隠岐島後における末期中新世隠岐アルカリ火山岩類の成因 [in Japanese] Origin of the Latest Miocene alkaline rocks from Oki-Dogo Island, SW-Japan [in Japanese]
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A large volume of lavas and pyroclastic rocks with varying composition erupted in the latest Miocene in the Oki-Dogo island. In this paper, the origin of shoshonite, trachyte, and low and high-SiO<sub>2</sub> rhyolites of the Oki Alkaline Volcanic Group is examined.<br> On the basis of model calculation, trachyte magma is inferred to be generated by fractional crystallization of plagioclase, clinopyroxene, magnetite and apatite from shoshonite magma, or by partial melting of crustal material or solidified shoshonite magma with complete melting of K-feldspar and/or biotite. From consideration of variation diagrams, low-SiO<sub>2</sub> rhyolites magma was not generated by fractional crystallization from trachyte magma or by magma mixing between trachyte and high-SiO<sub>2</sub> rhyolites magmas. High-SiO<sub>2</sub> rhyolites magmas were not generated by fractional crystallization from low-SiO<sub>2</sub> rhyolites magma.<br> Aphyric low- and high-SiO<sub>2</sub> rhyolites plot in restricted, but separate areas on the normative Qz-Ab-Or diagram. This suggests that these magmas were generated by partial melting of crustal materials under different P<sub>H<sub>2</sub>O</sub> conditions: that is, low-SiO<sub>2</sub> rhyolites were generated under higher P<sub>H<sub>2</sub>O</sub> than were the high-SiO<sub>2</sub> rhyolites.<br> Fe, Ce, Pb, Ba, Nb, Rb, Zr, Y and Th contents of both low- and high-SiO<sub>2</sub> rhyolites vary between factors of 1.1 to 6 times, compared with restricted SiO<sub>2</sub> variation of only 71.4 to 72.7 wt% for the low-SiO<sub>2</sub> rhyolites, and 75.0 to 75.3 wt% for high-SiO<sub>2</sub> rhyolites. The variations in element concentration are interpreted to be due to the following process: during partial melting at the cotectic point, in the some source areas following minerals such as apatite (affecting Y, Ce due to high partition coefficients), zircon (Zr, Th, Ce), sphene (Nb), allanite (Ce, Th), garnet (Y), magnetite (Fe) and biotite (Rb, Ba, Nb) remained as residual phases, in others they did not. This produced separate magmas with low or high concentrations of the elements concerned. This interpretation is supported by the observation that partial melt glasses in crustal xenoliths from Cenozoic basalts in Oki-Dogo Island also show similar compositional trends to the bulk composition of the low- and high-SiO<sub>2</sub> rhyolites. Similar compositional variation is recognized in one high viscosity rhyolite lava flow unit, suggesting that incomplete magma mixing took place between separate magmas generated by different degrees of partial melting.
- Journal of Mineralogical and Petrological Sciences
Journal of Mineralogical and Petrological Sciences 93(5), 162-181, 1998-05-05
Japan Association of Mineralogical Sciences