Igneous rocks emplacement and exhumation of sedimentary basement: Fission track age determination on the Osuzuyama volcano-plutonic complex and surrounding rocks, Miyazaki prefecture, Southwest Japan

Abstract

Fourteen fission track apatite ages and thirteen zircon ages are measured for the Osuzuyama volcano-plutonic complex (OVPC) and the surrounding Hyuga unit in Kyushu, Southwest Japan. The average apatite OVPC age is 12.6 ± 0.9 (±1σ) Ma, slightly younger than the average zircon age of 15.2 ± 0.5 (±1σ) Ma which is consistent with K-Ar ages reported before. Apatite ages of the nearby Hyuga unit are more or less older than the average apatite OVPC age, suggesting that the OVPC age would represent time of slow cooling after intrusion rather than the time of exhumation. Compared to the previously reported apatite data from the Shimanto accretionary complex at the north of the studied area, the age here is older and indicates less exhumation of the studied region. Given reasonable assumptions, the difference in the amount of exhumation since Miocene is estimated being larger than ∼1 km, probably controlled by the material to be exhumed and local geological setting.