Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1
Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1
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DOI：
10.1016/j.epsl.2005.06.015
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发表时间：
2005-09-15
影响因子：
5.3
通讯作者：
Dickin, AP
中科院分区：
文献类型：
--
作者：
Gibson, SA;Thompson, RN;Dickin, AP
洋岛玄武岩（OIB，例如，Kerguelen，Pitcairn and Walvis Ridge）被归因于上涌地幔柱的熔融：（i）浅层再循环脱层的次大陆岩石圈地幔或地壳;或（ii）深层再循环交代岩石圈、大洋高原或大洋地壳加上百分之几的远洋沉积物。我们提出了新的地球化学数据OIB样品从南大西洋中部，其中包括100至30马的碱性和拉斑玄武岩从沃尔维斯海岭和格兰德河上升和< 3马玄武岩和玄武岩从特里斯坦达库尼亚，Inaccessible和高夫。此外，我们还分析了来自非洲西南部和南美洲东部的白垩纪镁铁质-钾质岩浆，以确定冈瓦纳大陆解体期间可能分层的交代岩石圈地幔的成分变化。稀土元素反演和Sr、Nd、Pb同位素混合模型的结果表明，“贫化”地幔柱组分类似于FOZO，而南大西洋中部OIB“富集”地幔组分的组成在空间和时间上都是变化的，至少需要三种不同的富集地幔端元才能解释100 ~ 30 Ma OIB岩浆的组成范围。这些类似的源区镁铁质钾质岩浆来自：（一）刚果克拉通和西南非洲的达马拉带;（二）圣弗朗西斯科克拉通和巴西东南部的巴西利亚带;（三）巴西南部和巴拉圭的里约热内卢阿帕路易斯阿尔维斯克拉通。同位素最富集的EM-1玄武岩（Nd=-0.8 ~-4.5），形成于89 ~ 78 Ma之间的沃尔维斯海岭和格兰德河海隆，似乎含有10%~ 15%的贡献，来自低Nd，Pb-206/Pb-204和高[La/Nb]（n）的熔体源区，与巴西南部和巴拉圭之下交代的亚地幔岩石圈地幔成分相似。对板块运动的重建表明，在大陆分裂的时候，这将位于随后EM-1熔体生成地点的上方。来自西南非洲和巴西南部的交代岩石圈脱层的浅地幔再循环可以解释沃尔维斯海岭100 Ma和80 ~ 30 Ma玄武岩的微量元素和同位素比值的进一步变化。特里斯坦、英纳塞布尔和戈夫的现代岩浆含有来自富集地幔源的熔融贡献，具有高Pb-206/Pb-204，Pb-208/Pb-204和Sr-87/Sr-86比值以及相对于块体地球的低钕含量。这并不类似于镁铁质钾质岩浆的同位素组成，从邻近南大西洋的大陆，因此，我们建议，这是一个深循环地幔成分。最近特里斯坦地幔柱岩浆的高铅同位素比值与再循环洋壳加上百分之几的远洋沉积物的熔融贡献不一致。我们认为，深地幔成分是再循环的交代岩石圈地幔。变质交代岩石圈地幔，可变的组成，似乎只是一个内在的一部分，亚大洋特里斯坦地幔柱和块体岩石组成的现代特里斯坦地幔柱相关的熔体，因此，不适合模拟的贡献的地幔柱衍生的熔体的Parana-Etendeka大陆洪水玄武岩。这种关系也可能适用于其他大火成岩省和海洋岛屿沿着其相关的热点轨道。(c)2005 Elsevier B. V.保留所有权利。
The enriched mantle 1 (EM-1) component in ocean-island basalts (OIB, e.g., Kerguelen, Pitcairn and Walvis Ridge) has been attributed to melting in upwelling mantle plumes of either: (i) shallow-recycled delaminated subcontinental-lithospheric-mantle or crust; or (ii) deep-recycled metasomatised lithosphere, oceanic plateau or oceanic crust plus a few percent of pelagic sediment. We present new geochemical data for OIB samples from the central South Atlantic; these include 100 to 30 Ma alkali and tholeiitic basalts from the Walvis Ridge and Rio Grande Rise and < 3 Ma basanites and basalts from Tristan da Cunha, Inaccessible and Gough. Additionally, we have analysed Cretaceous mafic-potassic magmas from south-west Africa and eastern South America in order to establish the compositional variation of metasomatised lithospheric mantle that may have been delaminated during Gondwana break-up. The results of our rare-earth-element inversion and Sr-, Nd- and Pb-isotopic mixing models suggest that the 'depleted' mantle plume component resembles FOZO and that the composition of the 'enriched' mantle component in central South Atlantic OIB has varied both spatially and temporally.At least three different enriched mantle end-members are required to explain the compositional range of 100 to 30 Ma OIB magmas. These resemble the source regions of mafic-potassic magmas from: (i) the Congo craton and Damara belt of south-west Africa; (ii) the Sao Francisco craton and Brasilia belt of south-east Brazil; and (iii) the Rio Apa-Luis Alves craton of southern Brazil and Paraguay. The most isotopically enriched EM-1 basalts (epsilon Nd=-0.8 to -4.5), generated on the Walvis Ridge and Rio Grande Rise between 89 and 78 Ma, appear to contain a 10% to 15% contribution from a melt source region with low epsilon Nd, Pb-206/Pb-204 and high [La/Nb](n), similar in composition to metasomatised subcratonic lithospheric mantle beneath southern Brazil and Paraguay. Reconstructions of plate motions indicate that, at the time of continental break-up, this would have been located above the sites of subsequent EM-1 melt generation. Shallow-mantle recycling of metasomatised lithosphere delaminated from south-west Africa and south-cast Brazil may explain the further variations in trace-element and isotopic ratios of 100 and 80 to 30 Ma basalts from the Walvis Ridge, respectively.Recent magmas from Tristan, Inaccessible and Gough contain a melt contribution from an enriched mantle source with high Pb-206/Pb-204, Pb-208/Pb-204 and Sr-87/Sr-86 ratios and low epsilon Nd, relative to bulk-Earth. This does not resemble the isotopic composition of mafic-potassic magmas from continents adjacent to the South Atlantic and we therefore propose that it is a deep-recycled mantle component. The high Pb-isotopic ratios of recent Tristan plume magmas are inconsistent with a melt contribution from recycled oceanic crust plus a few percent pelagic sediments. We suggest that the deep mantle component is recycled metasomatised lithospheric mantle.Recycled metasomatised lithospheric mantle, of variable composition, only appears to have been an intrinsic part of the sub-oceanic Tristan mantle plume and the bulk-rock composition of present-day Tristan plume related melts are therefore inappropriate for modelling the contribution of plume-derived melts to the Parana-Etendeka continental flood-basalts. This relationship may also apply to other large igneous provinces and ocean islands along their associated hot spot tracks. (c) 2005 Elsevier B.V. All rights reserved.