A Middle Miocene basalt dike in the Tomikusa area in southern Nagano Prefecture, central Japan

Abstract

We discovered a NNW–SSE-trending, 85-cmwide basalt dike that was intruded into Early Miocene Tomikusa Group sediments in southern Nagano Prefecture, central Japan. K–Ar dating of plagioclase within this dike yielded an age of ca. 13.6 Ma (Middle Miocene), indicative of the timing of emplacement/cooling of the dike. Intrusion of the dike was coeval with late-stage volcanic activity that formed the Shitara igneous rocks a few tens of kilometers to the southwest of the study area.