Sr, Nd and Pb Isotopic Characteristics of Emeishan Basalt Province and Discussion on Their Source Region
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摘要: 选择峨眉山玄武岩区2个出露最全的云南永胜大迪里剖面和宾川上仓剖面进行了Sr、Nd、Pb同位素地球化学研究.结果表明, 少数样品的Pb同位素与Hanan和Graham定义的C组分相似, 而大多数样品则不在C组分范围之内, 说明除地幔柱物质外, 有岩石圈物质的加入.在多元同位素图解上, 峨眉山玄武岩位于EMⅠ、EMⅡ和DMM三端元之间, 表明其源区可以由地幔柱、富集的岩石圈地幔和地壳不同程度的混合来解释.结合已有的微量元素资料分析, 其中的地壳组分主要为下地壳, 而早期玄武质岩浆在上升过程中由于通道不畅通, 有较多的上地壳组分的混染.岩石圈地幔的富集作用可能与地幔柱释放出的小体积富Na、P而贫K的流体交代作用有关.粗面岩的同位素组成和玄武岩接近, 说明粗面岩是玄武质岩浆分离结晶作用形成的.
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关键词:
- 大陆溢流玄武岩 /
- Sr、Nd、Pb同位素 /
- 源区 /
- 地幔柱 /
- 峨眉山
Abstract: Two most completely outcropped geological sections, Dadili of Yongsheng County and Shangcang of Binchuan County, Yunnan Province, are chosen for Sr, Nd and Pb isotopic geochemical study. Minor samples have the same Pb isotopic composition as C component defined by Hanan & Graham, whereas most of the samples are without C component, suggesting lithospheric materials besides mantle plume. On the diagram of Sr-Nd-Pb, they are plotted within three endmembers, EMⅠ, EMⅡ and DMM. This can be easily explained by different degrees mixing of mantle plume, enriched lithospheric mantle and crustal components. These data, in combination with previous trace element data, indicate that the crustal components are chiefly from lower crust. Basaltic magmas erupted during the early stages of volcanism are susceptible to more crustal contaminations as they establish pathways to the surface. The fertile lithospheric mantle may be related to metasomatism of small melt fractions enriched in Na and P, but poor in K released from mantle plume. Trachyte has the same isotopic compositions as the basalts, implying that it is generated by fractional crystallization.-
Key words:
- continental flood basalt /
- Sr /
- Nd and Pb isotopes /
- source region /
- mantle plume
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图 2 C组分Pb同位素演化图解(底图据Hanan和Graham[62])
Fig. 2. Pb isotopic evolution diagram of C component
图 3 εNd(t)对(Th/Ta) PM和(La/Nb) PM图解
Fig. 3. Diagram of εNd (t) versus (Th/Ta) PM and (La/Nb) PM
表 1 峨眉山玄武岩Sr、Nd同位素成分
Table 1. Sr and Nd isotopic compositions of Emeishan basalts
表 2 峨眉山玄武岩Pb同位素成分
Table 2. Pb isotopic compositions of Emeishan basalts
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