Signatures of the source for the Emeishan flood basalts in the Ertan area: Pb isotope evidence

The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member.

峨眉山二滩玄武岩地球化学特征

峨眉山大火成岩省二滩地区玄武岩W（SiO2）／％=40．5～59．65；普遍高碱，w（K2O＋Na2O）：2．64％～7．67％。W（TiO2）：2．10％～6．53％，Ti／Y〉500，该区玄武岩属于高钛（HT）型玄武岩；其Mg^#：58～84，明显高于高于宾川高钛玄武岩（31～53），说明岩浆演化程度明显低于宾川高钛玄武岩浆。岩石样品中Sr显示明显负异常，暗示了二滩玄武岩经历了广泛的斜长石结晶分离；而Eu不显异常，则反映了玄武岩岩浆中有高Eu^3＋／Eu^2＋比值的存在，其环境为氧化环境。在二滩玄武岩和宾川高钛玄武岩中，Ni，Zr，TiO2和Mg^#均显示了明显差异，说明二滩玄武岩具有独立的地球化学特征。二滩玄武岩不相容元素（Rb，Ba，Th，U，Nb，Ta，La，Ce等）配分曲线与OIB相似，及其Ta／Yb vs Th／Yb双变量图解也显示出了富集地幔特征，这些特征反映了峨眉山二滩玄武岩源区为富集地幔源，玄武岩岩浆可能为地幔柱物质。此外，Ba／Th，Zr／Nb，La／Nb，Ba／Nb等比值介于EMI OIB和EMII OIB之间，以及Ce／Pb比值也说明：二滩玄武岩缺少HIMU OIB端元组分，是EMI OIB和EMII OIB两端元的混合产物。

峨眉山高钛玄武岩物源的不均一性研究

对二滩地区高钛玄武岩物源研究表明,该区岩石可分为高钛-1和高钛-2两种岩石类型。其反映了二滩高钛玄武岩浆物源的不均一性,这种不均一性主要表现在:1)微量元素Zr,Sr含量明显不同;2)部分熔融程度和熔融压力不同;3)岩浆的结晶分异不同;4)Pb同位素比值不同。这可能是深源地幔柱(来自核-幔边界或下地幔)在上升过程中携带的不同物质发生部分熔融所致。