藏南雅鲁藏布江缝合带放射虫研究进展与问题

PROGRESS AND PROBLEMS IN THE RADIOLARIAN RESEARCH ALONG THE YARLUNG-TSANGPO SUTURE ZONE

雅鲁藏布江缝合带是青藏高原上最年轻的一条缝合带,位于青藏高原南端,记录了随着印度大陆向北漂移并最终与欧亚大陆碰撞而消亡的新特提斯洋的演化历史。藏南地区的硅质地层呈东西向断断续续延伸达数千公里,分布于缝合带蛇绿岩南北两侧和特提斯喜马拉雅沉积区。在这些硅质岩地层中发现的放射虫时代从中三叠世安尼期至晚古新世坦尼特期,揭示了特提斯海盆从中三叠世开始,至早白垩世发展为成熟大洋,然后在晚古新世之后闭合的演化过程。不过,目前还欠缺阿普特阶之后到古新世早期的含放射虫深海沉积的确切证据。已报道的晚白垩世的放射虫时代以及萨嘎附近含古新世放射虫地层的时代和构造属性仍存在很大争议。缝合带南北两侧已报道的中-晚三叠世、早-中侏罗世和晚侏罗世-早白垩世早期的放射虫化石均来自于混杂岩中的硅质岩岩块。硅质泥岩和碎屑岩代表了新特提斯洋由远洋环境逐渐到达俯冲边界的重要阶段。目前硅质泥岩基质和碎屑岩中放射虫的研究并未引起足够重视。加强放射虫动物群的全球对比,尤其是东西特提斯洋放射虫动物群的对比,在藏南地区展开系统的高精度的放射虫生物地层研究对完善放射虫化石分带、阐明新特提斯洋的演化、分析特提斯洋俯冲过程和限定印度-欧亚大陆碰撞时限等都具有重要意义。

The Yarlung-Tsangpo Suture Zone(YTSZ),as the southernmost and youngest among the sutures in the Tibet Plateau,marks the place where the Neo-Tethys was finally consumed when the Indian continent moved northward and collided against the Eurasian continent.Siliceous rocks form a nearly continuous exposure which stretches east-west over several thousand kilometers.They are well distributed along the north and south sides of the YTSZ and the Tethys Himalaya.Radiolarian assemblages from these siliceous rocks range from Anisian(Middle Triassic)to Thanetian(Paleocene).It record the Neo-Tethys started to get marine sediments from Middle Triassic,developed into a mature ocean in Early Cretaceous,and closed after Paleocene.Reported radiolarian assemblages,which were assigned to the Late Cretaceous,are indicators of an Early Cretaceous age.There are no reports on radiolarian from Albian to early Paleocene along the YTSZ.Reported Middle to Late Triassic,Early Jurassic and Late Jurassic to Early Cretaceous radiolarian bearing marine sediments are all obtained from chert blocks in the mélange.Marine sediments in the Neo-Tethys was destroyed,disrupted and mixed together by off-scraping,accretion and following India-Eurasian collision.Siliceous mudstone and corse-grained sandstone record the significant stage when the Tethys approaches towards the trench of the convergent margin.Radiolarian assemblages in the siliceous mudstone and sandstone along the YTSZ have not been studied sufficiently.The stratigraphy and tectonic attributes of Paleocene radiolarian-bearing strata near Saga are still controversial.More comparison of radiolarians worldwidely,especially within the Tethys and detailed radiolarian biostratigraphy will make contributions to improving of radiolarian zonal scheme,elucidating the evolutionary history of Tethys,deciphering the subduction processes,and constraining the collision time between Indian and Eurasian continents.