Origin and tectonic setting of the Cenozoic basalts in Southwest Japan

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1.Origin of basaltLate 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 othe 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.2.Magmatic processes(1) Degree of partatial melting and produced magma : Kitamatsuura basakts (KMB) is divided into the alkali and the sub-alkaline (tholeiitic and calcalkaline) rock series. The primary magam of the alkali rock series was derived by a small degree of partial melting of a source mantle at 10-15 kbar and that of the tholeiitic rock series was produced by a large degree of partial melting of the same source at 5-10 kbar. (2) Open system magma chamber : In Higashi-Matsuura alkali basalts (HMB), three eruptive cycles are recognized. The magmas of 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. (3) Fractional crystallization of tholeiitic magma : 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. Each magmatic cycle shows a continuous trend on variation diagrams. Sr isotopic ratios gradually increase with increasing SiO2 contents. These lines of evidence suggest that the chemical variation of these rocks are controlled by fractional crystallization, with minor wall rock assimilation.3.Tectonic settingOrigin of these basalts were probably related to the opening of the Sea of Japan. Moreovere, the compositions of mantle diapir or magma have been modified by interaction with MORB or sub-continetal mantle beneath Southwest Japan.