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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Rheomorphic structures in a high-grade ignimbrite: The Nuraxi tuff, Sulcis volcanic district (SW Sardinia, Italy)
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Rheomorphic structures in a high-grade ignimbrite: The Nuraxi tuff, Sulcis volcanic district (SW Sardinia, Italy)

机译：高级火成岩中的变质构造：Sulcis火山区的Nuraxi凝灰岩（意大利西南撒丁岛）

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Deposits of the 15.8 Ma Nuraxi explosive eruption crop out in the Sulcis volcanic district, southwestern Sardinia. They consist of a decimeter-thick pumice fallout layer overlain, with no apparent temporal break, by several tens of meters thick, high-grade ignimbrite. The eruption was from a vent probably located ~20-30 km north of S. Pietro island, and it tapped a compositionally uniform, rhyolite magma reservoir (SiO_2 70-72 wt.%) bearing about 20 vol.% crystals. The ignimbrite consists of: (a) a Lower Ignimbrite (LI), <2-m-thick, fine-grained and glassy grading upward into a partly devitrified lapilli-tuff, and (b) an Upper Ignimbrite (UI) that is an intensely welded to lava-like lapilli-tuff. The deposit has an inferred maximum runout of about 80 km and overall dispersal/thickness characteristic of a low aspect-ratio ignimbrite. Over most of its outcrop the tuff rests on a flat, horizontal paleosurface and forms an ignimbrite plateau with uniform thickness and structural characteristics. However, in some areas it shows extensive folding and over-thickening as a result of rheomorphic flow into topographic depressions. Structural analysis of the ignimbrite reveals the presence of a pervasive foliation and lineation as well as macroscopic and microscopic flow structures. Occurrence of abundant, widely distributed stretched cavities in the tuff indicates that a substantial amount of gas was trapped in the tuff during aggradation. The observed pattern of structures of the ignimbrite plateau accords well with a conceptual model in which syn-depositional structures are formed during non-particulate, basal laminar flow due to agglutination of juvenile particles within the lower part of the pyroclastic density current. The flow was controlled by the substantial transfer of momentum from the high-speed, particulate upper part of the current to the top of the non-particulate part of the current and the onset of a complex relationship between welding, flow and sedimentation processes. It is proposed in particular that elongated gas pockets played a major role in guiding the laminar shear within the non-particulate part of the current in flat, horizontal areas, eventually promoting brittle failure of the deforming tuff mass along shear planes.
机译：15.8 Ma Nuraxi爆发性喷发的沉积物在撒丁岛西南部的Sulcis火山区中生长。它们由数十米厚的浮石覆盖层覆盖，没有明显的时间性断裂，由几十米厚的高级火成岩组成。喷发来自可能位于圣彼得罗岛以北约20-30公里的一个喷口，它喷出了成分均匀的流纹岩岩浆储层（SiO_2 70-72 wt。％），含约20 vol。％的晶体。着火物包括：（a）下火成岩（LI），厚度小于2米，细颗粒状和玻璃状逐渐向上渐变成部分失透的玻璃灰凝灰岩，以及（b）上火成岩（UI）是与熔岩状的凝灰岩紧密地焊接在一起。该矿床的最大径迹推断为约80 km，总的扩散/厚度特征是低纵横比的火成岩。凝灰岩的大部分露头位于平坦的水平古表面上，形成厚度和结构特征均一的火成岩高原。但是，由于流向地形凹陷的流变流，在某些地区它表现出广泛的折叠和过度增厚。对火成岩的结构分析表明存在普遍的叶状和线状以及宏观和微观流动结构。在凝灰岩中出现大量分布广泛的拉伸空腔，这表明在凝结过程中大量的气体被捕集在凝灰岩中。所观察到的火成岩高原结构的模式与概念模型非常吻合，在概念模型中，由于热碎屑密度流下部中的未成熟颗粒的凝集，在非颗粒的基础层流中形成了同沉积结构。流动是由动量从高速的，颗粒状的上部高速流动到电流的非颗粒状部分的顶部，以及焊接，流动和沉积过程之间复杂关系的开始而控制的。特别提出的是，细长的气穴在引导水的非颗粒部分内的平坦，水平区域中的层流剪切中起主要作用，最终促进变形凝灰岩块沿剪切平面的脆性破坏。

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