Acidic Plutonism in the Izu-Ogasawara (Bonin)-Mariana (IBM) Arc and Growth of Arc Crust: Petrological and Geochemical Characteristics of the Tonalite at the Komahashi-Daini Seamount and Difference From the Tanzawa Plutonic Complex

Recent seismic refraction and reflection data suggest that the continents are underlain by mafic lower crust and felsic middle crust. Petrogenesis of granitic middle crust layers is important for understanding the formation and evolution of continental crust. In modern tectonic regimes, tonalitic rocks and chemically equivalent volcanic rocks occur in island arcs and active continental margins. Thus, the petrogenesis of tonalite and related rocks in intra-oceanic arc settings is of great importance in understanding the processes of both recent island arc and continental crust formation. The Komahashi-Daini Seamount, in the northern Kyushu-Palau Ridge in the northern Philippine Sea plate, was investigated by the Japanese Geodynamics Project (GDP) cruises in the 1970's, and by the R/V Tansei-maru (Ocean Research Institute, University of Tokyo) in the 1990's. Plutonic rocks were dredged from the seamount, and have great importance for understanding the processes of island arc and continental crust formation. The petrographical and geochemical characteristics of the Komahashi-Daini Seamount tonalite are summarized as follows: (1) These tonalites are classified into biotite-hornblende tonalite and hornblende tonalite. Phenocrysts, especially plagioclase, show common lamellar twins and oscillatory zoning patterns; (2) This tonalite show low content of bulk LILE, and classified into low-K calc-alkaline, 1 to 8 wt.% MgO with 55 to 75 wt.% SiO2; (3) This tonalite shows roughly parallel and increasing total REE content with increasing SiO2 content, except for increasingly strong negative Eu anomaly at higher SiO2. These factors indicate that the Komahashi-Daini Seamount tonalite was produced by fractional crystallization. The parent magma of this tonalite is considered lower than 56 wt.% SiO2. Based on this relationship, we concluded that the source for the parental magma was arc mantle peridotite. We compared these tonalites with typical tonalite, i.e., Tanzawa Complex, central Japan. The Tanzawa complex is considered to represent the lower-middle crust of the IBM arc. One of the characteristics of these tonalites is that cumulate textures are common in the mafic rocks. And HFSE and REE vs. SiO2 diagrams of Tanzawa tonalites show inflection trends at 62 wt.% SiO2. This tonalite was derived from an intermediate (62 wt.% SiO2) parent magma by crystal fractionation (felsic part) and accumulation (basic part). The parent material of the intermediate magma is basaltic lower crust, and the gabbro is restite from the partial melting process (Kawate and Arima 1998). Differences between Komahashi-Daini Seamount and Tanzawa tonalite are; (1) Cumulate textures are not observed in the tonalite from the Komahashi-Daini Seamount; (2) Komahashi-Daini Seamount tonalite shows linear variation of Zr and REE vs. SiO2. These data and observations also support the interpretation that tonalite in the Komahashi-Daini Seamount was produced from basaltic magma. We suggest that this process of primary felsic plutonic activity predominated during the early stage of oceanic island arc activity, and later shifted to secondary granitoid activity, i.e., crystal fractionation and accumulation from andesitic magma derived from partial melting of basaltic lower crust, as represented by the tonalite in the Tanzawa Complex.