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首页> 外文期刊>Lithos: An International Journal of Mineralogy, Petrology, and Geochemistry >Hydrothermal alteration and melting of the crust during the Columbia River Basalt-Snake River Plain transition and the origin of low-delta O-18 rhyolites of the central Snake River Plain
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Hydrothermal alteration and melting of the crust during the Columbia River Basalt-Snake River Plain transition and the origin of low-delta O-18 rhyolites of the central Snake River Plain

机译：哥伦比亚河玄武岩-蛇河平原过渡期地壳的热液蚀变和融化以及蛇河平原中部低三角洲O-18流纹岩的成因

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摘要

We present compelling isotopic evidence from similar to 15 Ma rhyolites that erupted coeval with the Columbia River Basalts in southwest Idaho's J-P Desert and the Jarbidge Mountains of northern Nevada at that suggests that the Yellowstone mantle plume caused hydrothermal alteration and remelting of diverse compositions of shallow crust in the area where they erupted. These rhyolites also constitute the earliest known Miocene volcanism in the vicinity of the Bruneau-Jarbidge and Twin Falls (BJTF) volcanic complexes, a major center of voluminous (10(3) - 10(4) km(3)) low-delta O-18 rhyolitic volcanism that was previously defined as being active from 13 to 6 Ma. The Jarbidge Rhyolite has above-mantle delta O-18 (delta O-18 of +7.9 parts per thousand SMOW) and extremely unradiogenic epsilon(Hf) (-34.7) and epsilon(Nd) (-24.0). By contrast, the J-P Desert units are lower in delta O-18 (+4.5 to 5.8 parts per thousand), and have more moderately unradiogenic whole-rock epsilon(Hf) (-20.3 to -8.9) and epsilon(Nd) (-13.4 to 7.7). The J-P Desert rhyolites are geochemically and petrologically similar to the younger rhyolites of the BJTF center (the one exception being their high 8180 values), suggesting a common origin for J-P Desert and BJTF rhyolites. The presence of low-delta O-18 values and unradiogenic Nd and Hf isotopic compositions, both of which differ greatly from the composition of a mantle differentiate, indicate that some of these melts may be 50% or more melted crust by volume. Individual J-P Desert units have isotopically diverse zircons, with one lava containing zircons ranging from -0.6 parts per thousand to + 6.5 parts per thousand in delta O-18 and from -29.5 to -2.8 in epsilon(Hf). Despite this diversity, zircons all have Miocene U/Pb ages. The range of zircon compositions fingerprints the diversity of their source melts, which in turn allow us to determine the compositions of two crustal end-members which melted to form these rhyolites. These end-members are: 1) Archean basement with normal to high-delta O-18 and unradiogenic epsilon(Hf) and 2) hydrothermally altered, shallow, young crust with low-delta O-18 (0-1 parts per thousand.) and more radiogenic epsilon(Hf). We suggest that the shallow crust's low-delta O-18 composition is the result of hydrothermal alteration which was driven by a combination of normal faulting and high heat fluxes from intruding Yellowstone plume-derived basalts shortly prior to the onset of silicic magmatism. Furthermore, zircon diversity in the J-P Desert units suggests rapid assembly of zircon-bearing melts of varying isotopic composition prior to eruption, creating well-mixed magmas with heterogeneous zircons, We suggest that this hydrothermal priming of the crust followed by remelting upon further heating may be a common feature of intraplate mantle plume volcanism worldwide. (C) 2015 Elsevier B.V. All rights reserved.
机译：我们提供了令人信服的同位素证据，这些证据来自与爱达荷州西南部的JP沙漠和内华达州北部的Jarbidge山爆发的哥伦比亚河玄武岩相似的15个流纹岩，这表明黄石地幔柱羽引起了热液蚀变和重熔了浅壳的多种成分在他们爆发的区域。这些流纹岩还构成了布鲁诺-贾比治和特温福尔斯（BJTF）火山群附近的最早的中新世火山岩，该火山群是低三角洲O（10（3）-10（4）km（3））的主要中心-18流纹火山作用，以前被定义为活跃于13至6 Ma。 Jarbidge流纹岩具有高于表层的δO-18（δO-18含量为每千份SMOW 7.9份）以及极不辐射的epsilon（Hf）（-34.7）和epsilon（Nd）（-24.0）。相比之下，JP沙漠单位的三角洲O-18较低（+4.5至5.8千分之几），并具有中等程度的非放射源全岩石epsilon（Hf）（-20.3至-8.9）和epsilon（Nd）（- 13.4至7.7）。 J-P沙漠流纹岩的地球化学和岩石学特征与BJTF中心的较年轻的流纹岩相似（唯一的例外是其高8180值），这表明J-P沙漠和BJTF流纹岩的共同起源。低δO-18值以及不放射源的Nd和Hf同位素组成（二者与地幔组成的差异都很大）的存在表明，这些熔体中的某些熔体的体积可能超过50％。各个J-P沙漠单位具有同位素多样的锆石，其中一个熔岩包含的锆石在O-18三角洲中为-0.6千分之千至+ 6.5千分之几，在epsilon（Hf）中为-29.5至-2.8。尽管有这种多样性，但锆石都具有中新世U / Pb年龄。锆石成分的范围可识别出其源熔体的多样性，这反过来又使我们能够确定两个熔融形成这些流纹岩的地壳末端成员的成分。这些末端成员是：1）具有正常至高三角洲O-18和非放射性ε（Hf）的太古代基底，以及2）具有低三角洲O-18（0-1份千分之一）的热液蚀变浅浅新生硬皮。 ）和放射源性ε（Hf）更高。我们认为，浅层地壳的低δO-18成分是水热蚀变的结果，这是由正断层和高热通量共同作用驱动的，而正断层和高热通量是在硅质岩浆作用发生之前不久侵入黄石羽状玄武岩而引起的。此外，JP沙漠单位中的锆石多样性表明，在喷发之前，各种同位素组成的含锆锆石的熔体会迅速聚集，形成具有异质锆石的充分混合的岩浆。我们建议对地壳进行这种热液灌浆，然后在进一步加热时重熔是全球板内幔幔柱火山作用的共同特征。 （C）2015 Elsevier B.V.保留所有权利。

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