The Sudbury Igneous Complex : Viscous emulsion differentiation of a superheated impact melt sheet
The Sudbury Igneous Complex : Viscous emulsion differentiation of a superheated impact melt sheet
复制标题
萨德伯里火成岩复合体：过热冲击熔体片材的粘性乳液分化
DOI：
10.1130/b25579.1
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发表时间：
2005
影响因子：
4.9
通讯作者：
B. Marsh
中科院分区：
文献类型：
--
作者：
M. Zieg;B. Marsh
加拿大安大略省的萨德伯里火成岩杂岩是1.85Ga前一颗巨型陨石撞击大陆地壳在几分钟内产生的巨大熔片的残余物。瞬变空洞和熔融区到达莫霍面，瞬间(∼2分钟)松弛，形成一个更熟悉的大而浅的陨石坑，容纳着厚厚的过热(∼1700°C)熔体，覆盖着∼2公里的角砾岩。由此产生的双峰火成岩杂岩几乎不像成分相似的众所周知的片状天体的结晶。然而，玄武岩和花岗斑岩在同位素和微量元素组成上表现出显著的相似性，表明它们与周围地壳有着密切的共同亲缘关系。这个成岩之谜在这里被解释为快速形成过热的岩浆乳状液的冲击过程的自然、不可避免的结果，我们将其视为角砾岩的高温等价物。广泛的粘性离散、相互分散的镁铁质和长英质液体块形成了乳状液，反映了目标地壳物质的成分不均质性。在几天到几个月的时间里，这些乳液组分根据它们的相对密度分离成一个双峰的诺长石-花岗岩组合，形成了现在萨德伯里火成岩杂岩的基本结构。在最早的岩脉(即偏移量)中有明显的证据表明这种乳状液，这可能提供了新生熔融岩席的最早状态。乳状液分离后，强过热的双峰熔体在各层内立即发生强烈的热对流。这些对流运动使岩浆均匀并迅速冷却到液相线温度，随后对流停止。对流模式被火山口底部的凹陷地形固定在适当的位置，这反过来在将硫化物沉积引导到海湾中起到了关键作用。所有的进一步冷却都是通过上边界和下边界的热传导进行的，在此期间建立了凝固前沿，并从上边界和下边界向内传播。从地面向上，屋顶向下，都有明显的固化迹象。在整个冷却和凝固过程中，发生了最小的分化和成分改变。然而，在凝固阶段，火山口底板上的花岗岩碎屑和厚上覆的阿纳坪组的后备角砾石筏变得不稳定，进入熔体席状，部分熔融的残留物聚集在褐岩和花岗岩的界面上。玄武岩的一些间隙熔体也向上渗入，块体和熔体共同产生了异常过渡带的独特化学和物理特征。萨德伯里熔岩片本质上是一次全面的岩浆实验。相对于任何其他大型陆地岩浆，其形成条件是“精确的”已知的。因此，萨德伯里显然没有任何明显的模式分层，每个单元的整体均质性质，以及通过晶体分馏而缺乏任何显著的化学差异，使萨德伯里成为一个宝贵的例子，说明在大多数大型岩浆室形成时长期盛行的初始条件下不会发生什么。
The Sudbury Igneous Complex of Ontario, Canada, is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1.85 Ga ago. The transient cavity and melting zone reached the Moho and instantly (∼2 min) relaxed to form a more familiar large, shallow crater holding a thick, superheated (∼1700 °C) melt sheet covered by ∼2 km of breccia. There is little about the resulting bimodal igneous complex that resembles crystallization of well-known sheet-like bodies of similar composition. Yet, the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions, suggesting an intimate common parentage from the surrounding crust. This petrogenetic enigma is explained here as a natural, unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion, which we take as the high-temperature equivalent of breccia. A wide spectrum of viscously discrete, interdispersed parcels of mafic and felsic liquids, reflecting the compositional heterogeneity of the target crustal materials, formed the emulsion. Within days to months, the emulsion components separated according to their relative densities into a bimodal norite-granophyre assemblage that formed the basic structure of the present Sudbury Igneous Complex. There is clear evidence of this emulsion in the earliest dikes (i.e., offsets), which likely give the earliest state of the nascent melt sheet. Immediately following emulsion separation, the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer. These convective motions homogenized and rapidly cooled the magma to the liquidus temperatures, whereupon convection ceased. The pattern of convection was pinned in place by the embayment topography of the crater floor, which in turn played a pivotal role in directing sulfide deposition into the embayments. All further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins. There is clear evidence of solidification from the floor upward and the roof downward. Minimal differentiation and compositional modification took place throughout cooling and solidification. Nevertheless, during the solidification stage, granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying Onaping Formation became unstable and entered the melt sheet, and the partially melted remnants collected at the interface between the norite and granophyre. Some interstitial melt from the norite also percolated upward, and, altogether, the blocks and melt produced the distinctive chemical and physical characteristics of the unusual Transition Zone. The Sudbury melt sheet is, in essence, a full-scale magmatic experiment. The conditions of formation, relative to any other large terrestrial magma, are “precisely” known. Thus the clear lack of any significant modal layering, the overall homogeneous nature of each unit, and the lack of any significant chemical differentiation through crystal fractionation establish Sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers.