Megacryst-rich Miocene alkali basalt dikes intruded in the Sanbagawa metamorphic rocks from Sakurasanri, Ehime Prefecture

Abstract

Miocene alkali basalt dikes have been reported in a few locations in the Outer Zone of Southwest Japan. We report a new location where alkali basalt dikes have intruded the Cretaceous Sanbagawa metamorphic rocks in Sakurasanri, Northwestern Shikoku. Middle Miocene igneous rocks of the Ishizuchi Group are also found in this region. K-Ar dating of groundmass feldspar in a Sakurasanri alkali basalt dike gave an age of 15.7±0.5 Ma, which is close to or older than the age of igneous rocks of the Ishizuchi Group. The trace element characteristics of the Sakurasanri alkali basalt are similar to those of oceanic island basalt (OIB), although the former was affected by the assimilation of felsic crustal rocks. The Sakurasanri alkali basalt contains abundant megacrysts and gabbro xenoliths. The megacrysts include crystallization products (Cr-rich diopside, olivine, and Cr-spinel) from a primitive basaltic magma and antecrysts (Ti-rich diopside, less-magnesian olivine, and calcic plagioclase) from earlier pulses of alkali basalt magma. Prolonged OIB-type magmatism could have been the heat source of some of the forearc magmatism in Southwest Japan during the middle Miocene.