Petrogenesis of the lavas of the Usu volcano, Hokkaido, on the basis of some fractional crystallization process

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Detailed petrographic descriptions of the lavas from Usu volcano were made already by several researchers, but there has been no satisfactory petrological consideration on the remarkable difference in chemical composition between somma and dome-lavas. The somma-lavas, forming a stratocone, are tholeiitic basalt or mafic andesite (SiO2=49.36-53.46%); on the other hand, the succeeding dome-lavas and pumice are dacite or rhyolite (SiO2=68.89-73.04%). The present paper have applied Osborn's (1959) idea, which proposes two fractional crystallization trends depend on the Po2 conditions, to interprete the genesis of the Usu lavas. If the chemical compositions of the lavas are plotted in the FeO+Fe2O3/FeO+Fe2O3+MgO-SiO2 diagram, the sommalavas lie on a steep ascending trend while the dome-lavas lie on a gentle one. With some assumptions, the idealized fractional crystallization of the parental magma can be calculated as follows; (1) The genesis of somma-lavas; Fractional crystallization on the tholeiite trend. (2) The genesis of dome-lavas; Fractional crystallization on the talc-alkali rock trend. For the dome-lavas, the trend of FeO and Fe2O3 contents could not be shown as a straight line; to maintain Po2, 0.4% oxygen must be added to the andesitic magma. After all, primary magma is solidified as crystal aggregates and residual felsic lava in proportions as shown in Figure 4.
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