The Rock Sequences And Geochemistry Features Of The Zhonggang Ocean Island Volcanic-Sedimentary Strata,Gaize,Tibet

The research area is located at the center of the Tibetan Plateau, within the area of the Gaize city of the Ali region. As the primary target in my research,the Zhonggang ocean island is in the western part of the Bangong Co-Nujiang River suture zone. In order to further explore the evolutionary history of the Bangong Co-Nujiang RiverOcean during the Triassic and Jurassic, I have set characteristics and structural properties of the Zhonggang ocean island formation, through field route survey, the measured profile, thin section identification, rock geochemistry analysis and indoor comprehensive study. The main finding of this work are summarized as the followings:1.Afterfield route survey and measured sections, the contact relationships between the Zhonggang ocean island formation and the Muga Kangri group are found both sedimentary contact and unconformitycontact. The contact relationships between the Zhonggang ocean island formation and the Dong Tso ophiolite is faulted contact only.2.Someaquatic animals and corals, that existed during late jurassic, have been found in the Zhonggang ocean island limestone. What's more, conodont have also been found, which means the Zhonggang oceanisland already exists in Triassic.3.Undersea basalt eruption and breccia basalt(or breccia limestone) can be regard as the sign of basement in the Zhonggang ocean island formation. Limestone can be regard as the sign of overlying strata. Rocks,which were oxidized in to red, can indicate the changing of sea level. Shale and siliceous ooze can can be regard as the sign of the end of one cycle. In breif, basalt-breccia basalt- breccia limestoneLimestone- biohermal limestone-shale can be seen as an integrated cycle in the Zhonggang ocean island formation.4 Through the rock geochemical analysis, it is concluded that the Zhonggang ocean island formation basic volcanic rocks belongs to calc-alkaline series and high in potassium calc-alkaline series. Distribution curves of REE chondrite standardized the whole change law is consistent, light and heavy rare earth fractionation characteristic is obvious; the total REE of basic volcanic rock is higher than volcanic rocks' s. LREE is more enrichment than HREE in both of them,which show that the right curve of light rare earth element enrichment, no obvious Eu anomaly.5.Bycontrast with the characteristic of hy-gromagmatophile elements, the Zhonggang ocean island basalt can be classified as HIMU OIB, and geotectonic setting of the Zhonggang oceanisland is ocean island but Island arc.6.Based on Gaizeregional geological survey projects, we derived the evolution of Bangongco-Nujiang neo-tethys oceanic by sedimentary environmentsand geochemical characteristics:Opened in Late Permian, expanded rapidly in Early Triassic, and in the late Triassic- Jurassic period to reach maturity,gradually closed in late Jurassic- early Cretaceous, eventually completely closed in the early Cretaceous.