Formation of the Troodos Ophiolite at a triple junction: Evidence from trace elements in volcanic glass
Formation of the Troodos Ophiolite at a triple junction: Evidence from trace elements in volcanic glass
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DOI：
10.1016/j.chemgeo.2014.08.006
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发表时间：
2014-10
期刊：
影响因子：
3.9
通讯作者：
M. Regelous;K. Haase;S. Freund;M. Keith;C. Weinzierl;C. Beier;P. Brandl;T. Endres;H. Schmidt
中科院分区：
文献类型：
--
作者：
M. Regelous;K. Haase;S. Freund;M. Keith;C. Weinzierl;C. Beier;P. Brandl;T. Endres;H. Schmidt
来自Akaki峡谷的Troodos蛇绿岩喷出部分的新鲜火山玻璃的成分为拉斑玄武岩和玄武质至英安岩。与正常MORB相比，它们具有极低的分馏校正的Na 8、Fe 8和Ti 8，并且相对于具有类似不相容性的非流体移动的元素，富集流体移动的微量元素，包括U、Ba、Rb、Sr和Pb。Akaki熔岩的微量元素组成定义了一个阵列之间延伸的“弧后熔岩”的组合物，并从熔岩序列的上部Troodos玻尼岩定义的字段。Troodos熔岩来自地幔源，经历了早期熔融亏损，后来富集的流体和小程度的熔体。这些过程可以解释Troodos挤压岩中Pb/Ce与Zr/Nb和Ba/Nb异常的负相关关系。虽然一些Troodos熔岩在成分上与弧后扩张中心的熔岩相似，但熔岩堆上部的玻安岩似乎与弧前、弧后或其他构造环境中的熔岩成分不完全相同，在这些环境中已经发现了类似的岩石类型。我们认为，地球化学演化推断的地幔来源的Troodos熔岩，地质证据是最一致的起源Troodos蛇绿岩在一个扩展中心靠近一个脊-沟-沟，或脊-沟-转换三重连接点，高度亏损，俯冲修改，流体富集地幔楔材料能够上涌，并在一个“弧前”位置的浅深度。在这样的构造环境中，弧火山活动被扩张中心捕获，解释了特罗多斯缺乏陆上弧岩浆活动的证据。三重连接点的快速侧向迁移可以解释其他特提斯俯冲带上蛇绿岩的相似年龄。
Fresh volcanic glasses from the extrusive section of the Troodos Ophiolite in Akaki Canyon are tholeiitic and basaltic to dacitic in composition. Compared to normal MORB they have extremely low fractionation corrected Na8, Fe8and Ti8and are enriched in fluid-mobile trace elements, including U, Ba, Rb, Sr and Pb, relative to non-fluid mobile elements of similar incompatibility. Trace element compositions of Akaki lavas define an array extending between ‘back-arc lava’-like compositions, and the field defined by Troodos boninites from the upper part of the lava sequence. Troodos lavas were derived from a mantle source that underwent early melt depletion, and later enrichment by both fluids and small degree melts. These processes can explain the unusual negative correlation of Pb/Ce with Zr/Nb and Ba/Nb in Troodos extrusives. Although some Troodos lavas are similar in composition to lavas from back-arc spreading centres, the boninites from the upper parts of the lava pile do not appear to have exact compositional equivalents among lavas from fore-arcs, back-arcs or other tectonic settings where similar rocktypes have been recovered. We suggest that the geochemical evolution inferred for the mantle source of Troodos lavas, together with geological evidence is most consistent with an origin for the Troodos Ophiolite at a spreading centre close to a ridge–trench–trench, or ridge–trench–transform triple junction, where highly depleted, subduction-modified, fluid-enriched mantle wedge material was able to upwell and decompress to shallow depths in a ‘fore-arc’ location. In such a tectonic setting, arc volcanism is captured by the spreading centre, explaining the lack of evidence for subaerial arc magmatism in Troodos. Rapid lateral migration of the triple junction could account for the similar ages of other Tethyan supra-subduction zone ophiolites.