Textural and compositional evidence for magma mixing and its mechanism, Abu volcano group, southwestern Japan
Textural and compositional evidence for magma mixing and its mechanism, Abu volcano group, southwestern Japan
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日本西南部阿布火山群岩浆混合及其机制的结构和成分证据
DOI：
10.1007/bf00963583
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发表时间：
1986
影响因子：
3.5
通讯作者：
T. Koyaguchi
中科院分区：
文献类型：
--
作者：
T. Koyaguchi
第四纪玄武岩，安山岩和英安岩从阿布单成火山群，日本西南部，（由40多个单成火山）显示两个不同的化学趋势，特别是在FeO*/MgO与SiO2图。一种趋势的特征是FeO*/MgO富集，SiO2含量略有增加（Fe型趋势），而另一种则显示出明显的SiO2富集，FeO*/MgO比值相对恒定（Si型趋势）。铁型趋势是由分离结晶扣除橄榄石和普通辉石从原始碱性玄武岩岩浆。硅型倾向的岩石的特征是部分熔融或再吸收的石英和钠质斜长石斑晶和/或细粒玄武岩包裹体。它们很可能是含橄榄石斑晶的原始碱性玄武岩岩浆与含石英、钠质斜长石和角闪石斑晶的英安岩岩浆混合的产物。岩石学的变化，以及从玄武岩到英安岩的硅型趋势的化学变化是由玄武岩和英安岩岩浆的各种混合比例。安山岩和英安岩中的部分角闪石和单斜辉石斑晶可能是在岩浆混合过程中从玄武岩浆中结晶出来的。在混合之前，橄榄石和尖晶石，石英，钠质斜长石和普通角闪石分别在玄武质和英安质岩浆中结晶。在熔岩流中，玄武岩包裹体的丰度从火山口附近向熔岩流周边逐渐减少，而被再吸收斑晶的数量则相反，表明岩浆房中存在分带现象。混合方式的变化取决于混合比。在镁铁质混合物中，玄武岩和英安岩岩浆可以液态混合（液-液混合）。另一方面，在玄武岩混合物中，玄武岩岩浆被淬火并形成包裹体（液-固混合）。在混合过程中，解体的玄武岩岩浆和寄主英安岩岩浆很快达到热平衡。只有当平衡温度充分高于玄武岩岩浆固相线时，混合岩浆的成分才会均匀化。硅型趋势在化学和岩相学上与钙碱性趋势相似。因此，区别于分离结晶趋势（如Fe型趋势）的钙碱性趋势可能是岩浆混合的产物。
Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.