Dredge petrology of the boninite- and adakite-bearing Hahajima Seamount of the Ogasawara (Bonin) forearc: An ophiolite or a serpentinite seamount?

摘要

During the Hakuho-Maru KH03-3 cruise and the Tansei-Maru KT04-28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest-southeast elongated, rectangular massif, 60 km x 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on-land ophiolite outcrops. Volcanic rocks included mid-oceanic ridge basalt (MORB)-like tholeiitic basalt and dolerite, calc-alkaline basalt and andesite, boninite, high-Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB-type and island arc-type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Crsharp 50-80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra-subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast-southwest-oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu-Ogasawara (Bonin)-Mariana forearc, the Hahajima Seamount may be better interpreted as a fault-bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (< 2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave-eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body.