Study on the Tertiary Volcanic Rocks in the Izu-Mariana Arc

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1. Study on the Tertiary volcanic rocks in the Bonin Islands.(1) Sheeted dike complex at Ishi-ura on the east coast of Chichi-jima.The dikes and associated pillow lavas are composed mainly of clioenstatite-bearing boninites, which are the most primitive and undifferentiated among the Bonin Island boninites. This confirms the early suggestion that the parallel dike swarm on Ishi-ura, located at the lowest of the pillow lava sequences in Chichi-jima, represents a part of the sheeted dike complex common in many ophiolites.(2) Augite boninites in the Tatsumiwan area of southeastern Chichi-jima.The augite boninites contain augite phenocrysts attaining the highest Mg value 89.3, most magnesian so far reported from the Bonin Island boninites. These rocks crystallized augite prior to orthopytoxene and belong to cpx-HMA, which is the first discovery from the Bonin Island boninites.(3) Island arc tholeiites similar to those occurring in Haha-jima in chemistry were discovered on John Beach in southwestern Chichi-jima.2. Comparative study on boninites and other magnesian andesites.(1) Trace elements in the Tertiary volcanic rocks from western Setonaikai.The Setouchi magnesium andesites have remarkably high Rb, while Sr is rather low. The Setouchi volcanic rocks form by addition of water containing Rb, K, Ba and Zr, as well as boninites.(2) Sanukitoids quite identical with the Setouchi sanukitoids in chemical and mineral compositions were discovered in Choshi of eastern Honshu Nagasaki of western Kyushu.