Hawaiian basalt and Icelandic rhyolite: Indicators of differentiation and partial melting
Hawaiian basalt and Icelandic rhyolite: Indicators of differentiation and partial melting
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DOI：
10.1007/bf01829378
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发表时间：
1991-06
期刊：
影响因子：
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通讯作者：
B. Marsh;B. Gunnarsson;R. Congdon;R. Carmody
中科院分区：
文献类型：
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作者：
B. Marsh;B. Gunnarsson;R. Congdon;R. Carmody
尽管夏威夷岛上有大量的玄武质火山活动，但并没有产生流纹岩。另一方面，冰岛的流纹岩火山活动占其表面岩石的10-12%。这种对比研究使用的基本火成岩过程表现出的片状夏威夷熔岩湖和Shonkin凹陷岩盖在蒙大拿州。高度分化的残余熔体通常位于向内推进的凝固前沿内，通常无法进入喷发过程。只有当大量的初始斑晶群存在时，厚的基底堆晶才能迅速形成，才有可能将高度分化的熔体供应到活性区（即，岩浆房的一部分。虽然在夏威夷，橄榄石的沉降对分异有长期的控制，但在凝固前沿内会发生进一步的分异。只有通过反复的运输和保持，才有可能区分超过凝固前沿前沿的临界组成（10.7% MgO和51.5% SiO2）。通过对夏威夷和Shonkin凹陷的晶体粒度分布（CSD）分析，揭示了玄武岩体凝固过程中的物理化学过程，包括结晶动力学过程。正是这种基本的双峰特征是理解冰岛火山活动的关键。冰岛的火山岩主要以双峰形式出现，玄武岩与中央火山有关。流纹岩、花岗斑岩和霏细岩是常见的，侵入体通常是层状的。火山灰流和真正的花岗岩类侵入体是罕见的。在托尔法冰盖中央火山的大量熔岩包含不平衡斑晶组合。这一点，以及流纹岩和玄武岩之间的氧同位素值的分歧，反映了冰岛的异质玄武岩地壳广泛的部分熔融产生这些流纹岩。相对较小的、化学性质不同的、空间上紧密的岩体是由早期凝固循环中的花岗斑岩偏析集中而成。这一过程也反映在冰岛东部Slaufrudalur的层状花岗斑岩侵入体中。Slaufrudalur是一个没有通风口的地下破火山口，在火成岩过程和风格上与陆上Torfajokull破火山口相当。夏威夷主要是由于晶体沉降导致的分离结晶，不产生流纹岩。冰岛的地质构造允许薄而热的玄武岩地壳不断地进行大规模的再加工，通过集中原始的玄武岩偏析和矿脉以及部分熔融的中间喷出物，这些喷出物已经深入地壳，从而产生流纹岩。
In spite of the voluminous basaltic volcanism on the island of Hawaii, rhyolite is not produced. Iceland, on the other hand, exhibits common rhyolitic volcanism amounting to some 10–12% of its surface rocks. This contrast is investigated using the fundamental igneous processes exhibited by sheet-like Hawaiian lava lakes and Shonkin Sag laccolith in Montana. Highly differentiated, residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes. Only when a large initial phenocryst population is present, from which a thick basal cumulate can rapidly form, is it possible to supply highly differentiated melt into the active (i.e., eruptable) portion of the magma chamber. Although there is protracted control of differentiation at Hawaii by settling of olivine, further differentiation occurs within the solidification fronts. Only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front (∼ 7% MgO and 51.5% SiO2). Crystal size distributions (CSDs) for Hawaii and Shonkin Sag are used to demonstrate the inferred physical and chemical processes of solidification, including the kinetics of crystallization.A ubiquitous feature of these basaltic bodies is the formation of coarse veins and segregations of refined melt and granophyres within the upper solidification front. It is this fundamental bimodal feature which is the key to understanding Icelandic silicic volcanism.Rhyolites in Iceland occur mainly as a bimodal population with basalts associated with central volcanoes. Rhyolites, granophyres, and felsites are common, with the intrusions often being layered. Ash flows and true granite-like intrusions are rare. The voluminous silicic lavas at Torfajokull central volcano contain disequilibrium phenocryst assemblages. This, and the disagreement in oxygen isotopic values between rhyolites and basalts, reflects extensive partial melting of the heterogeneous basaltic crust of Iceland to produce these rhyolites. Relatively small, chemically distinct, and spatially intimate silicic bodies are formed by concentrating granophyric segregations from earlier cycles of solidification. This process is also reflected in the layered granophyric instrusion of Slaufrudalur in eastern Iceland. Slaufrudalur is an unvented subterranean caldera, equivalent in igneous processes and style to the subaerial Torfajokull caldera.Hawaii is dominated by fractional crystallization due to crystal settling and does not produce rhyolite. Iceland's tectonics allow continual and extensive reprocessing of thin, hot basaltic crust which produces rhyolite by concentrating original silicic segregations and veins and by partially melting intermediate extrusives, which have subsided deep into the crust.