Neo-Granitic Pluton and Geothermal Reservoir at the Kakkonda Geothermal Field, Iwata Prefecture, Japan

Abstract

A neo-granitic pluton which is petrographically composed of tonalite and quartz diorite has been found at 2.2 km-2.8 km depth at the Kakkonda geothermal field, Iwate Prefecture, Japan. Pre-Tertiary, Miocene strata and Pliocene dykes of tonalite and dacite were thermally metamorphosed by the granitic pluton. The youngest metamorphic protolith is 4.9 ± 1.0 Ma in age. The width of metamorphosed area is over 2.0 x 2.5 km.The metamorphic minerals are green or brown biotite, muscovite, cordierite, anthophyllite, andalusite, hastingsite, cummingtonite, tremolite, pyrrhotite and magnetite. The biotite and cordierite isograds envelope the granitic pluton above ca.1.0 km and 0.7 km respectively, which sink toward the southwest in 15-30 degrees. The roof of the granitic pluton is inferred to be parallel with the surface of the mineral isograds. This estimation was proven by two deep geothermal wells.The productive fractures exist at the margin of the granitic pluton and its neighboring pre-Tertiary formation. The deep and high temperature (300-350°C≤) geothermal fluid, which is slightly acid water (pH 3.5-4.5), exists in these fractures. The depth of the roof of the granitic pluton can be estimated from the isograds of biotite and cordierite.