藏南古近纪甲查拉组物源分析及其对印度-欧亚大陆碰撞启动时间的约束

位于特提斯喜马拉雅北亚带的江孜地区古近纪甲查拉组角度不整合于晚白垩世宗卓组之上,系该地区最高(时代最晚)海相地层。运用岩石学和地球化学方法对其进行分析研究结果表明该组物源区主要为近源再旋回造山带,岩屑的母岩类型主要是岩浆弧成因的中性、中酸性安山质火山岩。新生代以前,特提斯喜马拉雅属于印度板块的被动大陆边缘,从特提斯喜马拉雅南亚带向北亚带显示了一种从浅水陆棚到深水盆地的变化,在侏罗-白垩纪时其陆源碎屑物主要是成熟度极高的石英砂岩,所以甲查拉组的碎屑物质只能来源于当时的冈底斯弧地区,所获有限的古水流证据也指示了这一点。从欧亚大陆侵蚀下来的碎屑物质被带到原印度大陆地区沉积,暗示该区的特提斯洋壳已经完全消失,印度与欧亚大陆在特提斯喜马拉雅中、东部产生了初始的陆-陆碰撞,其碰撞的启动时间为甲查拉组开始沉积的65 M a±。

PROVENANCE ANALYSIS OF PALEOGENE GYACHALA FORMATION IN SOUTHERN TIBET :IMPLICATION FOR THE INITIATION OF COLLISION BETWEEN INDIA AND ASIA

Gyachala Formation is Paleogene marine strata in Gyangze area,which belongs to the northern subzone of the Tethyan Himalaya.The paper studied the lithology of sandstones and the type of disperision pattern of sediment,analyzed the chemical composition of volcanic grains found in the sandstones by electron microprobe analysis.The data show that the provenance of the Gyachala Formation is proximal recycled orogen,with the lithic detritus comprised by andesitic igneous rocks,derived from an erosoin of a volcanic arc.The Tethyan Himalaya tectonically belongs to the India passive continental margin.The sedimentary facies on this margin vary from shallow water shelf in the southern subzone of the Tethyan Himalaya to deep water basin in the northern subzone of the Tethyan Himalaya.In the period from Jurassic to Cretaceous,the terrigenous clastics of the Tethyan Himalaya were mainly comprised by quartzose sandstones of very high maturity,derived from cratonic areras of India continental plate.In contrast,the composition of volcaniclastic sandstone of Gyachala Formation was from Gangdese volcanic arc area,which is located on the Asian continental margin.This interpretation is also supported by the south-direction palaeocurrent data.During the Gyachala Formation deposition time the detrital material eroded from Gandese volcanic arc was transported across the Yarlung Zangbo Suture Zone and deposited on the India continental plate passive margin.Such development requires presence of a foreland basin or remanent basin.Therefore,the Gyuchala Formation provides evidence that at the time of its deposition Tethyan oceanic crust was already completely subducted and India and Asia continental plates collided,resulting in the development of a foreland basin or remanent basin.Therefore the presence of Gyachala Formation provides evidence that the initial collision between India and Asia plate occurred at about 65 Ma.