Timing of hydrothermal activity estimated by radiometric ages of Cretaceous granitic rocks at the Watari Peninsula, Fukutsu City, Fukuoka Prefecture, northern Kyushu

Abstract

The Cretaceous granitic rocks, the Kitazaki Tonalite and Shikanoshima Granodiorite, and veneer Oligocene sedimentary rocks, the Tsuyazaki Formation, are distributed in the Watari Peninsula, Fukuoka Prefecture. There are mineral veins composed by calcite and zeolites in those rocks. Rb-Sr, and fission track geochronological analyses were carried out for granitic rocks, in order to determine the cooling process of granitic rocks and timing of hydrothermal activity. Biotite and felsic fraction separated from the Kitazaki Tonalite and Shikanoshima Granodiorite give Rb-Sr isochron age of 108.6±2.5 Ma and 107.0±0.7 Ma, respectively. Fission track ages from the Kitazaki Tonalite are 97 Ma (titanite), 89−88 Ma (zircon), 14 Ma (apatite). Fission track ages from the Shikanoshima Granodiorite are 95 Ma (titanite), 87–84 Ma (zircon), 15 Ma (apatite). Fission track ages of apatite from granitic rocks are younger than the Tsuyazaki Formation, which suggests a thermal event at about 15 Ma to reset the fission track apatite age. The homogenization temperature of fluid inclusions in calcite are 89–111 °C (Kitazaki Tonalite), 95–118 °C (Shikanoshima Granodiorite) and 85–91 °C (Tsuyazaki Formation). Sr isotopic compositions of calcite and zeolites in granitic rocks and Tsuyazaki Formation are overlap with one another. These data suggest that vein minerals in granitic rocks and zeolites and calcite in the IC Member of the Tsuyazaki Formation were formed by a series of hydrothermal activity at about 15 Ma. © 2005, Japan Association of Mineralogical Sciences. All rights reserved.