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首页> 外文期刊>Contributions to Mineralogy and Petrology >Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano
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Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano

机译：巴拿马弧带硅质火山作用的起源：埃尔瓦勒火山火山有两阶段分馏过程的证据

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In the Central American Volcanic Arc, adakitelike volcanism has often been described as volumetrically insignificant. However, extensive silicic adakitic volcanism does occur in the Panamanian arc and provides an opportunity to evaluate the origin of this magma-type as well as to contrast its origin with other Central American silicic magmas. The Quaternary volcanic deposits of El Valle volcano are characterized by pronounced depletions in the heavy rare earth elements, low Y, high Sr, high Sr/Y, relatively high MgO, and low K_2O/Na_2O, when compared with other Quaternary Central American volcanics at similar SiO_2. These chemical features are also diagnostic of adakitic signatures. Our new ~(40)Ar/(39)Ar ages of lava flows and ash flows that compose the volcanic edifice of El Valle volcano illustrate that the eruptive volume of adakitic-like volcanism is substantial during the Quaternary (~ 120 km3). Adakitic-like magmas dominate the strati-graphic record. Common to all models for the origin of an adakite geochemical signature is the involvement of garnet, as a residual or fractionating phase. The stability of garnet in hydrous magmas has been recently reevaluated with important consequences; garnet is a stable primary igneous phase at pressure and temperature conditions expected for magma differentiation at the roots of a mature island arc. Moreover, adakite-like volcanism erupted at El Valle volcano displays the middle rare earth element depletion observed in other Panamanian volcanic centers that has been attributed to significant amphibole fractionation. Extensive amphibole fractionation may have occurred in two stages. The first stage of fractionation, garnet + amphibole fractionation, occurs from hydrous basaltic-andesitic parental magmas that have ponded at the base of an overthickened crust. The second stage occurs at mid-lower crustal levels where abundant amphibole + plagio-clase and minor sphene crystallized from water-rich magmas. These two stages combined may have resulted in an amphibole-rich cumulate layer. This amphibole layer is likely the source of the abundant amphibole-rich cumulate enclaves and blobs found in volcanic products across the Panamanian arc. Stalling of water-rich magmas during this two-stage fractionation process could drive the interstitial liquids to the evolved compositions typical of continental crust, while leaving behind amphibole-rich cumulate rocks that may eventually be returned to the asthenosphere. Differentiation of H_2O-rich magmas under the conditions appropriate for the roots of island arcs may therefore be a key process in developing a better understanding of the generation of continental crust in island arc environments.
机译：在中美洲火山弧中，经常将adakitelike火山活动描述为体积微不足道的。但是，巴拿马弧确实发生了广泛的硅质adakitic火山作用，并提供了一个机会来评估这种岩浆类型的成因，并将其成因与其他中美洲硅质岩浆进行对比。与中美洲其他第四纪火山岩相比，埃尔瓦尔火山第四纪火山岩沉积物的特征是重稀土元素，低Y，高Sr，高Sr / Y，相对高MgO和低K_2O / Na_2O的贫化。类似的SiO_2。这些化学特征还可以诊断adakitic签名。构成El Valle火山火山岩的熔岩流和灰分流的新的〜（40）Ar /（39）Ar年龄表明，第四纪（〜120 km3）期间象阿达基特火山的喷发量很大。象岩质的岩浆占主导地位的地层记录。石榴石作为残存或分馏阶段的参与，是所有与adakite地球化学特征成因有关的模型的共同点。石榴石在含水岩浆中的稳定性最近被重新评估，具有重要的意义。石榴石在压力和温度条件下是稳定的初级火成相，预期在成熟岛弧的根部发生岩浆分化。此外，在埃尔瓦尔火山喷发的类似akakite火山活动显示出在其他巴拿马火山中心发现的中等稀土元素耗竭现象，这归因于明显的角闪石分馏。广角闪石的分级分离可能发生在两个阶段。石榴石+闪石的分级分离的第一阶段是从含水的玄武岩-安山岩母岩浆中抽出的，这些岩浆堆积在增厚的地壳底部。第二阶段发生在地壳中下部，从富含水的岩浆中结晶出丰富的角闪石和斜长石和少量的蝶骨。这两个阶段的结合可能导致了富含闪石的堆积层。该闪石层很可能是巴拿马弧上火山产物中大量闪石丰富的累积飞地和斑点的来源。在这两个阶段的分馏过程中，富水岩浆的失速可能将间隙液驱至大陆壳典型的演化成分，而留下了富含闪石的堆积岩石，这些岩石最终可能返回软流圈。因此，在适合岛弧弧根的条件下，富H_2O岩浆的分化可能是对岛弧环境中陆壳产生的更好理解的关键过程。

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