Magmatic Evolution of Zao Volcano, Northeast Japan (<Special Section>Models of Volcano (I))

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  • 東北日本,蔵王火山におけるマグマ系の変遷(<特集>火山のモデル(I))
  • 東北日本,蔵王火山におけるマグマ系の変遷
  • トウホク ニホン ザオウ カザン ニ オケル マグマケイ ノ ヘンセン

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Lavas and pyroclastics from the Zao volcano are primarily calc-alkaline andesite which is associated with minor volumes of basalt, basaltic andesite, and dacite of the same magma series. Small amounts of basaltic pyroclastics of low-alkali tholeiitic series are also recognized. Evolutional history of the volcano is divided into four stages based on geological characteristics. Low-alkali tholeiitic magmas erupted in stage 1, whereas calc-alkaline magmas repeatedly erupted during stages 2 (a, b), 3, and 4. Moreover, in each evolutional stage, the magma displays distinctive trend of compositional variation. Major- and trace-element variations of the low-alkali tholeiitic suite are successfully reproduced by crystal fractionation models of observed phenocrystic phases. On the other hand, there is much evidence for magma mixing in the calc-alkaline rock samples, such as disequilibrium Fe-Mg partitioning between Mg-rich olivine and Fe-rich reversely zoned pyroxene phenocrysts. These strongly suggest that the calc-alkaline rocks were generated by mixing of (olivine-phyric) mafic magma and (pyroxene-phyric) felsic magma. K/Zr ratios for the inferred mafic end-member magmas varied from 70 in stage 2b, up to maximum value, around 100, in stage 3, then down to < 80 in stage 4. On the other hand, the felsic end-member magmas had the K/Zr ratios greater than 90 throughout the stages 2b, 3, and 4. The low-alkali tholeiitic magmas of stage 1 have range of the K/Zr ratios from 57 to 67. Therefore, felsic end-member magmas in stages 2b to 4 and mafic end-member magma in stage 3 can not be derived from these low-alkali tholeiitic magmas simply by fractional crystallization process. Temperature of the felsic end-member magma prior to mixing tends to increase from 900℃ to 1070℃, as the evolutional stage proceeds. The rise of temperature probably suggests that the chamber received continued thermal input through the injection of mafic magmas.


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