Abstract: Analyzed the geochemistry features of Caiyuanzi diabase from Dongchuan area and found that: the content of rare erath elements was much higher,w(SiO₂)was 39.19%~53.31%; w(La)/w(Sm) and w(LREE)/w(HREE) was low which showed the fractionation of light-heavy rare earth was not obvious; w(La)/w(Yb) was 1.90~4.34 wich showed light enrichment of light rare earth; Eu was no abnormalities which showed no plagioclase fractional crystallization in the diagenetic process; Nb and Ta was no obviously abnormalities which showed less contamination of primitive magma by crust materials.Geochemistry analysis showed obviously ocean island basalts, the formation age was at  later Early Proterozoic end-eraly mesoproterozoic, and a large number of mantle matter upwelling.Dongchuan diabase should be product of continental rift extensional, massive upwelling of mantle material, asthenosphere upwelling of lithospheric tectonic background.

Key words: Diabase, Extensional tectonic background, Geochemistry, Oceanic island basalt, Dongchuan area