峨眉山溢流玄武岩省高钛玄武岩的两种不同地幔源特征

Two Distinct Mantle Sources for High-Ti Basalts in the Emeishan Overfall Basalt Province

为探讨和揭示峨眉山高钛玄武岩的幔源特征,以二滩高钛玄武岩为研究对象进行了主要元素、微量元素和Sr-Nd-Pb同位素的系统研究。研究表明:二滩高钛玄武岩可分为A和B两组玄武岩;两组岩石间的微量元素(Rb、K、Ba、Th、Nb和Ta)富集程度和微量元素比值(Ba/Nb、Ba/Th、Zr/Nb、Th/La、Zr/Hf)以及同位素比值(87Sr/86Sr、208Pb*/206Pb*)均存在较为明显的差异。造成这种差异的原因不是岩浆过程(结晶分异、地壳混染、部分熔融)的不同,而是A组和B组具有不同的地幔源。A组具有EMII特征,可能为富含辉石岩的交代地幔部分熔融所形成;B组则具有EMI和C组分的混合特征,可能为交代谱系较宽的地幔物质熔融所形成。

In order to probe and reveal the characteristics of mantle sources for high-Ti basahs in the Emeishan overfall basalt province, major elements, trace elements and Sr- Nd- Pb isotopes of the Ertan high-Ti basalts are systemic studied. It＇s indicated that they can be divided into group A and group B types. Two types of basalts shows such distinct geochemical characteristics as different concentrations of the trace elements（Rb, K, Ba, Th, Nb, Ta） and trace-element ratios（Ba/Nb, Ba/Th, Zr/Nb, Th/La, Zr/Hf） as well as different ^87Sr/^86 Sr and ^208 Pb^＊ /^206 Pb^＊ ratios. The difference between group A and group 13 is resulted from distinct mantle sources instead of magma evolution processes （fractional crystallizztion, crustal contamination, assimilation-fractional crystallization, and partial melting）. Group A, with the feature of EM Ⅱ, is probably generated by partial melting of metasomatic mantle enriched pyroxenite. Group B, with the mixed features of EM Ⅰ and ＂C＂ component, is probably derived from partial melting of mantle-derived materials with wide metasomatic spectrum.