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Mutsu Hiuchidake Volcano has an erosion caldera which shows a horseshoe like geomorphological feature toward east. Tertiary strata as a basement are distributed in the caldera. Tertialy strata and a part of pyroclastic deposits of the Mutsu Hiuchidake Volcano have altered strongly to moderately by hydrothermal activities related to the volcanism. This study is to clarify a progress history of the alteration by using a geological mapping, thermoluminescence (TL) dating, x ray diffraction analysis, an infrared reflection absorption analysis and a fluid inclusion study.Highly altered zone is recognized in the area of midstream to upstream along the Ohakagawa and the Koakagawa within erosion caldera. The argillic alteration zone surround a silicification zone in the highly altered area shows a circular distribution. The strongly altered areas are along NNW SSE to NNE SSW fractures. The alteration areas were divided into the smectite, kaolinite, alunite and pyrophyllite zones.The kaolinite and alunite zones give the TL ages of quartz 67 13 ka (KG 5), 88 18 ka (OG 4) and 91 23 ka (OG 1). The smectite zone within the argillic alteration zone of outside of collapse caldera, yield the ages 752 215 ka (SO 2) and 615 197 ka (KG 1). These TL ages suggest the hydrothermal activity end at 70 to 90 ka.The existence of pyrophyllite suggests that hydrothermal temperatures were 200 to 250 C in these area. This is supported from the homogenization temperatures of fluid inclusions in calcite, 242 C in average.

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The high temperature hot springs such as in Yunomine and Tosenji and the hydrothermal alteration zones of the Hongu area are distributed in the southern part of Kii peninsula, though Quaternary volcanoes which can be their potential heat sources are not distributed in the area. This research has aimed to understand the thermal history of the area by using several dating methods.Thermoluminescence (TL) dating method was applied according to the distance from dykes or gushing out point of the hot springs which thought to be the center of alteration. The results show that the alteration age of Yunomine alteration zone is younger as it approaches the gushing out point of hot spring. No clear tendency was observed in other places. These results suggest that the alteration ages near Yunomine hot spring are controlled by the distance from the gushing out point. However, no clear tendency was observed at other hot springs between the alteration ages and the distance from the center of alteration when two or more activities of alteration have occurred.The results of the TL, Fission Track (FT) and K Ar dating show that after the high temperature hydrothermal alteration (until tens of millions of years ago), relatively low temperature alteration (from at least hundreds of thousands of to tens of thousands of years ago) occurred in Hongu and Totsukawa area.

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