A detailed K-Ar and 40Ar-39Ar geochronological study was conducted on the Kinpu-san body, which is a part of the Kufu plutonic complex. We collected a sample from every 3 km x 3 km area in the body. The chronological study based on this high density sampling clearly illustrates that the body systematically becomes younger from north to south. The oldest age in the body could be considered to restrict the initiation time of the Philippine Sea plate subduction. Thus, we conclude that the subduction started 16 Ma age at latest. Previous paleomagnetic studies have revealed that the adjacent region north of the Kanto Mountains rotated 60° clockwise between 15 Ma and 12 Ma. Our data demonstrate that the Kinpu-san body cooled during this same interval. Hence, the Kinpu-san body is a candidate for providing information about the severe deformation accompanied with the collision between the Izu-Bonin and Honshu arcs.
K-Ar age studies in the Ashigawa and the southern part of the Tokuwa granodioritic bodies, which consist of the southern part of the Kofu plutonic complex, revealed that they formed between 12 and 9 Ma. Quite a narrow range of ages obtained from the Ashigawa southernmost part of the Tokuwa pluton implies that they cooled rapidly. The southern part of the Tokuwa pluton, as a whole, shows a systematic age distribution with a decrease in age to the north. Compilation of currently available plutonic ages in the South Fossa Magna suggests that the plutonic activities occurred three times in this region. Episodic activity like this could be argued in relation to the tectonic development of this region.
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