Abstract
K-Ar and FT analyses were carried out on biotite, zircon and apatite from the Middle Miocene Ichifusayama Granodiorite pluton in Kyushu, Japan in order to reveal the cooling history. The core and rim of the pluton revealed K-Ar biotite ages of 13.39-13.36 and 13.31-13.54 Ma, FT zircon ages of 13.3-13.1 and 12.7-12.3 Ma, and FT apatite ages of 13.1-13.7 and 10.6-10.8 Ma respectively, suggesting that the core cooled, to 100 °C from 300 °C, faster than the rim, of which the cooling rate is ∼ 100 °C/Ma. The track-length analyses also suggest extremely rapid cooling without a late stage annealing process. The faster cooling of the core in comparison with the rim is unusual. The difference in cooling velocity between the core and rim was possibly caused by an influence of paleotopography.
