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Volcanological and petrological evolution of Barren Island (Andaman Sea, Indian Ocean)

机译：贫瘠岛（印度洋安达曼海）的火山学和岩石学演化

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摘要

The Island of Barren (Andaman Sea, Indian Ocean) consists of a horseshoe-shaped caldera opened to the west that was formed by either a giant, lateral landslide of the original volcanic cone or a vertical collapse of a surficial magma chamber due to a paroxysmal eruption. The reconstruction of the pre-and post-cald-eric activity indicates that the subaerial volcanic history of Barren Island was characterized by effusive activities accompanied by subordinate explosive (scoria fall) eruptions. The collapse of the central portion of the edifice terminated this mainly effusive phase. The inner flanks and the rim of the depression are covered by a 20 m thick sequence of breccias and tuffs. Beds are structurally massive with steep fronts, likely due to secondary mass flowage. The lack of caldera-forming eruption together with the absence of chemically evolved magmas could suggest a lateral landslide as the most probable process responsible for the caldera formation. The active polygenic tuff cone, presently occupying the inner portion of the caldera, consists of at least five coalescent summit craters and four eccentric spatter cones at the base. These features are attributed to the historic activities that are dominated by the emplacement of lava flows and pyroclastic deposits (e.g. 1803, 1991, 1995 and, possibly, 2000 eruptions). Lava flows moved westward from lateral spatter cones and occasionally reached the sea. A few months after the 2004 tsunami and Sumatra earthquake, Barren Island volcano violently resumed its eruptive activity that lasted for almost two years. The volcanic products ejected from May 2005 eruption have covered the historic lava flows and pyroclastic deposits.rnPetrologically, the lava flows show a narrow compositional variation, from low-K basalts to basaltic andesites. Pre-caldera magmas display a general less evolved composition and a certain variability of the Sr isotopic ratios (0.70385-0.70400); post-caldera magmas are compositionally more homogeneous and have an almost constant Sr isotopic ratio (0.70399-0.70403). Petrographic and geochemical characteristics indicate an important role played by a crystal-liquid fractionation process. However, the presence of large phenocrysts of xenocrystic origin, the small but significant variations of Sr isotopic ratios in the pre-caldera magmas, the relationships between ~(87)Sr/~(86)Sr and trace elements and the ratios of incompatible trace elements in the basaltic rocks point to the occurrence of additional evolutionary processes and/or to a role of a heterogeneous magma source. The most primitive magma (Mg# 71, Ni 218 ppm, Cr 557 ppm), displaying the lowest Th content (0.39 ppm), found in the Barren Island is also the most primitive in the Burma-Andaman-Java Arc, thus representing an important reference composition for the region.
机译：贫瘠之岛（印度洋安达曼海）由向西敞开的马蹄形破火山口组成，该破火山口由原始火山锥的巨大横向滑坡或由于阵发性造成的地表岩浆腔垂直倒塌形成喷发。火山前和火山后活动的重建表明，巴伦岛的海底火山史的特征是爆发性活动，伴有从属爆炸（火山灰坠落）爆发。建筑物中央部分的塌陷终止了这个主要的喷射阶段。凹陷的内侧面和边缘被20 m厚的角砾岩和凝灰岩覆盖。床体结构庞大，前缘陡峭，可能是由于二次质量流所致。火山口形成喷发的缺乏以及化学演化的岩浆的缺乏可能表明侧滑是造成火山口形成的最可能过程。活跃的多基因凝灰岩锥体，目前占据火山口的内部，由至少五个聚结的山顶陨石坑和位于底部的四个偏心飞溅锥组成。这些特征归因于以熔岩流和火山碎屑沉积为主的历史活动（例如1803年，1991年，1995年，可能还有2000次喷发）。熔岩流从侧面的飞溅锥向西移动，偶尔到达海中。在2004年海啸和苏门答腊地震发生后的几个月，巴伦岛的火山猛烈地恢复了持续了将近两年的喷发活动。从2005年5月喷发喷出的火山产物覆盖了历史性的熔岩流和火山碎屑沉积物。在岩石学上，熔岩流显示出从低K玄武岩到玄武安山岩的狭窄成分变化。破火山口前岩浆显示出一般较少演化的组成和Sr同位素比率（0.70385-0.70400）的某些变化；火山口后岩浆在成分上更均匀，并且具有几乎恒定的Sr同位素比（0.70399-0.70403）。岩石学和地球化学特征表明，结晶液分馏过程起着重要作用。但是，存在异种起源的大表晶，破火山口前岩浆中Sr同位素比率的微小但显着变化，〜（87）Sr /〜（86）Sr与微量元素之间的关系以及不相容痕量的比率玄武岩中的元素指出了额外的演化过程的发生和/或非均质岩浆源的作用。在Barren岛发现的最原始的岩浆（Mg＃71，Ni 218 ppm，Cr 557 ppm）显示出最低的Th含量（0.39 ppm），也是缅甸-安达曼-爪哇弧中最原始的岩浆，因此代表着该地区的重要参考成分。

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  • 来源
    《Journal of Asian earth sciences》 |2009年第5期|469-487|共19页
  • 作者

    Dornadula Chandrasekharam; Alba P. Santo; Bruno Capaccioni; Orlando Vaselli; Mohammad Ayaz Alam; Piero Manetti; Franco Tassi;

  • 作者单位

    Department of Earth Sciences, Indian Institute of Technology Bombay, 400076 Mumbai, India;

    Department of Earth Sciences, Via C. La Pira, 4, 50121 Florence, Italy CNR-IGG Institute of Geosciences and Earth Resources, Via C. La Pira, 4, 50121 Florence, Italy;

    Department of Earth and Geological-Environmental Sciences, Piazza Porta San Donato, 1, 40126 Bologna, Italy;

    Department of Earth Sciences, Via C. La Pira, 4, 50121 Florence, Italy CNR-IGG Institute of Geosciences and Earth Resources, Via C. La Pira, 4, 50121 Florence, Italy;

    Department of Earth Sciences, Indian Institute of Technology Bombay, 400076 Mumbai, India;

    Department of Earth Sciences, Via C. La Pira, 4, 50121 Florence, Italy;

    Department of Earth Sciences, Via C. La Pira, 4, 50121 Florence, Italy;

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  • 正文语种 eng
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  • 关键词

    barren island; field volcanology; petrology; radiogenic isotopes; fluid geochemistry;

    机译：贫瘠的岛屿野外火山学岩石学放射性同位素流体地球化学;

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