Study On The Tectonic Evolution Of The Tuolai River Area In The Northern Margin Of The Central Qilian Block

The central Qilian block,represented by the Tuolai River area near the Yushigou phiolite,underwent complex multi-phase and multi-stage tectonic evolution which yielded different types of magmatic rocks,thus involving in major issues about continental dynamics.In the 1:50000 regional geological survey,we carried out field data,petrographic and mineralogic examinations,observations on microscopic structure,geochemical analysis,zircon U-Pb dating,etc.Therefore,we've got some new discoveries and new insights on the issues of the tectonic evolution of the northern margin of the central Qilian block.(1)We redefined the Tuolai Group of the basement of the cen Qiltral Qilian block,and thus firstly determined its diagenetic age of 1178~897 Ma by the results from LA-ICP-MS zircon U-Pb dating.The detrital zircon age distribution characteristic(peaked at ca.1780Ma)is comparable to the Mesoproterozoic and Neoproterozoic detrital zircons from northern and eastern North China Craton,suggesting the Qilian Block has a closer affinity with North China Craton.(2)We discovered retrograded eclogites in the metamorphic basement of the Dayanglong area of the northern margin of the central Qilian block.Petrographic and mineralogic examinations,together with coesite pseudomorph revealed ultrahigh-pressure(UHP)metamorphism at the peak metamorphic conditions which estimated 690±7? in case of the pressure up to 30±3Kbar,while the estimated metamorphic conditions revealed a clockwise p-T path,suggesting the subducted slab has undergone a process of rapid subduction and exhumation.Results from zircon U-Pb dating show that the age of 485±22Ma may represent the top of the UHP metamorphic time.According to geochemical analysis and tectonic discrimination,the protolith of retrograded eclogites is MORB,which likely came from the North Qilian oceanic crust.The above results,combined with arc magmatic rocks in the northern margin of the central Qilian block,confirm that the North Qilian oceanic plate had experienced south-north bi-dive subduction during the Late Cambrian-Middle Ordovician.(3)We found a high Sr syenite granite belonging to the shoshonite series,whose age is 440.3±2.1 Ma from zircon U-Pb dating.The tectonic environment is identified as collisional granite,which may be formed from slab breakoff.It is speculated that it is the product of about 440~410 Ma forward arc-continental collision,the same as the thrust ductile shear zone on the north slope of Tuolaishan.410~380 Ma is the period of oblique arc-continental collision,forming a huge right-lateral strike ductile shear zone and regional post-collision granite in the northern margin of the central Qilian block.(4)Combined with the detrital zircon U-Pb geochronology analysis,provenance and filling sequence,together with the subsidence and uplift of the Carboniferous-Triassic basin had been inferred.In the Jurassic-Cretaceous period,the strike-slip faults controlled the development of intermontane basins and the rapid accumulation of sediments,which also caused further uplifting and exhumation of the Tuolainanshan and Tuolaishan mountains.(5)Three Cenozoic fault tectonic systems were established in the Tolainanshan fault zone,the Tolaihe fault zone and the Tolaishan fault zone.The former is the imbricate backward propagation thrust nappe,while the latter two inherited the pre-existing faults.All the faults were responses of the collision of the Indian-Eurasian plate on the northern margin of the Qinghai-Tibet Plateau.Combined with previous studies on tectonochronology,three stages of Cenozoic tectonic evolution are determined: the backward propagation thrust faulting of the Tuolainanshan fault zone,the re-activity of the Tuolaihe and Tuolaishan fault zone,and the neotectonic movement.