U-Pb Dating And Hf Isotopic Compositions Of Detrital Zircons In The Sediments Of Tuotuo River In Northern Qiangtang Block

Earth scientists are dedicated to studying the formation and evolution of the continental crust,which are the enduring topic of geology field.The modern rivers are ideal tools uniformly for collecting crust samples because of its strong erosive force and wide draining area.With the development of modern in situ analytical technology,U-Pb dating and Hf isotopic compositions of detrital zircons of the sediments of rivers may be used to trace magma events and continental crust evolution of their provenances.The Qiangtang block is located in hinderland of the Tibet plateau,whose north is adjacent to the Songpanganzi block along with Jinshajiang suture zone and south is connecting to the Lhasa block bound by Bangonghu-Nujiang suture zone.The high-temperature and low-pressure belt of metamorphic rocks developed in the central zone of the Qiangtang block,which is known as the Longmucuo-Shuanghu suture zone so that the Qiangtang block is divided into two micro blocks.Many earth scientists are interested in the Qiangtang block because of the complex tectonic background and evolution,which is known as an important window to monitor the evolution of the Tethys in northern Tibet.The drainage of Tuotuo river is located in the Northern Qiangtang block,with the carboniferous-Jurassic strata.The river sand samples are collected in the downstream areas of the Tuotuo river.The main-trace elements of sand sample show the characteristics of the upper continental crust.The detrial zircon U-Pb age results show six populations:(1)2683~2442 Ma;(2)2031~1498 Ma;(3)1205~751 Ma;(4)613~468Ma;(5)426~102 Ma;(6)39-37 Ma.This paper collects the zircon ages of the Qiangtang block,which stem from the carboniferous to Jurassic strata.Comparing to the Tuotuo river samples,the Northern Qiangtang block contributes all materials for the Tuotuo river.Through analyzing the U-Pb age characteristics of Tuotuo river and combining previous research,this paper concludes that the Northern Qiangtang block not only experienced some major global events,such as the aggregation and cracking event of Rodinia supercontinent as well as pan-african movement,but recorded the magma events of the Tethys ocean evolution as well as the Cenozoic intracontinentalsubduction caused by Tibet Plateau Uplift.Since Neoproterozoic(~1000 Ma),the Northern Qiangtang block turned into a period of continuous magmatism,related to the aggregation and cracking event of Rodinia supercontinent as well as the Tethys ocean evolution and the Tibet Plateau Uplift.During the Mid-Paleoproterozoic(2.0 Ga-1.7 Ga),the magmatism of the Northern Qiangtang bolck is related to the strong tectonic thermal events of the Qiangtang basement in this period,whose materials mainly come from the recycling of old crust.Hf depleted mantle model ages reveal that by the end of Lately Paleoproterozoic,the Northern Qiangtang block has formed the half of the earth's crust.