Chronology Of Detrital Zircons From The Precambrian Basement Rocks In The Qilian Orogenic Belt And Its Tectonic Implications

The Qilian Orogenic Belt(QOB)is a significant segment of the Central China Orogenic System that traverses from the eastern to the western of China.It is surrounded by the Alxa block in the eastern North China Craton and the Qaidam block.This orogenic belt has experienced complex history of tectonic evolution in Precambrian.Generally,the QOB is divided into three distinct tectonic parts: the North Qilian,the Central Qilian,and the South Qilian belts.The Precambrian low–grade meta–sedimentary strata have been sparsely distributed in these three belts,but their depositional ages and provenance still remain controversial.These problems hinder the reconstruction of the tectonic affinities and the comparison research on the evolution history between its three belts,and thus hamper to discuss the Precambrian tectonic evolution of the QOB.According to these problems,we present new LA–ICP–MS zircon U–Pb ages and Lu–Hf isotopes of detrital zircons from the Mesoproterozoic and Neoproterozoic meta–sedimentary rocks from the three belts of the QOB.Three main conclusions have been obtained below:(1)The depositional age and tectonic settings of the Mesoproterozoic and Neoproterozoic meta – sedimentary rocks have been constrained.The U – Pb chronology results of detrital zircons from the six Precambrian samples show that the two Mesoproterozoic zircon samples exhibit the youngest age population peaking at?1209 Ma and have deposited in an extensional sedimentary basin.For the other four Neoproterozoic samples,the youngest detrital zircon populations were dominantly produced in early Neoproterozoic(1.0–0.8 Ga),and these samples were most likely formed in a convergent sedimentary basin.(2)The three belts of the QOB have same basement and show close affinities to the South China block from Mesoproterozoic to Early Neoproterozoic.Two Mesoproterozoic zircon samples from the North Qilian and the Central Qilian display similar age spectra and Hf isotopic compositions of detrital zircons,which are consistent with the characteristics of that from the coeval rocks in the South Qilian.This reveals that the three belts might be of a united block in the late Mesoproterozoic.In addition,compared with the contemporaneous sedimentary rocks from the neighboring blocks,the QOB is suggested to have tectonic affinity to the South China block.Similarity,four Neoproterozoic meta–sedimentary samples,from the Gaolan and Huangzhong groups in the Central Qilian and the Hualong and the Wulan groups in the South Qilian,also share same basement and have a close relationship with the South China block.(3)The juvenile crust materials increased during 1.6–1.2 Ga,which reacts to the characteristics of the exterior orogenic belt.This change was likely caused by the breakup of the Columbia supercontinent.In 1.2–0.8 Ga,however,the percent of old crustal reworking materials raised,which is consistent with the features of the interior orogenic belt and closely related with the assembly of the Rodinia supercontinent.Combining with the cumulative curves of our detrital zircons samples,it is indicated that the tectonic setting of the QOB in Mesoproterozoic and Early Neoproterozoic had changed from the extensional setting to the convergent setting.According to these changes,we suggest that the QOB has been involved by the exterior and interior orogens from Mesoproterozoic to Early Neoproterozoic sequentially,which was corresponded to the transition from the breakup of Columbia to the assembly of Rodinia.