Caldera formation in the Rum Central Igneous Complex, Scotland
Caldera formation in the Rum Central Igneous Complex, Scotland
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苏格兰朗姆中央火成岩群的火山口形成
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
C. Donaldson
中科院分区：
文献类型：
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作者：
V. Troll;C. H. Emeleus;C. Donaldson
抽象的。苏格兰西北部拉姆中央火成杂岩的北方边缘区代表了火山早期长英质阶段的一部分。边缘带是早期破火山口底部的遗迹，是沉积岩和火成岩的充填物。它的形成是通过对复合体的环形断裂系统及其火山碎屑和火山岩内火山岩相的实地考察来探索的。一系列的岩浆膨胀和腔室的增长造成了最初的隆起，随后形成了一个崩溃的结构，而没有伴随着火山作用。这个塌陷结构，在平面上是圆形的，在起源上类似于一个盐盆地，它是由上升的底辟之上的地壳伸展而形成的。我们称之为原始破火山口。塌陷角砾岩是最初的充填物，它代表了形成盆地壁的托里东期砂岩的大块、大块和大圆石的坍塌和滑动。对这些沉积物的分析表明，复合体中角砾岩的厚度（80-170 m）存在相当大的变化，表明原始破火山口的底部不均匀，与零星坍塌一致。在累积了厚度超过70米的角砾岩之后，薄的互层流纹英质结晶凝灰岩（10-30厘米）记录了破火山口中长英质岩浆的最早喷发。火山口的形成随后被一段静止期所打断，这是由当地几米厚的震积砂岩的存在所记录的，该砂岩是由角砾岩中的细粉冲刷而成的。随后的复苏创造了一个断裂模式的圆顶，沿沿着流纹英安岩岩浆上升的堤坝和爆发高达约10公里3的流纹英安岩intracaldera熔结凝灰岩。这次大规模的喷发导致火山口底部进一步下沉，形成了一个现在部分被清空的岩浆房。熔结凝灰岩中的镁铁质包裹体表明，喷发是由基性岩浆多次注入长英质岩浆所占据的岩浆室而引发的。
Abstract. The Northern Marginal Zone of the Rum Central Igneous Complex in NW Scotland represents part of the early, felsic phase of the volcano. The marginal zone is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks. Its formation has been explored through field examination of the ring fracture system of the Complex and its pyroclastic and epiclastic intracaldera facies. A sequence of magmatic tumescence and chamber growth caused initial doming, followed by the formation of a collapse structure without accompanying volcanism. This collapse structure, circular in plan, is akin in origin to a salt basin formed by crustal stretching above a rising diapir. We call this the proto-caldera. Collapse breccias, which represent the slumping and sliding of megablocks, blocks and boulders of the Torridonian sandstones which form the walls of the basin, were the original infilling. Logs of these deposits reveal considerable variation in thickness of the breccias (from 80–170 m) in the Complex, indicating an uneven floor to the proto-caldera, consistent with piecemeal collapse. Following accumulation of up to >70 m thickness of breccia, thin interbedded rhyodacitic crystal tuffs (10–30 cm) record the earliest eruptions of felsic magma in the caldera. Caldera formation was then interrupted by a period of quiescence, recorded by the presence of an epiclastic sandstone of locally several metres thickness, formed by washout of fines from the breccias. Subsequent resurgence created a fracture pattern characteristic of doming, along which rhyodacite magma rose in dykes and erupted up to perhaps 10 km3 of rhyodacitic intracaldera ignimbrites. This major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber. Mafic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma.