We report on the petrography, whole-rock chemical composition, Sr isotope ratio, and K–Ar age of borehole core samples (310–630 m depth) of basement rocks from the northwestern caldera floor of Aso volcano, central Kyushu. The basement core is composed of granodiorite and hornfelsic pelitic-psammitic schist. Both the granodiorite and schist contain mainly quartz, plagioclase, K-feldspar, biotite, and opaque minerals. The granodiorite also contains rare hornblende. The schist has a banded structure, and the schist and granodiorite are migmatitic at a depth of 553 m. The granodiorite yields a K–Ar biotite age of 105 Ma. The petrographic characteristics and age of the granodiorite are similar to those of other basement granitic rocks in and around the Aso area (93–108 Ma). These observations indicate that the basement granitoids of the Aso area cooled at the same time as Cretaceous granitoids in central Kyushu.
