White mica K–Ar ages from lawsonite–blueschist facies Hakoishi sub–unit and from prehnite–pumpellyite facies Tobiishi sub–unit of the Kurosegawa belt, Kyushu, Japan

摘要

White mica K–Ar age dating was carried out for metapelites in the Hakoishi sub–unit and a meta–pillow basalt in the Tobiishi sub–unit of the Kurosegawa belt (Kamimura et al., 2012), Yatsushiro district, Kyushu, Japan. The Hakoishi sub–unit suffered the lawsonite–blueschist facies metamorphism with peak conditions of 0.7–0.9 GPa and <300 °C. Phengite separate with K content of 5.323 wt% from OD 28 gives 299.0 ± 6.1 Ma and those with K content of 1.222 wt% and 2.142 from OD 113 give 280.2 ± 5.8 Ma and 245.3 ± 5.1 Ma, respectively. The Tobiishi sub–unit suffered the prehnite–pumpellyite facies metamorphism with the peak conditions round <0.4 GPa and <350 °C. White mica named as alumino–celadonite with a composition range of Si from 3.79 to 3.94 and Fe~(3+) from 0.30 to 0.48 for Oxygen = 11 basis is developed in amygdules of meta–pillow basalts, closely associated with pumpellyite and chlorite. Alumino–celadonite separates obtained from pillow basalt, SOD01–B, vary K content from 5.601 to 2.343 wt% and give K–Ar ages from 181 to 173 Ma. The peak P–T conditions of the all studied samples were much lower than the closure temperature of Ar in white micas, so those data can be interpreted as the peak metamorphic timing of each sub–unit, i.e., the existence of both late Paleozoic (299–245 Ma) lawsonite blueschist facies metamorphic rocks and Mesozoic (181–173 Ma) prehnite–pumpellyite facies metamorphic rocks are newly identified from the Kurosegawa belt in Kyushu, although Mesozoic epidote–blueschist facies metamorphic rocks have been reported in the relevant area. Obtained metamorphic ages and grade from the Hakoishi and Tobiishi sub–units are almost similar with those of the Osayama area in the Renge belt (Tsujimori and Itaya, 1999) and of the Katsuyama area in the Suo belt of the Inner Zone of Southwest Japan, respectively (Hashimoto, 1968; Nishimura, 1998). These data support the Inner Zone origin of the Kurosegawa belt proposed by Isozaki and Itaya (1991).