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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Petrogenesis of alkaline basalts at the tip of a propagating rift: Evidence from the Heimaey volcanic centre, south Iceland
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Petrogenesis of alkaline basalts at the tip of a propagating rift: Evidence from the Heimaey volcanic centre, south Iceland

机译：裂谷传播时碱性玄武岩的成岩作用：来自冰岛南部海迈火山中心的证据

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摘要

Heimaey is the volcanic centre in the Vestmannaeyjar Volcanic System, situated at the tip of Iceland's propagating Eastern Volcanic Zone (EVZ). The Heimaey lavas are slightly alkaline, ne-normative, basalts with plagioclase, olivine and Ti-magnetite as phenocryst phases. Inversion modelling using REE suggests that a primary magma is generated by fractional melting over a depth interval of 100-65 km beneath Heimaey of a source similar to that of MORB. The primary magma fractionated 31% olivine and clinopyroxene en route to ponding at the base of the crust (30-35 km depth). Tectonic events as a result of the extensional stress exerted by the southwards propagating EVZ causes magmas to be emplaced into different levels of the crust where they evolve by fractional crystallization independently of each other (i.e. polybaric fractionation). During residence in a parental magma chamber at the mantle/crust boundary the fractionating assemblage is dominated by olivine and clinopyroxene, whereas olivine and plagioclase dominates the fractionating assemblage during residence in crustal magma chambers. The most evolved magma composition can be related to a parental Heimaey melt by 73% fractional crystallization of predominantly plagioclase, clinopyroxene and olivine. The residence times in crustal magma chambers are short as indicated by the absence of equilibrium phenocryst assemblages (e.g. lack of cpx-phenocrysts). Oxygen and radiogenic isotopes suggests that no crustal contamination occurred, and that the role of magma mixing is insignificant in the evolution of the Heimaey lavas. The polybaric evolution of the Heimaey lavas in small, isolated, magma chambers at different levels in the crust fits well with a southward propagation of the EVZ, with the rift-tip currently located beneath the Vestmannaeyjar Volcanic System.
机译：Heimaey是Vestmannaeyjar火山系统中的火山中心，位于冰岛蔓延的东部火山区（EVZ）的尽头。 Heimaey熔岩为弱碱性，正规范的玄武岩，具有斜长石，橄榄石和钛磁铁矿作为隐晶相。使用REE的反演模型表明，一次原始岩浆是通过在Heimaey下方100-65 km的深度间隔内分馏融化而产生的，该震源类似于MORB。初级岩浆在进入地壳底部（深度30-35 km）的过程中，将31％的橄榄石和斜辉石分馏。由向南传播的EVZ施加的伸展应力导致的构造事件导致岩浆进入地壳的不同水平，在那里它们通过分步结晶而彼此独立地演化（即多气压分馏）。在地幔/地壳边界处的母岩浆室中居住期间，分馏组合主要由橄榄石和斜辉石占主导地位，而在地壳岩浆室中，橄榄石和斜长岩则在分馏组合中占主导地位。最主要的岩浆成分可能与母体Heimaey熔体有关，主要是斜长石，斜辉石和橄榄石的73％分级结晶。地壳岩浆室内的停留时间很短，这是由于缺乏平衡的隐晶组成（例如缺乏cpx-隐晶）所表明的。氧气和放射性同位素表明没有发生地壳污染，并且岩浆混合的作用在Heimaey熔岩的演化中并不重要。 Heimaey熔岩在地壳中不同水平的小而孤立的岩浆室内的多弧形演化与EVZ的向南传播非常吻合，裂谷尖端目前位于Vestmannaeyjar火山系统下方。

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