Trachytic pyroclastics from Agua de Pau volcano, Sao Miguel, Azores: evolution of a magma body over 4,000 years
Trachytic pyroclastics from Agua de Pau volcano, Sao Miguel, Azores: evolution of a magma body over 4,000 years
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亚速尔群岛圣米格尔岛阿瓜德波火山的粗面质火山碎屑岩：4000 多年来岩浆体的演化
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发表时间：
1982
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通讯作者：
M. Storey
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作者：
M. Storey
最近的圣米格尔地层学记录了大量由次-普林宁期火山喷发产生的粗质火山碎屑沉积。Walker和Croasdale（1971）以及Booth等人（1978）进行的火山年代学研究表明，在过去5000年里，圣米格尔岛上三座平流层活火山之一的Agua de Pau火山口发生了5次这样的喷发。对由此形成的火山碎屑序列进行了地球化学和电子探针研究，揭示了单个矿床之间浮石碎屑化学和矿物学的显著差异；大多数这些变异表现出时间控制。例如，Sr和Eu/Eu{Sr *}的价值随着演替而逐渐降低，而La、Zr和Nb等不相容元素则逐渐富集，在最近的矿床中达到最高浓度。这5个气落浮石矿床代表了一个演化的粗溶岩浆体的连续样品，其中碱长石的分馏在很大程度上控制了液体的下降线。这种晶体分馏导致了熔体中过碱度的发展，这是第二年轻的矿床福戈D火山喷发时形成的。一些矿物学和化学特征的存在表明，粗溶性熔体周期性地受到演化程度较低的岩浆的污染。
The Recent stratigraphy of Sao Miguel records large numbers of trachytic pyroclastic deposits produced by sub-plinian to plinian eruptions. Tephrochronological studies by Walker and Croasdale (1971) and Booth et al. (1978) have shown that in the last 5,000 years there have been five such eruptions from the caldera of Agua de Pau, one of the three active stratovolcanoes on Sao Miguel.A geochemical and electron microprobe study made on the resultant pyroclastic succession, revealed significant variations in pumice clast chemistry and mineralogy between the individual deposits; most of these variations show temporal control. For example, Sr and Eu/Eu{sr*} decrease in value up through the succession, whereas incompatible elements such as La, Zr and Nb show stepwise enrichment, attaining highest concentrations in the most recent deposit. It is proposed that the five air fall pumice deposits represent successive samples of an evolving trachytic magma body in which fractionation of alkali feldspar has largely controlled the liquid line of descent. This crystal fractionation had resulted in the development of peralkalinity in the melt by the time Fogo D, the second youngest deposit, was erupted.The presence of some mineralogical and chemical peculiarities suggest that the trachytic melt has been periodically contaminated by less evolved magmas.