島根県隠岐，島後火山岩類のストロンチウム同位体組成の時間的変化

Abstract

Fifty-three volcanic rocks from Dogo island, Oki, Shimane Prefecture, southwestern Japa were analyzed for Sr isotopic compositions with two basement rocks. The rock samples consit of talc-alkali rock suite, Nagaoda shoshonite-banakite suite, Oki trachyte-rhyolite suite, Dogo mugearite suite, Hei trachyte and Tsuzurao rhyolite series, and Daimanjiyama, Ohmine, Kuroshima, Shiroshimazaki, Saigo, and Misaki alkali basalt groups in the order of probable eruption sequence. The volcanic rocks of calc-alkali suite and shoshonite-banakite suite were produced before Japan Sea opening (ca. 15 Ma), and both have 87Sr/86Sr ratios higher than 0.7068. Long after Japan Sea opening Oki trachyte-rhyolite suite was erupted (ca. 6.6 Ma); they have rather low 87Sr/86Sr ratios (0.7066-0.7081). Mugearites followed and have similar Sr isotopic composition, whereas 4.6 Ma old Daimanjiyama basalts have clearly low 87Sr/86Sr ratios (0.7050-0.7051). The rocks erupted 3-4 Ma seem to have the lowest 87Sr/86Sr ratios: they are Ohmine, Kuroshima, Shirosimazaki alkali basalt suites (0.7044-0.7048). The 87Sr/86Sr ratios of the Saigo basalts erupted 0.84 Ma are higher than those erupted 3-4 Ma. The latest volcanic products in Dogo island, Misaki basalt suite has even higher 87Sr/86Sr ratios (0.7054-0.7057) than the Saigo basalt suite. Thus, temporal and systematic variation of Sr isotopic compositions of the volcanic rocks from Dogo can be recognized. The 87Sr/86Sr ratios of the rocks were once as high as 0.7066 or even higher than 0.708, but they started decreasing down to ca. 0.7044-0.7048 4-3 Ma ago. Since then the ratios rebounded to 0.7049-0.7055. The Hei trachyte and Tsuzurao rhyolite series are not included in this temporal and systematic change. The mantle diapir associated with Japan Sea spreading might have caused the decrease in the ratios, and either Pacific Ocean plate or Philippine Sea plate subduction may be responsible for this rebound.