Abstract
Volcaniclastic rocks such as volcanic breccia, volcanic conglomerate, volcanic sandstone, and mudstone with volcanic rock gravels are found in small bodies scattered sporadically in the Shimanto Supergroup in the east coast area of the Hata Peninsula, western Shikoku, SW Japan. The volcaniclastic rocks are contained in chaotic facies made up mainly of mudstone and sandstone of the Late Oligocene-Early Miocene (Burdigalian) Shimizu and Arimisaki Formations, and are occasionally accompanied by Middle Eocene limestone blocks. Most of the volcaniclastic rocks were originally deposited from sediment gravity flows such as debris flow and turbidity flow, although, the mechanism of their emplacement in the chaotic facies still remains unsolved. Volcanic rocks are composed of alkali basalt, trachyandesite, trachyte and comendite, which are all classified into alkaline volcanic rock series. Amygdules filled with chlorite and calcite are abundant in alkali basalt and trachyandesite, indicating that they were formed at a shallow depth. Most volcanic rocks have been subjected to some degree of alteration. Pseudomorphs of olivine are commonly found in the alkali basalts and all mafic minerals broke down to mixture of chlorite and Fe-Ti oxide minerals. The alkali basalts have geochemical affinity to within-plate basalts of seamount origin judging from some discrimination diagrams using alteration-resistant elements. All observations suggest that collision of a seamount would have occurred during the Late Oligocene-Early Miocene at the convergent margin of western Shikoku. And the fragments of it incorporated into the accretionary prism.
