The most representative volcanisms of Paleo-Izu-Bonin-Mariana Ridge were subduction zone-related boninitic magmatisms (49 - 46 Ma). During KT04-28 cruises, adakitic high Mg andesites-dacites (Ar-Ar age: 43.46 ±3.29 Ma, in this study) were firstly recovered from Bonin Ridge forearc seamounts. Adakitic high Mg andesites contain extremely high Sr contents respect to low Y contents, and they show strongly enriched in LREE respect to MREE, and much depleted in HREE, like to typical adakitic rocks; however, the marked positive anomaly in Zr and Hf relative to Sm is much similar to boninites. In adakitic high Mg andesites, we firstly found the presence of the chromian diopside xenocrysts. Based on geochemical modeling calculation, these were chromian diopsides in residual peridotite which had been extracted 10 wt % melt from the original fertile mantle of the Philippine Sea Plate. In the core of chromian diopside xenocrysts, bronzite and pargasite lamellae were crystallized accompanying with Si-rich glass inclusions. By EPMA and SIMS analysis, these Si-rich glass inclusions are dacitic glass inclusions (average: 65.33 wt % SiO2, 0.4 wt % MgO, 2.83 wt % of total alkali) characterized by adakitic-boninitic features (average trace elements compositions: (La/Sm)n = 6.12, (Eu/Tm)n = 9.6, (Hf/Sm)n = 2.4). The co-existing feature of bronzite-pargasite exsolution lamellae and adakitic glass inclusions in chromian diopside xenocrysts is the most direct evidence which showed the chemical reaction between the slab-related adakitic melt and depleted mantle peridotite. These glass inclusions were the primary melts derived from this reaction event. The most important result about petrological studies mentioned above is that the boninites and adakitic high Mg andesites could be produced by the partial meting of the depleted residual mantles which had been reacted with slab melt. According to the slab thermal structure (Iwamori, 2006), the phase diagrams of MORB + H2O and peridotite + H2O system (Maruyama and Okamoto, 2006), the slab melt only can be generated by the partial melting of subducted a young oceanic crust (< 5 Ma). Although now that the tectonic evolution of the Western Pacific during the Eocene is not clear yet, the boninites and adakitic high Mg andesites are the best evidences to demonstrate the existence of an active ridge which was subducting beneath the West Philippine Sea Plate around 50-45 Ma.
