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首页> 外文期刊>Contributions to Mineralogy and Petrology >Origin of oceanic phonolites by crystal fractionation and the problem of the Daly gap: an example from Rarotonga
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Origin of oceanic phonolites by crystal fractionation and the problem of the Daly gap: an example from Rarotonga

机译：大洋菱沸石的晶体分级成因和戴利间隙的问题：以拉罗汤加岛为例

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摘要

Felsic alkalic rocks are a minor component of many ocean island volcanic suites, and include trachyte and phonolite as well as various types of alkaline and peralkaline rhyolite. However, there is considerable debate on the nature of their formation; for example, are they formed by partial melting of anomalous mantle or the final products of fractional crystallization of mafic magmas. The phonolites and foidal phonolites on Rarotonga were formed by low pressure crystal fractionation of two chemically distinct parental magmas. Low silica and high silica mafic magmas produced a basanite–foidal phonolite series and an alkali basalt–phonolite series, respectively. The foidal phonolite composition evolved from the low silica mafic magmas by approximately 60% fractionation of titanaugite + leucite + nepheline + magnetite + apatite. Fractionation continued with the crystallization of aegirine-augite + nepheline + kaersutite + magnetite + apatite. The phonolites formed from the alkali basalts by approximately 40% fractionation of kaersutite + titanaugite + Fe–Ti oxide + plagioclase + apatite and continued to evolve further by fractionation of anorthoclase + nepheline + aegerine-augite + Fe–Ti oxides. As the magmas fractionated in both suites, their overall viscosities (solid + liquid) increased until a point was reached whereby viscosity inhibited the eruption of magmas with compositions intermediate between the mafic rocks and the felsic rocks. However, the magmas continued to fractionate under static conditions with the residual fluid becoming foidal phonolitic in the low silica suite or phonolitic in the high silica suite. These phonolitic liquids, as a result of an increase in volatiles and enrichment of alkalis over aluminum, would actually have a lower viscosity than the intermediate liquids. This decrease in viscosity and the switch from a magma chamber being predominantly a liquid with suspended solids to a solid crystalline network with an interstitial liquid enabled phonolitic liquids to migrate, pool, and eventually erupt on the surface.
机译：长英质碱性岩石是许多大洋岛火山岩组中的次要组成部分，包括速滑岩和方沸石，以及各种类型的碱性和过碱性流纹岩。但是，关于其形成的性质，存在很多争论。例如，它们是由异常地幔的部分熔融或镁铁质岩浆的部分结晶的最终产物形成的。拉罗汤加岛上的方沸石和非方体方沸石是由两种化学性质不同的母体岩浆的低压晶体分级形成的。低硅镁铁质岩浆岩和高硅镁铁质岩浆岩分别产生了玄武岩-卵形辉石系列和碱玄武岩-辉石系列。从低硅质镁铁质岩浆演化出的辉石状辉石岩成分约为60％的钛铁矿+白云母+霞石+磁铁矿+磷灰石。分馏继续进行，同时生成了aegirine-augite +霞石+ kaersutite +磁铁矿+磷灰石。碱玄武岩形成的语音沸石通过约40％的玄武岩+钛铁矿+ Fe–Ti氧化物+斜长石+磷灰石分馏而形成，并通过分馏正长石酸+霞石+爱琴海碱-辉石+ Fe–Ti氧化物而继续发展。随着岩浆在这两个组中的分馏，它们的总体粘度（固体+液体）增加，直到达到一个点，从而粘度抑制了岩浆岩的喷发，其成分介于铁镁质岩和长英质岩之间。但是，岩浆在静态条件下继续分馏，残余流体在低氧化硅组中变成为类胶质的，或在高氧化硅组中为胶质的。这些挥发性液体由于挥发性增加和碱在铝上的富集而增加，因此其粘度实际上要比中间液体低。这种粘度的降低和从岩浆腔室的切换主要是具有悬浮固体的液体到具有间隙液体的固态晶体网络，从而使声液体在表面迁移，聚集并最终喷发。

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  • 来源
    《Contributions to Mineralogy and Petrology》 |2001年第3期|336-346|共11页
  • 作者

    G. Thompson; I. Smith; J. Malpas;

  • 作者单位

    Department of Earth Sciences University of Hong Kong Hong Kong;

    Department of Geology University of Auckland New Zealand;

    Department of Earth Sciences University of Hong Kong Hong Kong;

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