EVOLUTION HISTORY OF THE KAKKONDA MAGMA-HYDROTHERMAL SYSTEM, JAPAN, ESTIMATED THROUGH SIMPLIFIED-MODEL NUMERICAL SIMULATIONS

摘要

At the Kakkonth geothermal area, Japan, a large Quatemary granitic body is lstributed below ca. 2.5 km depth from the surface, associating with thick high-temperature contact metamorphic zones in the overlying formations, and a meteoric-water-thminated type hydrothermal system is developed at present time. During the "Deep-Seated Geothennal Resouites Survey", WD-la well was drilled to 3729 m depth, and a thermal conduction zone with a maximum temperature over 500 deg C was observed in the Quatemaiy granitic body. Evolution history of the Kakkonda geothermal system was analyzed by the numerical simulation method lsed on the simplified one-dmensiona1 transient thermal condection models, in which high 'extended thermal conckictivity' was assumed for geothermal reseivoirs (Shigeno, 1999a). The time-space temperature distribution data obtained from the WD-Ia well could be explained by the following dynamic environment model (KR1 model): (1) At first, the thick contact metamorphic zones were produced through heat storage by thermal condection from the magma chamber emplaced probably ca. 0.20 Ma, (2) Iater (e.g. 0.03 Ma), reservoir formation caused the present convective temperature profile with the high condective temperature gradient in the cooling Quaternary granitic body. A modfied three-stage scenario (KM2 model) that assumed contribution of magmatic fluid convection in the granitic body for the first stage improved fitting of the observed and simulated Lemperature profiles for the contact metamorphism.