动力学和对新特提斯洋的演化意义。结果显示：1）以拉孜浪错一带为代表的古海山形成于中二叠世（Wordian-Capitanian期），在仲巴纳久和偏吉一带的残留古海山建造于白垩纪中期（Barremian晚期-Coniacian期），最晚可到~83 Ma；2）两期古海山的基座野外产出以枕状玄武岩为主，出露有集块岩和块状（杏仁和气孔）玄武岩，均具有OIB（夏威夷）型拉斑玄武岩的主量元素和微量元素配分模式，而仲巴的白垩纪古海山基座部分样品具有MORB型样式；3）拉孜浪错的中二叠世海山盖层有砾岩相、页岩相、泥灰岩相和灰岩相四种，其中的灰岩课区分出10种微相；相应地，仲巴白垩纪海山盖层存在角砾岩、泥岩、硅质岩和灰岩四种岩相，但灰岩中只分辨出两种颗粒灰岩相关的微相；4）拉孜古海山在基座与盖层之间可分辨出正常、冲刷侵蚀、残积角砾和暴露喀斯特面四种接触关系，仲巴古海山也具有前三种接触关系类型，指示它们都经历过海山和海岛过程。差别在于，白垩纪位于海平面之下的古海山基座顶部水深可超过500-1000 m，而中二叠世海山基座顶部的古水深不超过100 m；5）拉孜地区中二叠世古海山是一个复合类型，由8个以上的次级古海山叠加而成；仲巴地区的白垩系则可能含有12（6+6）个以上相对独立的古海山。综合上述结果表明，雅鲁藏布江缝合带的这两期古海山都是洋壳板内构造背景产物，新特提斯洋洋中脊扩张和地幔柱上涌是造成海山过程的关键驱动力，它们分别代表了新特提斯洋最早和最晚时期板内洋壳记录，既指示了新特提斯洋的发生早于中二叠世即初始打开可能在早二叠世甚至更早，又说明在白垩纪中-晚期（Barremian晚期-Santonian）直到~83 Ma新特提斯洋洋壳增生还在继续。