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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?
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Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?

机译：稀有的高Nb玄武岩熔岩的地球化学：它们是否源自由板状熔体交代的地幔楔？

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摘要

Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb. >. 20. ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.
机译：从成分上讲，高Nb玄武岩类似于HIMU（高U / Pb）海洋岛玄武岩，大陆碱性玄武岩和在平板窗口上方形成的碱性熔岩。来自西印度群岛东部牙买加东部的碱性碱性玄武岩熔岩的成分类似于高Nb玄武岩（Nb> 20. ppm）。 Halberstadt高Nb玄武岩分为两个组成亚组，其中第1组熔岩相对于第2组具有更丰富的不相容元素浓度。这两个组均来自同位素不同的尖晶石橄榄岩地幔源区，它们都需要石榴石和闪石作为交代物。残留相。哈尔伯斯塔特（Halberstadt）地球化学表明，熔岩不能通过下部地壳超镁铁质复合体，交代的地幔岩石圈，俯冲板，大陆壳，地幔柱羽源区或由富集和枯竭成分组成的上地幔源区的部分熔融而得到。相反，它们的成分，特别是负Ce异常，高的Th / Nb比和与附近的Adakite熔岩相似的同位素比，表明Halberstadt岩浆来自于成分可变的尖晶石橄榄岩源区，这些源区由板状熔体交化而成。沉淀的石榴石，角闪石，磷灰石和锆石。建议将高Nb玄武岩归类为Nb> 20 ppm的独特岩石类型，板内碱性玄武岩成分，但在俯冲带通过岩浆作用而在俯冲带中产生，这不同于产生其他板内熔岩的岩浆。

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