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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Morphology and growth style of a Miocene submarine dacite lava dome at Atsumi, northeast Japan
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Morphology and growth style of a Miocene submarine dacite lava dome at Atsumi, northeast Japan

机译：日本东北部厚中的中新世潜艇陶粒熔岩穹顶的形态和生长方式

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A Miocene submarine lava dome at Tate-iwa, Atsumi, Yamagata, Japan, displays well-preserved primary morphological features. The dome currently rises above a wave-cut coastal platform and is 55 m high and 90-180 m across. It comprises a massive core and a lava lobe-hyaloclastite rim, both of which are composed of compositionally uniform, feldspar-phyric dacite (SiO_2 = 64 wt.%). The boundary between these two zones is distinct but gradational. The massive core consists of homogeneous, coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin. The lava lobe-hyaloclastite rim encircles the massive core and consists of a complex of 80-90% dacitic lava lobes and 10-20% hyaloclastite. The lava lobes are 1-6 m thick, 3-12 m wide and more than 5 m long. Each lobe consists of a radially columnar-jointed core and a glassy rim 10-30 cm thick. The hyaloclastite comprises polyhedral dacite clasts 5-100 cm across in a matrix of dacite fragments up to 5 mm across. The lava lobes and hyaloclastite have gradational contacts in places. Paleobathymetric studies based on foraminifera in sediment beneath the dome, and the K-Ar age of the dome, suggest that it was extruded in a middle-bathyal environment (1500-2000 m below sea level) at 12.9 +- 0.6 Ma. The internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes, and endogenous growth involving a continuous magma supply and simple expansion from the interior. The presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion, in spite of its high silica content. Magma temperatures calculated by two-pyroxene and Fe-Ti oxide geothermometers were 999-1042 and 957-1005℃, respectively. The inferred low viscosity may be attributed to the high temperature of the magma and/or a high confining pressure resulting from the deep-sea environment.
机译：日本山形县厚水市立石洼的中新世海底熔岩穹顶表现出保存完好的主要形态特征。该穹顶目前升至经过波浪切割的沿海平台上方，高55 m，跨度90-180 m。它由块状岩心和熔岩叶-破硅质岩边缘组成，两者均由成分均匀的长石-闪锌矿组成（SiO_2 = 64 wt。％）。这两个区域之间的边界是不同的，但是是渐变的。块状岩心由均质，连贯的陶粒组成，其特征是沿边缘的流动带状分布和从中心向边缘辐射的柱状节理。熔岩叶-破硅质岩边缘环绕块状岩心，由80-90％的菊状熔岩叶和10-20％的破硅质岩组成。熔岩裂片厚1-6 m，宽3-12 m，长5 m以上。每个瓣由放射状的柱状接合核和10-30厘米厚的玻璃状边缘组成。透明质岩包括5-100cm宽的多面体辉晶碎片，其基质为达5mm宽的辉石碎片基质。熔岩裂片和透明质岩在某些地方具有渐变接触。基于穹顶以下沉积物中有孔虫的古生物学研究以及穹顶的K-Ar年龄表明，它是在中等海底环境（海平面以下1500-2000 m）中以12.9±0.6 Ma挤压的。穹顶的内部结构表明，它是由外生生长（包括小的达克特叶的挤压）和内生生长（包括连续的岩浆供应和从内部简单扩张）的结合形成的。沿轮缘存在小的熔岩裂片表明，尽管岩浆中二氧化硅含量高，但岩浆在挤出时的粘度相对较低。用二-和Fe-Ti氧化物地热仪计算的岩浆温度分别为999-1042和957-1005℃。推断的低粘度可以归因于岩浆的高温和/或深海环境导致的高围压。

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