Transition of magma genesis estimated by change of chemical composition of Izu-bonin arc volcanism associated with spreading of Shikoku Basin

Arc volcanism in the Izu-Ogasawara arc is separated into first and latter term at the separate of Shikoku Basin. Middle to late Eocene early arc volcanism formed a vast terrane of boninites and island arc tholeiites that is unlike active arc systems. A following modern-style arc volcanism was active during the Oligocene, along which intense tholeiitic and calc-alkaline volcanism continued until 29Ma, before spreading of the back- arc basin. The recent arc volcanism in the Izu-Ogasawara arc have started in the middle Miocene, and it is assumed that arc volcanism were decline during spreading of back-arc basin. In the northern Kyushu-Palau Ridge, submarine bottom materials were dredged during the KT95-9 and KT97-8 cruise by the R/V Tansei-maru, Ocean Research Institute, university of Tokyo, and basaltic to andesitic volcanic rocks were recovered during both cruise except for Komahashi-Daini Seamount where recovered acidic plutonic rocks. Komahashi-Daini Seamount tonalite show 37.5Ma of K-Ar dating, and this age indicates early stage of normal arc volcanism. These volcanic rocks are mainly cpx basalt to andesite. Two pyroxene basalt and andesite are only found from Miyazaki Seamount, northern end of the Kyushu-Palau Ridge. Volcanic rocks show different characteristics from first term volcanism in the Izu-Ogasawara forearc rise and recent arc volcanism. The most characteristic is high content of incompatible elements, that is, these volcanics show two to three times content of incompatible elements to Komahashi-Daini Seamount tonalite and former normal arc volcanism in the Izu outer arc (ODP Leg126), and higher content than recent Izu arc volcanism. This characteristic is similar to some volcanics at the ODP Leg59 Site448 in the central Kyushu- Palau Ridge. Site448 volcanic rocks show 32-33Ma of Ar-Ar ages, which considered beginning of activity of Parece Vela Basin. It is considered that the dredged volcanic rocks are uppermost part of volcanism before spreading of the Shikoku basin, and these rocks show about 26Ma, at the beginning of activity of Shikoku Basin, of Ar-Ar ages (Ishizuka, pers comm.). Therefore, it is considered that these volcanics are associated with rifting acrivity before spreading of back-arc basin. Based on these observations, it is considered that the chemical characteristics of Izu-Ogasawara arc volcanism were changed at spreading of Shikoku Basin. That is, low incompatible element content activity had continued from tholeiitic activity in the Haha-Jima during early arc volcanism to normal arc volcanism in the Izu forearc region recovered during ODP Leg126 operation. High incompatible element activity had begun at the beginning of back-arc basin activity, and incompatible element content of Izu arc magma was decreasing after spreading of Shikoku Basin to recent activity. We considered these characteristics are interpreted that mantle beneath island arc were change depleted composition to enrich composition at the back arc basin activity. And we assumed that this mantle movement is associated with back-arc basin activity.