西南日本の新生代火成活動とテクト二クス

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(1) Cenozoic magmatism in southwest Japan was reviewed and some ideas of origin of island arc magmatism were proposed.(2) Late cenozoic basalts are widely distributed along the southwest Japan arc. In the Chugoku district, an enriched mantle plume with an EM I signature ascended from deep in the mantle. On the other hand, mantle diapirs characterized by an EM II signature rose slowly and were entrained an overlying N-MORB like depleted mantle below the north Kyushu district.(3) The magmas of Higashi-Matsuura alkali basalts (HMB) attract attention by their three cyclic variation in composition (magmatic cycles). It has been found that each magmatic cycle starts with the eruption of primitive basalts followed by fractionated ones. This suggests that the magma chambers of HMB were compositionally stratified.(4) The rocks of Uku island represent four magmatic cycles. Each cycle began with the eruption of basic magma, followed by eruption of andesitic to dacitic magma. The chemical variatio … More n of these rocks are controlled by fracitonal crystallization, with minor wall rock assimilation.(5) Adakite-like magmas in Daisen volcanic zone were derived from partial melting of lower crust or mixing of mantle derived magma and adakite-like magma.(6) A dacite dome at Unzen volcano grew mainly exogeneously when it was small and the effusion rate was high, but endogeneously when the dome became large and the effusion rate declined.(7) In Cenozoic southern Kyushu, moderate to high potassic "flood andesites" were derived from high-magnesian andesite and voluminous low potassic andesites were produced by fractional crystallization of basalt magmas.(8) Many high-magnesian andesites (HMA) in Cenozoic southwest Japan were petrographically described and the origin of HMA magmas was discussed.(9) Mechanisms of Cretaceos to Paleogene plutonism were explained by the plate motions.(10) The low potassium analytical method with ordinary flame photometry in convenience and high precision was developed. This method was applied to the K-Ar dating of igneous and metamorphic hornblendes from ophiolites and coexisting hornblende and clinopyroxene from volcanic rocks. Less