岩手県，葛根田地熱地域に伏在する第四紀葛根田花こう岩

Abstract

The Kakkonda geothermal field is one of the most prominent liquid-dominated geothermal systems belong to the Sengan geothermal area in Northeast Japan. A granite pluton is confirmed at the bore holes of 1, 950-2, 770m depth, which is considered to be a possible heat source of the area. The pluton is named the Kakkonda granite. The samples examined were obtained bore holes in which temperatures over 350°C were measured. K-Ar ages of minerals of the pluton have the range from 0.068 to 0.34 Ma, which may indicate the ages subsequent passing the closure temperature of each mineral. The intrusive age of the pluton might have been older than the obtained data because of high temperatures, and might have been slightly younger than 0.7-1.0 Ma, from the relation between the ages of the adjacent Tamagawa Welded Tuffs. At any rate, obtained ages are the youngest of all the plutons heretofore reported. Chemical composition of the Kakkonda Granite ranges from quartz diorite and tonalite of SiO2, 65% to granite of SiO2 75%, and is similar to that of rocks of the adjacent older volcanoes of the age 0.5-1 Ma, but shows slight differences from that of the Tamagawa Welded Tuffs ; i. e., the former two are lower in A12O3, P2O5 and higher in MgO contents than the Tamagawa Welded Tuffs. Major and trace element chemistry of the pluton also shows similar characteristics of that of the Pleistocene dacitic and rhyolitic pyroclastic flows from southern part of northeast Honshu, Japan.The genetic relation between the pluton and the Tamagawa Welded Tuffs or the older volcanoes should be further studied in detail.