K-Ar ages and cooling history of the Kaikomagatake granitoid pluton, and their bearing on tectonic evolution of the Akaishi Mountains, central Japan

Abstract

K-Ar mineral ages were determined for the Miocene Kaikomagatake and associated Yakejizo plutons emplaced in the northern end of the Shimanto Belt within the Akaishi Mountains, central Japan. Coexisting hornblende, biotite and K-feldspar from the Kaikomagatake pluton yielded 13.7 Ma, 11.7 Ma and 10.3 Ma, respectively. The age data combined with closure temperatures and geological data in the area indicate that the pluton was emplaced at about 15 Ma and cooled with a rate of about 110°C/Ma in the temperature range of 100-500°C. The granitoid plutons dated here could have been emplaced in relation to a deep fault, presently recognized as the Itoigawa-Shizuoka Tectonic Line along which the western block uplifed mainly in Quaternary period to form the Akaishi Mountains. Geologic structure of the Akaishi Mountains characterized by north-south trend appears to have formed before the emplacement of these plutons, probably in late Early Miocene. Granitoid activity in the early Middle Miocene, including the Kaikomagatake may have taken place just after the back-arc spreading that formed the Japan Sea Basin. -from English summary