ABSTRACT

Ca, Sr and Ba contents in various volcanic rocks (basalts, andesites, rhyolites and their cognate inclusions) from Quaternary volcanoes in Toshima, Udonejima, Niijima and Kozushima, the Izu Islands, Japan, have been determined by an ICP-OES method. Basalts and andesites from Toshima and Niijima make two different series of Sr/Ca-Ba/Ca systematics (SB systematics), indicating that there exist two different primary magmas derived from a common mantle peridotite with different degrees of partial melting. The various series of SB systematics of volcanoes in the Izu Islands so far obtained, suggest that the common mantle peridotite has a chondritic composition in terms of Sr/Ca and Ba/Ca ratios. The various series of SB systematics defined by basalt-andesite-dacite series in the Izu Islands converge into a rhyolitic composition of Jinaijima type. The silicic rock may be an end product of a crystal fractionation process, and might have formed a thin crust under the Niijima and Kozushima region. The majority of rhyolites which characterize the volcanoes of Niijima and Kozushima display quite different SB systematics, suggesting that the rhyolites are formed by remelting of a thin crust, and substraction of Na-rich plagioclase from the silicic magma. The rhyolite volcanism may be triggered by the heat of basaltic magma intruded under the thin crust, as indicated by the presence of cognate inclusions with basaltic composition in the rhyolites.