The Establishment Of "Zhikong-Songduo Ocean" In The Eastern Gangdese,Tibet

The discovery of oceanic island volcanic rocks in Zhikong-Songduo area,the eastern Gangdese,Tibet is a great significance to the understanding of the ocean evolution in this area.In the field geological characteristics,the Pairigang ocean island has the typical binary structure characteristics of ocean island,including basalt and overlying carbonate cap,and the landslide accumulation characteristics of ocean island can be seen locally.On the basis of field geological survey,this paper makes a preliminary study on the Pairigang ocean island basalt found in the Zhikong-Songduo area,the results show that the basalt in this area has high content of TiO₂?Fe₂O₃T?P₂O₅?the average is 2.05%?10.57%?0.43%?,high rare earth content(the total is213.0×10^-6289.2×10^-6),strong fractionation of light rare earth and heavy rare earth?LREE/HREE:7.16⁸.63?,and the rare earth elements and trace elements are similar to the standard oceanic island basalt?OIB?.The results of structural discriminant diagram show that the basalt in the Pairigang area is formed in the oceanic island environment within the oceanic plate.The results of multiple trace elements La/Nb,Th/Nb,Zr/Nb,Zr/Y,Ce/Pb,Nb/U ratio and?Th/Ta?_PM-?La/Nb?_PMM diagrams show that basalt magma may be derived from mantle thermal plume upwelling of asthenosphere mantle and has suffered from obvious mixing of lower crust and subcontinental lithosphere mantle.The Zr/Nb-Ce/Y and La/Sm-Sm/Yb diagrams indicate that basalt magma may have suffered a degree of partial melting of garnet-spinel peridotite.The dating analysis shows that the granite porphyry La-ICP-MS is 190.4±2.8Ma,which belongs to the early Jurassic.The La-ICP-MS zircon U-Pb dating results of the ocean island basalt and the ocean island volcanic rocks are 274Ma and 275M respectively,which are consistent with the dating results of the ocean island rocks found in Songduo area of the same belt,indicating that the Pairigang ocean island is formed in the early Permian.According to the discriminant diagram of tectonic environment and dating analysis results,at least in the late Palezozic?early Permian?,there is a mature oceanic environment in the Zhikong-Songduo area,and Pairigang ocean island basalt should be the remnant of Zhikong-Songduo ocean.Pairigang OIB in this study and other dates incluing OIB mafic volcanic rocks and MORB and OIB-type eclogites suggest that Zhikong-Songduo area exsits a mature ocean basin in the early Permian,and may experience the completely evolutionary process,such as expansion,subduction and collision.