A K-Ar Investigation of the Chichibu Quartz Diorite and Some Discussions on Its Cooling History

抄録

Thermochronological investigations have been carried out on the Chichibu Quartz Diorite in order to elucidate the detailed timing of intrusion and its cooling history. The Chichibu Quartz Diorite is a small pluton (6 × 3 km) situated in the central part of the Kanto Mountains, Japan. Newly obtained K-Ar ages from 6 biotites, ranging from 5.6 to 6.8 Ma, and from 2 hornblendes, ranging from 6.0 to 6.1 Ma, strongly suggest that the Chichibu Quartz Diorite experienced a considerably rapid cooling. Combining with previously reported paleomagnetic results, we can conclude that the body formed and subsequently cooled around Chron C3A of Cande and Kent's (1995) geomagnetic polarity time scale.