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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Subsurface geology, ancient hydrothermal systems and crater excavation processes beneath Lake Rotomahana: Evidence from lithic clasts of the 1886 AD Rotomahana Pyroclastics
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Subsurface geology, ancient hydrothermal systems and crater excavation processes beneath Lake Rotomahana: Evidence from lithic clasts of the 1886 AD Rotomahana Pyroclastics

机译：罗托玛哈纳湖下面的地下地质，古代热液系统和火山口开挖过程：来自1886年公元罗托玛哈那热碎屑岩的碎屑碎屑的证据

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The craters associated with the 1886 AD phreatomagmatic Rotomahana eruption, Okataina Volcanic Centre, New Zealand, and the near-vent geology are now hidden beneath Lake Rotomahana and its post-eruptive sediment fill. Lithic clasts from the near-vent lithic lapilli ash deposits of the Rotomahana Pyroclastics are used in this study to trace geological and geothermal conditions before the eruption, as well as vent excavation dynamics. Near-vent deposit characteristics were described in the field, representative lithic clasts were documented petrographically, and unaltered clasts were analysed for major and trace element compositions. The majority of the lithic clasts were rhyolites with subordinate ignimbrites and hydrothermally altered clasts, and trace siltstone and silicified clasts. The rhyolites were classified into four petrographic groups according to phenocryst content and assemblage and were more diverse with respect to geochemical compositions. Most of the rhyolite lithics in the Rotomahana Pyroclastics did not match the rhyolite domes exposed subaerially around the lake, but did have affinities with the pre-Matahina caldera Wairua Rhyolite, and potentially other older non-exposed domes. Ignimbrites most likely correlated either to the Matahina ignimbrite or older non-exposed units. Hydrothermally altered rhyolite and ignimbrite lithic clasts are common and suggest that there has been a long-lived hydrothermal system in this sector, possibly dating back to early activity of the Okataina Volcanic Centre. The diversity in lithic types indicate a spatial variation in country rock lithology and strength, which probably contributed to the vent position and morphology along the Rotomahana fissure. (C) 2015 Elsevier B.V. All rights reserved.
机译：如今，与旋转岩浆有关的火山口，新西兰奥卡塔纳火山中心以及临近火山口的地质坑现在都隐藏在Rotomahana湖及其喷发后的沉积物之下。在这项研究中，使用了罗托玛哈那火山碎屑岩近于火山岩的火山灰灰岩沉积物中的岩屑，以追踪喷发前的地质和地热条件，以及通风孔的开挖动力学。在该领域描述了近乎成矿的沉积特征，岩石学记录了代表性的碎屑，并分析了未改变的碎屑的主要和微量元素组成。大部分岩性碎屑为流纹岩，其具有次要的火成岩和热液蚀变的碎屑，以及微量的粉砂岩和硅化的碎屑。根据表晶的含量和组合，流纹岩被分为四个岩石学组，并且在地球化学组成方面也更加多样化。 Rotomahana火山碎屑岩中的大多数流纹岩石块与湖底裸露的流纹岩穹顶不匹配，但与前Matahina破火山口Wairua流纹岩以及可能的其他较老的未暴露穹顶有亲缘关系。火成岩很可能与Matahina火成岩或较老的未暴露单位相关。热液蚀变的流纹岩和火成岩碎屑岩很常见，这表明该领域存在着长寿的热液系统，这可能可以追溯到Okataina火山中心的早期活动。岩性类型的多样性表明，乡村岩石的岩性和强度存在空间变化，这可能有助于沿Rotomahana裂隙的出气孔位置和形态。 （C）2015 Elsevier B.V.保留所有权利。

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