Rb-Sr and K-Ar ages of the Higo metamorphic rocks and related granitic rocks, Southwest Japan.

An adakitic pluton on Kyushu Island, southwest Japan arc

“…250 and ca. 100 Ma (Osanai et al, 1993Nakajima et al, 1995;Nagakawa et al, 1997). The Ryuhozan metamorphic rocks experienced low-to medium-grade metamorphism up to amphibolite facies (Yamamoto, 1962;Sakashima et al, 1999), and are mainly derived from mafic and felsic tuff, carbonate rocks and minor clastic sediments.…”

Staurolite-bearing gneiss and re-examination of metamorphic zonal mapping of the Higo metamorphic terrane in the Kosa area, central Kyushu, Japan

“…They interpreted that the Higo metamorphic rocks reached the peak of metamorphism at about 250 Ma and were then locally re heated by the intrusion of the Shiraishino granodiorite at about 120 Ma (the age after Kamei et al, 1997). They stated that the age of 100 Ma reported by Nakajima (1995) represents the cooling age of the Shiraishino granodiorite and the Higo metamorphic rocks. On the contrary, Nagakawa et al (1997) obtained very similar age (103 108 Ma) for biotite from Zones B to E of Obata et al (1994) using K Ar dating.…”

“…The age of the Higo metamorphic and plutonic rocks has been under debate. Nakajima (1995) dated biotites from both the Shiraishino granodiorite and the Higo metamorphic rocks to be 100 106 Ma by K Ar dating. He also determined the same ages for these rocks by Rb Sr whole rock isochron method.…”

Diatexite and metatexite from the Higo metamorphic rocks, west-central Kyushu, Japan

“…These intrusives are, on a gross scale, distributed semi parallel to the metamorphic belt, and the contact is roughly concordant with the internal structure of the metamorphic rocks and, therefore, the intrusion is considered to be syntectonic and synmetamorphic. This scheme is supported by radiometric dating of these intrusives, both by the K Ar method (Nagakawa et al, 1997;Nakajima et al, 1995;Kamei, 1997), and the Rb Sr method (Shibata and Yamomoto, 1965;Hayase and Ishizaka, 1967). Throughout the Manotani and Higo metamorphic rocks, the metamorphic grade increases monotonically southward and the whole terrane has been subdivided into five zones of different metamorphic grade, A, B, C, D and E, in the order of increasing metamorphic grade (Nagakawa et al, 1992).…”

“…The age of the Higo metamorphic rocks has been determined by the K Ar radiometric method, analyzing metamorphic biotite, muscovite and hornblende. A limited range of age, 100 to 108 Ma, was obtained from a wide area of the metamorphic grades, covering zone B to zone E (Nakajima et al, 1995;Nagakawaet al, 1997). However, significantly older ages (206 to 214 Ma) were obtained from some samples from the Manotani metamorphic rocks (zone A), where high pressure mineralogy (lawsonite, prehnite and crossite) had been reported (Karakida et al, 1989), thereby suggesting the possibility of polymetamorphism of the whole metamorphic terrane.…”