The Nature Of The Lithosphere Mantle In Souther China Craton:Constraints From Cenozoic Basalts And Mantle Xenoliths

The South China Craton lithospheric mantle has attenuated gradually since Cenozoic, and the understanding of lithospheric mantle characteristics and ambient condition desires constraints from geochemical data. This article studies Cenozoic basalts and mantle xenoliths, of which samples collected from Dayang Chao and Bailin, Fujian; Jiucai Di, Jie Yang, Guang Dong. The research contains major and trace elements analysis of basalt whole rocks and analysis of fluid compositions of mantle xenoliths, by which the magmatism, lithospheric mantle characteristics and environment conditions of South China Cenozoic Craton are discussed. The main conclusions are listed as follow:1.The cenozoic basalts in South China are mainly alkaline series, including basanite、trachybasalt and alkaline basalts.with SiO240.21%～49.15%; Na2O+K2O2.08%～5.87%, mainly lying between4.0%～6.0%; Mg#52.45%～72.10%, suggesting parts of the samples collected may be close to primary magma. In Harker diagram, MgO-Al2O3、TFe2O3、K2O、TiO2are negatively correlated, which may indicates a fractional crystallization process of olive, but an unobvious crystallization process of plagioclase and ilmenite.2.The distribution patterns of trace elements and REE of South China Cenozoic shows OIB characteristics, with Ta and Nb positive anomaly, La-La/Sm positively correlated in the diagram. The samples locate near EM II oceanic island basalt, indicating that trace element change in the basalt mainly depend on partial melting process, and fractional crystallization and continental matter contamination is low during basalt magma uplifting.3.Fluid compositions of the mantle xenoliths are released at200-600℃,600-800℃, and800-1100℃intervals in the stepwise heating process. The fluids released in800-1100℃mainly derive from volatiles and fluid compositions traped during mineral crystallization, with an average of CO54.6%, CO230.0%; in600-800℃, fluids mainly derive from subsequent metasomatism, with an average of CO236.9%, CO32.9%; and in200-600℃, mainly from alteration, with CO241.5%,CO23.5%.4.Incompatible element ratio in the basalts of South China Craton in Cenozoic is similar to that of EM II oceanic island basalt. The basalt derives from asthenospheric mantle and has compositions fo lithospheric mantle, suggesting that the formation environment is tension continental rift and that the attenuation of lithosphere may relate with the uplifting of asthenospheric mantle.5.According to the chemical compositions, the fluid can be divided into three groups:primary fluid, mantle metasomatism fluid and subsequent volatiles. The fluid of lithosphere in primary evolution stage is mainly CO, and metasomatism fluid changes from weak reductive to weak oxidative, which may be relate with the oxidative compositions caused by lithosphere attenuation. With the combination of trace and major element of basalt characteristics, South China Craton basalt magmatism in Cenozoic may be relate with the recycle of lithospheric matter, which is caused by Pacific plate subduction.