雅鲁藏布江缝合带南缘混杂岩放射虫组合及其地质意义

Radiolarian Assemblage from the Mélange in the Southern Margin to Yarlung-Zangbo Suture and Its Geological Significance

在西藏南部雅鲁藏布江缝合带南侧的吉隆地区出露有一套混杂岩,该混杂岩中见大量的放射虫化石。其研究成果可为恢复缝合带南缘混杂岩地层层序、探讨特提斯古海洋盆地演化、揭示印度-亚洲板块的全面碰撞过程等提供重要的生物地层学证据。将吉隆地区桑单林剖面自下而上划分为宗卓组、桑单林组和者雅组3个岩石地层单元;并在其中发现有晚白垩世—始新世放射虫化石,鉴定出50属、72种,划分了4个放射虫化石带:Immersothorax cyclops带(晚白垩世Campanian期),Spongurus irregularis带(早古新世,RP1—RP3),Buryella dumitricai带(古新世中晚期,RP4—RP6),Phormocyrtis turgida带(始新世早期,RP7—RP8)。通过对古近纪时期特提斯喜马拉雅北亚带的地层、沉积等特征进行横向上的对比分析,认为古新世时处于特提斯喜马拉雅沉积带北亚带西段的吉隆地区为前渊环境,东段的江孜地区则为滨浅海沉积环境,表明西藏境内特提斯洋的闭合并不是同时发生的,而是沿雅鲁藏布江缝合带自东向西发生。

A set of mélange is exposed in Sangdanlin,Gyirong,south to the Yarlung-Zangbo Suture (YZS). The measured Sangdanlin section is lithostratigraphically divided into Zongzhuo Formation,Sangdanlin Forma-tion and Zheya Formation from bottom to top.Abound radiolarian fossils were gained from the mélange and 72 species of 50 genera were identified,which allows the recognizing of four radiolarian assemblages:Immersotho-rax cyclops,Spongurus irregularis,Buryella dumitricai and Phormocyrtis turgida zones.The radiolarian assem-blage points to a late Cretaceous to early Eocene age for the strata of the Sangdanlin section.The research re-sults provide the important paleontologic evidence to restore the stratigraphic sequence of “mélange”in the southern margin of YZS,and they also explore the evolution of the Neotethys and reveal the comprehensive colli-sion process between the India and the Asia plates.