Detrital Zircon Geochronology And Its Tectonic Significance In The Ancient North Qilianoceanic Subduction Zone

The North Qilian orogenic belt is located in the northeastern margin of the Tibetan Plateau,its tectonic evolution is based on the convergence and splitting of Neoproterozoic Rodiniasuper continent,and the experienced the development of Qilian ocean in Early Cambrian Ordovician,the ocean crust subduction,the Silurian and Devonian transition,andthe collision orogeny.At present,there is a lot of controversy about the subduction stage of oceanic crust in the North Qilian orogenic belt,and the provenance of the sedimentary rocks in the high-pressure metamorphic belt is not further studied.The research objects here are the sedimentary rocks in the North Qilian high pressure metamorphic belt.Based on the field geological survey,the detrital zircon U-Pb dating of Ordovician sedimentary rocks was studied;the regional tectonic evolution is combined with the previous understanding of the North Qilian orogenic belt of ocean continent converging,to study the key geological problems,such as the provenance of surrounding continental blocks during the stage from North Qilian ocean-continent collision to uplifting and associated magmaticactivities.Ocean-continent convergent stage formed in the sedimentary rocks include: sample Q15-04(molasse),Q15-124(meta-greywacke),Q15-129(blueschist facies metamorphic sandstone)and Q15-165(meta-siltstone).The detrital zircon U-Pb age spectra reveal that the North Qilian orogenic belt containssedimentaryprovenance across the Archean and early Paleozoic era,with the peak ages of 2.5Ga,1.7Ga,1.4Ga,1.0Ga,0.76 Ga and 0.47 Ga.Comparison with the adjacent plate tectonic magmatic events indicates that the Archean detrital zircon mainly originated from northern Alashan block metamorphic basement;Late Proterozoic Neoproterozoic detrital zircon mainly originated from the Qilian terrane metamorphic crystalline rock series,withcontribution from the Southern Yangtze;Early Paleozoic detrital zircons were from the North Qilian arc volcano rock.The peak age of 0.76 Ga in the age spectrum is consistent with previous studies on the age of rift volcanic rocks developed by Rodinia supercontinent cleavage in this area.It shows that the initial extension of Qilian Yang were formed at 0.76 Ga.The detrital zircon U-Pb age spectrums of samples Q15-80(fine sandstone),Q15-98(metasandstone)and Q15-102(blueschist facies metamorphic sandstone)show that the detrital zircons were mainlyderived from the North Qilian arc volcano rock,ophiolite and subduction complex,indicating the occurrence of island arc volcanic rocks at 540 ~ 430 Ma,Which is consistent with the subduction time of the ancient North Qilian oceanic crust.The sedimentary rock formed in the collision stage: the detrital zircon age spectra of samples Q15-78(Triassic sandstone)and Q15-79(Permian sandstone)reveal that the source of the sedimentary rocks is mainly from the Alashan block,the North Qilian island arc and the eastern Kunlun area.Based on above studies,zircons in the sedimentary rocks of the high-pressure metamorphic belt in the north Qilian orogenic belt was mainly derived from the adjacent Precambrian metamorphic crystalline base of the Qilian block,the Precambrian metamorphic crystalline base of the Alashan block,and the North Qilian island arc.The late orogenic stage was mixed with the East Kunlun region material.Theage spectra of the North Qilian orogenic beltshow similarity characteristics of Yangtze plate.