U-Pb Dating And Hf Isotopic Composition Of Detrital Zircon In The Sediments From Three Rivers In Southwestern China And Its Geological Significance

The formation and evolution of continental crust have always been one of the themes for Earth Science. In recent years, floodplain detrital zircon has become a powerful tool to study crustal growth and evolution.The Qinghai Tibet Plateau is the world’s top, the formation and evolution history has been concerned by domestic and foreign geologists. The Jinsha River, Lancang River and Nu River are flowing along three important suture zones of the Tibetan Plateau, which are Xijin Wulan-Jinshajiang suture zone, Longmucuo-Shuanghu-Lancangjiang suture zone and Bangonghu-Nujiang suture zone. Three suture zones are block boundaries between Songpan-Ganzi block, North Qiangtang block and Lhasa block. We try to study possible source region, time of major geological events occurred, the collision time of adjacent blocks, and the Paleo Tethys ocean history from detrital zircon U-Pb age. We try to explore the time of crustal growth, and the formation of crust from Hf isotopic composition of detrital zircons. The results are as follows:1) The populations of detrital zircon U-Pb ages of the Jinsha River are: 98 Ma, 175~355 Ma, 402~639 Ma, 730~1200 Ma and 1409~3160Ma. The Lancang River’s detrital zircons exhibit populations of <177 Ma, 201~418 Ma, 428~544 Ma, 581~1189 Ma and 1406~2533 Ma. The Nu River’s detrial zircons are consisted of 50~74 Ma, 84~235 Ma, 275~413 Ma, 451~569 Ma, 657~1429 Ma and 1602~3436 Ma.2) The source region of the Jinsha River’s and Lancang River’s sediments may be the North Qiangtang block. The source area of the Nu River River’s sediments is mainly from the middle Lhasa block, with a small amount of North Lhasa block.3) The Jinsha Ocean has a history from early Carboniferous to late Triassic The Qiangtang block and Songpan- Ganzi block collided.during the early Triassic and middle Triassic. The Lancang Ocean evolved from Devonian to the end of Triassic.The North and South Qiangtang blocks collided in Permian. The Nu ocean evolved from Triassic to early Cretaceous, which closed bythe collision between the Lhasa block and Qiangtang block.4) The period from Neoarchean to Mesoproterozoic is the continuous stage of crustal growth from the depleted mantle in the North Qiangtang block, which accounts for more than 70% of the whole crust. The most rapid period of crustal accretion of the Eastern-middle Lhasa block is 2.0~1.2 Ga.