Abstract
The Early Miocene Shijujiyama Formation in the Cape Muroto area is presumed to have accumulated in a trench-slope basin formed on the Hioki accretionary complex. The lower part of the formation is dominated by mudstone intercalated with pillow lava, volcanic breccia, and volcanic sandstone. A dolerite (Shijujiyama dolerite), inferred to be a dyke, occurs in the mudstone, and a dyke (Maruyama dolerite) intrudes the Hioki melange along the Maruyama coast. The pillow basalt, volcanic breccia (basaltic andesite and dolerite clasts), and dolerite dykes are geochemically sub-alkaline basalt/andesite. The basaltic andesite breccia and pillow basalt (together referred to as the Shiina volcanic rocks), and Shijujiyama dolerite have geochemical affinities with island arc basalt, whereas the Maruyama dolerite and dolerite breccia have affinities with MORB. The MORB magmas originated from the subducting, shallowly buried, active spreading center of the Shikoku Basin. The genesis of the Shiina volcanic rocks, which involved being extruded upon the floor of a trench-slope basin, can be attributed to the assimilation of accreted mudstone by MORB magma and fractional crystallization. In conclusion, the Maruyama and Shijujiyama dolerites and the Shiina volcanic rocks of the Cape Muroto area could be explained as products resulting from near-trench magmatism related to ridge subduction at about 15 Ma.
