Genesis of the plutonic-hydrothermal system around Quaternary granite in the Kakkonda geothermal system, Japan

The Kakkonda plutonic-hydrothermal system has as its heat source the Quaternary Kakkonda granite. The Kakkonda granite has a thick (similar to 1.3 km) contact-metamorphic zone, known mainly from the geothermal survey well WD-1a (total depth: 3729 m) drilled by the New Energy and Industrial Technology Development Organization (NEDO). The Kakkonda granite is a stock several tens of square kilometers in area with an upper contact about 1.5-3 km deep. It is a composite pluton varying from tonalite to granite. The early-stage granitic rocks are slightly metamorphosed to biotite grade by late-stage granitic rocks. K-Ar ages of separated minerals from the granitic rocks in both stages show the same cooling ages of 0.24-0.11 Ma for hornblende, 0.21-0.02 Ma for biotite, and 0.14-0.01 Ma for potassium feldspar. These are the youngest ages for granite in the world. The K-Ar ages become almost zero at similar to 580 degrees C for biotite and potassium feldspar, and at similar to 350 degrees C for illite. The Kakkonda granite intruded into a regional stress field in which the minimum principal stress was ENE-WSW and nearly horizontal. The regional stress field coincides with that of a previously recognized F2 fracture system before similar to 0.4-0.3 Ma. Both stages of the Kakkonda granite and the contact aureole are fractured by recent tectonism, resulting in a zone of hydrothermal convection from about 2.5-3.1 km depth up to the surface. The boundary between the zone of hydrothermal convection and the underlying zone of heat conduction occurs similar to 250-550 m below the upper contact of the Kakkonda granite, and has a temperature of similar to 380-400 degrees C. (C) 1998 CNR. Published by Elsevier Science Ltd.