Geochemistry And Geochronology Of Eclogites From Altyn Tagh And Its Geological Significance

The ultrahigh pressure metamorphism (UHPM) have been initially treated as a petrographic oddity by the geologic community, otherwise, they have now recognized as a normal feature of continental plate collisional orogen and important to understand how deep the upper part of the continental lithosphere can be subducted. Taken the eclogite developed in the Altyn Tagh UHPM belts and their wall rocks as objects, the purpose in this study is to present the fresh conclusions listed below on the basis of the geological field work, geochemistry, zircon genesis mineralogy and chronology in the study area. In addition, the comparison has been done for the other UHP rocks and eclogite from the North Qaidam.The eclogite distribution, assemblage and their field relationship geologically had been determined, and their wall rocks are composed mainly of granitoid gneisses, garnet-bearing biotite gneisses and marble.The isotopic and geochemical characteristics of the eclogite and their wall rocks in the Altyn Tagh suggests that the primary rocks of the eclogite could be OIB or E-MORB with positive ε_Nd values, implying the magmas derived from a depleted mantle source, also indicate an important contribution of EM components. Whereas, the wall rocks are characterized by a closely affinity to the rocks formed in the continental margin setting.The new zircon dating yields 484±6Ma for the UHP eclogites and 499±27Ma for biotite-gneiss in the Altyn Tagh, respectively, which not only imply the UHP eclogite and their wall rock had undergone the ultrahigh pressure Metamorphism simultaneously, but also indicate the UHP rocks was formed in the east and west region in the study area, constructing a typical UHP belts.On the basis the discovery of stipoverite in the argillaceous schist, the primary features of eclogite and the consistent age between the UHP eclogite and their wall rocks, this paper proposals that the UHP eclogite in the Altyn Tagh is the production of the continental lithosphere deep subduction accompanied by the old oceanic crust, which had uplifted to the crust formerly.By the analysis of the primary rocks and metamorphic age in the Altyn Tagh and North Qaidam, the UHP eclogites in the North Qaidam area were derived from the continental slab subduction, and then back up to the surface. Whereas, the UHP eclogites belts in the Altyn Tagh may be the part of Xitieshan UHP rock as a result of the west extending of the North Qaidam UHP belts.