中部九州内帯花崗岩類のRb-Sr全岩アイソクロン年代  [in Japanese] Rb-Sr whole rock isochron ages of granitic rocks from the central Kyushu, Japan  [in Japanese]

Rb-Sr whole rock isochron ages were determined for Middle Cretaceous granitic rocks and S-type tonalites in central Kyushu, southwest Japan. The former granites are divided into two age groups; the older group with a range of 140-160 Ma (Kunisaki tonalite: 142.2±2.1 Ma and Yamanaka granodiorite: 164.0±14.7 Ma) and the younger group with a range of 100-120 Ma (Watada granodiorite: 101.6±7.2 Ma, Nioki granite: 116.9±7.3 Ma, Haki granodiorite: 118.0±11.3 Ma, Kusano granite: 100.6±3.7 Ma, Kikuchi granodiorite: 121.3±8.4 Ma and Tamana granite: 116.8±12.7 Ma). The older granitic rocks would have been formed related to the subduction of the Farallon-Izanagi ridge, and the younger ones to that of the Kula-Pacific ridge along the eastern margin of the Eurasia plate. The S-type tonalite in the Higo area, which were derived from anatexis of pelitic granulite in the Higo metamorphic terrane, also shows a well defined whole rock isochron of 229 Ma, but the genesis of the tonalite was clearly different from those of the other granitic rocks in central Kyushu. Sr initial ratios of these granitic rocks show low values of 0.7044〜0.7056 except very for Kusano granite (Sr I=0.7065) and Higo S-type tonalite (Sr I=0.7068). They are very simliar to those of the San'in region, but different from those of the Ryoke-Sanyo region in southwest Japan, although the granitic and related metamorphic rocks excluding Sangun metamorphic rocks in central Kyushu have been thought to be a western extension of Ryoke belt. Newly analysed Rb-Sr whole rock isochron ages are harmony with a thought that the Cretaceous granitic igneous activities were migrated from central Kyushu toward the east in the Ryoke-Sanyo region, whereas Sr initial ratios of granitic rocks shed a light on the difference of mantle-crust composition between central Kyushu and the Ryoke-Sanyo region.