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Mapping a hidden terrane boundary in the mantle lithosphere with lamprophyres

机译:使用煌斑岩在地幔岩石圈中隐藏的地形边界图

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摘要

Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Throughout southwest Britain, post-Variscan lamprophyres are (ultra)potassic with strong incompatible element enrichments. Here we show that they form two distinct groups in terms of their Sr and Nd isotopic compositions, occurring on either side of a postulated, hitherto unrecognized terrane boundary. Lamprophyres emplaced north of the boundary fall on the mantle array with εNd −1 to +1.6. Those south of the boundary are enriched in radiogenic Sr, have initial εNd values of −0.3 to −3.5, and are isotopically indistinguishable from similar-aged lamprophyres in Armorican massifs in Europe. We conclude that an Armorican terrane was juxtaposed against Avalonia well before the closure of the Variscan oceans and the formation of Pangea. The giant Cornubian Tin-Tungsten Ore Province and associated batholith can be accounted for by the fertility of Armorican lower crust and mantle lithosphere.
机译:煌斑岩代表含水的碱性地幔熔体,是有关大陆岩石圈组成信息的唯一来源。在整个英国西南部,瓦里斯卡纳后的煌斑岩(超)呈钾质,元素富集度强。在这里,我们表明,就其Sr和Nd同位素组成而言,它们形成了两个不同的组,它们出现在假定的,迄今无法识别的地层边界的两侧。在边界以北安置的煌斑岩落在地幔阵列上,εNd-1至+1.6。边界以南的那些富含放射性Sr,其初始εNd值为-0.3至-3.5,在同位素上与欧洲Armorican断层的类似年龄的煌斑岩没有区别。我们得出的结论是,在瓦里斯卡纳海洋关闭和Pangea形成之前,一个Armorican地形就与Avalonia并置了。巨大的Cornubian锡-钨矿省和相关的岩床可以由Armorican下地壳和地幔岩石圈的肥力来解释。

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