Rb–Sr Geochronology of the Hida metamorphic belt, Japan

Abstract

Rb–Sr whole-rock and mineral isochron ages have been determined for metamorphic and granitic rocks of the Hida metamorphic belt. The results indicate that an extensive metamorphic event together with plutonic activity took place within the belt during the latest Paleozoic – early Mesozoic period. The older ages of 220–250 m.y. represent an earlier phase of the metamorphism, whereas the younger ages of 170–180 m.y. represent a later phase. The Funatsu granitic rocks yielded a whole-rock isochron age of 176 m.y. with an initial 87Sr/86Sr ratio of 0.7056. This age is believed to indicate the time of original emplacement, and the rocks are considered to represent late-kinematic intrusion in the Hida belt.Some information on the middle Paleozoic metamorphism in the Hida Mountains was obtained from the isochron study. The whole-rock isochron age of 412 m.y. for the metamorphic rocks of the Fujibashi area may be considered, although not confirmed, to indicate the time of older metamorphism. The Omi Schist of the Circum–Hida crystalline schist belt, which belongs to the glaucophanitic type of metamorphism, gave a mineral isochron age of 350 m.y. thereby providing evidence of mid-Paleozoic metamorphism.The initial 87Sr/88Sr ratios for the whole-rock samples of the Hida metamorphic belt are found to be generally low, i.e. 0.705–0.708. This is especially so for the metamorphic rocks from the northern part of the belt where the lowest values were found.

Rb-Sr whole-rock and K-Ar mineral ages of granitic rocks in Japan.

“…observed for the Cretaceous granites corresponds to the period during which the temperature of granitic masses decreased from 500'C to 300"C. On the assumption of a normal geothermal gradient of 30°C/km, the granitic masses that were emplaced less than 10 km in depth would have cooled rapidly, giving rise to concordant ages. Thus the ob served slight discordance may indicate that the granitic masses were held at a temperature above 300"C for some period after the emplace KAGAMI, 1973; (4) (5) (6) (7) (8) KAWANO and UEDA, 1964, 1966a, 1966b(9) K'uBo, 1977; (10) SHIBATA, 1971, 1974, unpublished, (20) SHIBATA and KARAKIDA, 1965(21) SHIBATA and MILLER, 1962;(22) SHIBATA and NozAWA, 1966;(23) (24) (25) SHIBATA et al, , 1970SHIBATA et al, , 1978 SHIGENO and YAMAGUCHI, 1976;(27) UENO, 1977;(28) YANAGI, 1975;(29) YANAGI et al, 1971. KAWANO and UEDA, 1967; (2) (3) (4) (5) SHIBATA, 1968SHIBATA, , 1978SHIBATA, , 1979 (7) SHIBATA and ISHIHARA, 1974, 1979, (8) SHIBATA et al, 1971 ment.…”

Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime.

“…170 Ma) in the Hida terrane, central Japan (e.g. Shibata et al, 1970). The 260 and 180 Ma ther mal events recognized in the Hida terrane are likely to have been extensive in Northeast Japan as well as in Southwest Japan.…”

New fossil discovery from the Hida metamorphic rocks in the Unazuki area, central Japan.

“…The eastern wing and a part of the western wing were thermally metamorphosed by plutons (Hiroi, in press). The mineral age of the western wing ranges from 212 to 248 million years according to the Rb-Sr method (Yamaguchi andYanagi, 1968, 1970;Shibata et al, 1970). The Plutonic complex has been considered as a part of Funatsu plutonics of Late Paleozoic or Early Mesozoic Era (Kano, 1973).…”