The hydrothermal alteration and contact metamorphism on the tonalite and volcanics of the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge

At the Komahashi-Daini Seamount (KDS) in the northern Kyushu-Palau Ridge (KPR), acidic plutonic rocks (tonalite) were recovered by research cruises of Japanese Geodynamics Project (GDP) during 1970's and R/V Tansei-Maru, Ocean Research institute, during 1990's. Haraguchi et al. (2003) considered that this tonalite was produced by fractional crystallization of the basaltic magma during the arc volcanism before spreading of the Shikoku backarc basin. This study was selected flesh samples. However, many altered plutonic rocks and volcanics were recovered by these cruises. These volcanics show highly alteration. In this study, we research these highly altered plutonics and volcanics by petrographical, geochemical and mineralogical processes using chemical analysis by electron microprobe analyses (EPMA) and identification by X-ray diffraction (XRD). And we consider alteration processes during intrusion of acidic plutonic body. Alteration of tonalites is prominent in colored minerals. Many colored minerals are replaced into chlorite identified by EPMA and XRD analyses. Fresh tonalites not altered of colored mineral are about 1/10 amount of all recovered plutonic rocks. Plagioclase resist from alteration compared to colored mineral, however, albitization is observed in some high-altered tonalites. K-feldspar is rare in altered tonalites. Many volcanics show completely replacement into secondary phases and some rocks display relic igneous textures. Plagioclase phenocrysts exhibit albitization, and albite and chlorite are identified by XRF and EPMA, similar to plutonic rocks. Quartz is also identified. Altered colored minerals are identified into epidote by XRD and EPMA analyses. Fine secondary phases are difficult to identify by microscope observation, and minor secondary phase are difficult to identify by XRD. We considered that the analyses of mineral composition by EPMA are the effective tools of identification of fine mineral phases. The mineral assemblage of chlorite, albite and quartz in the altered plutonic rocks indicate alteration under 150 to 200°C or higher than this temperature. We considered this alteration was caused by hydrothermal circulation between intrusive rock and host rocks. We also considered that the volcanic rocks had effected under contact metamorphism because these volcanics exhibits prominent re-crystallization. The mineral assemblage of epidote, chlorite and albite in the altered volcanic rocks indicate alteration under higher temperature of plutonic rocks. We considered that this secondary mineralization is the effect of contact metamorphism by intrusion of tonalitic magma. Volcanics were also affected under hydrothermal alteration by fluid circulation between intrusion and host rock. Therefore, we considered that these volcanics are the products of arc volcanism before rifting activity, assumed to earliest stage volcanics of the KPR. The earliest stage of arc volcanism in the KPR was only reported from the Palau Islands. Therefore, this volcanics is important to indicate the environment of early stage arc volcanism in the KPR.