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Tectonic Evolution of the North Qilian Orogenic Belt from the Late Ordovician to Devonian:Evidence from Detrital Zircon Geochronology

XU Ya-jun;DU Yuan-sheng;YANG Jiang-hai;State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences;Faculty of Earth Sciences,China University of Geosciences;  
The Upper Ordovician-Devonian strata are distributed asymmetrically in the North Qilian orogenic belt(NQOB)and are record of the collision process between Central Qilian block and North China plate.Age spectra of detrital zircons from the Upper Ordovician strata in the eastern of belt suggest that sediments in the basement of Upper Ordovician originate from the North Qilian arc and detritus in the upper part of Ordovician come from Central Qilian block and the North Qilian arc to the South and North China plate to the north.This change of source,integrated with ages of synsedimentary zircons,implies that the time of initial collision between Central Qilian block and North China plate is between 470and 450Ma.The Lower Silurian strata in the western part of belt preserve a large number of grains from Central Qilian block and North Chinaplate,with the infill of sediments from the North Qilian arc and synorogenic granites,which imply the time of collision in the western part of belt.Compared to Ordovician lithofacies and the time of collision in the eastern and western parts,the asynchronous collision between Central Qilian block and North China plate suggest a"diachronous,unequal margin collision"model.This collisional model resulted in the Central Qilian block thrust onto the North Qilian arc and missed from sediments in the Lower Silurian in the eastern part of belt.Sediments form the North Qilian arc re-appeared in the Lower-Mid Devonian.Combined the distribution and deformation of Silurian and Devonian in the whole belt,the orogeny of NQOB is diachronous in the trending direction due to the"diachronous,unequal margin collision".The eastern sector had stronger tectonic intensity compared to the western sector,which resulted in the re-exposure of the North Qilian arc covered by Central Qilian block during the Early Silurian,infilling of synorogenic granites and recycle of the Mid-Upper Silurian.The Hexi-Corridor basin completed the transition from retroarc basin to remnant retroarc basin,to foreland basin during the Late Ordovician-Devonian.
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