SR isotope geochemistry of voluminous rhyolite of the Tamagawa Welded Tuffs, related rocks and andesite of the young volcanics, in Hachimantai geothermal area, Northeast Japan |
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| Authors: | Shigeko Togashi Teruo Shirahase Shiro Tamanyu |
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| Abstract: | The volcanic rocks from the Hachimantai geothermal area, Northeast Japan, are divided into the following five groups: (1) rhyolite of the Tamagawa Welded Tuffs (about 500 km3, 2.0–1.4 Ma); (2) andesite related to the Tamagawa Welded Tuffs (about 10 km3, 1.5 Ma); (3) andesite of the Matsukawa Andesite (about 100 km3, 1.8-1.2 Ma); (4) dacite of the Kashinai Formation (about 100 km3, 1.1 Ma); and (5) andesite of the Young Volcanics (about 300 km3, younger than 1 Ma).
ratios of less-altered samples of the five groups range from 0.7038 to 0.7046. There is no significant difference in ratios among the five groups in spite of differences in age and chemical composition. Therefore, fusion of or contamination by old acidic crust can be ruled out for the genesis of the voluminous acidic magma which produced the rhyolite of the Tamagawa Welded Tuffs and the dacite of the Kashinai Formation.The low
ratios and the chemistry suggest two possibilities for the genesis of the acidic rocks. One is a low degree of partial melting of the basic crust, which has a low
ratio, under high temperature. Another is a high degree of fractional crystallization from basic magma derived from the mantle. |
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