Origin of the latest Miocene alkaline rocks from Oki-Dogo Island, SW-Japan

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Authors:Kobayashi, Shinji; Sawada, Yoshihiro
Author Affiliations:Primary:
Tohoku University, Graduate School of Science, Institute of Mineralogy, Petrology, and Economic Geology, Sendai, Japan
Shimane University, Japan
Volume Title:Ganko Journal of Mineralogy, Petrology and Economic Geology
Source:Ganko = Journal of Mineralogy, Petrology and Economic Geology, 93(5), p.162-181. Publisher: Mineralogical Society of Japan and the Japanese Association of Mineralogists, Petrologists and Economic Geologists, Sendai, Japan. ISSN: 0914-9783
Publication Date:1998
Note:In Japanese with English summary. 34 refs.; illus., incl. strat. cols., 8 tables, geol. sketch map
Summary: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-SiO2 rhyolites of the Oki Alkaline Volcanic Group is examined. 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-SiO2 rhyolites magma was not generated by fractional crystallization from trachyte magma or by magma mixing between trachyte and high-SiO2 rhyolites magmas. High-SiO2 rhyolites magmas were not generated by fractional crystallization from low-SiO2 rhyolites magma. Aphyric low- and high-SiO2 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 PH2O conditions: that is, low-SiO2 rhyolites were generated under higher PH2O than were the high-SiO2 rhyolites. Fe, Ce, Pb, Ba, Nb, Rb, Zr, Y and Th contents of both low- and high-SiO2 rhyolites vary between factors of 1.1 to 6 times, compared with restricted SiO2 variation of only 71.4 to 72.7 wt% for the low-SiO2 rhyolites, and 75.0 to 75.3 wt% for high-SiO2 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-SiO2 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.
Subsections:Igneous rocks
Subjects:Alkali basalts; Basalts; Cenozoic; Fractional crystallization; Igneous rocks; Magmas; Miocene; Mixing; Neogene; Partial melting; Rhyolites; Shoshonite; Tertiary; Trace elements; Trachytes; Upper Miocene; Volcanic rocks; Asia; Far East; Japan; Oki-Dogo Island
Coordinates:N361000 N362500 E1332500 E1324500
Abstract Numbers:00M/3943
Record ID:2001025715
Copyright Information:GeoRef, Copyright 2020 American Geosciences Institute.
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