抄録
A geological study on the “Young Volcanoes” in the Matsukawa-Kakkonda area, northeast Japan was carried out to evaluate the heat source in this area. The products from the “Young volcanoes” are divided into the Early stage volcanics (erupted in Matsuyama reversed epoch or older epoch) and the Late stage volcanics (erupted in Brunhes normal epoch) by accumulated paleomagnetie and K-Ar age data in the Sengan field including the studied area. Matsukawa Andesites, Obukadake, Iwate-Ojiromori and Omatsukurayama volcanoes belong to the Early stage volca.nics; and Mitsuishiyama, Iwate, Nyutozan and probably Kurikigahara volcanoes belong to the Late stage volcanics. The results of the K-Ar age determination in this study are as follows: Matsukawa Andesites 1.67±0.12, 1.39±0.08 and 1.29±0.15 Ma, Obukadake volcano 1.57±0.06 Ma, Omatsukurayama volcano 1.07±0.50 Ma, Mitsuishiyama volcano 0.46±0.05 Ma. The estimated volume of products from each volcanoes in cubic kilometers are as follows: Matsukawa Andesites 15.3, Iwate-Ojirornori 0.02, Obukadake 3.8, Omatsukuravama 1.9, Kurikigahara 0.2, and Mitsuishiyama 0.5. It is difficult to evaluate precisely the volume of the magma reservoir from the volume of the eruptive material. In this paper, the youngest eruption age and the total volume of the erupted material for each volcano were plotted on the Smith and Shaw (1975) diagram to compare the relative present temperature of each magma reservoir. The evaluated temperature of the magma reservoir of the Matsukawa Andesites is higher than that of Mitsuishiyama volcano, which shows the youngest K-Ar age data. Futherrnore, the estimated present temperature of the magma reservoir which erupted the Tamagawa Welded Tuffs, 2 Ma and 1 Ma large scale felsic pyroclastic flow deposits, is much higher than those of all andesite volcanoes in the studied area. So it is difficult to restrict the heat source of each geothermal reservoir to each magma reservoir which erupted the andesite volcano in this area. The heat source is considered to be the accumulated compound heat energy from the magma reservoirs in the recent 3 million years including those of Tamagawa Welded Tuffs.
