SHRIMP U-Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan: Implications for late Paleozoic granitic activity in Japanese Islands

The new result of SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan, provides 281.5 ± 1.8 Ma. The age is 30 m.y. older than the available age of the Kinshozan Quartz Diorite obtained by hornblende K–Ar method. The new U–Pb zircon age represents the time of crystallization of the Kinshozan Quartz Diorite. The hornblende K–Ar age indicates the time that the Kinshozan Quartz Diorite cooled down to 500 °C which is the closure temperature of the systematics. Permian granites are found in small exposures in Japan, and frequently referred to as 250 Ma granites. The Kinshozan Quartz Diorite is considered as a type of the 250 Ma granites, and the age was influential in establishing a model of Paleozoic tectonic evolution for the Japanese Islands. The new age of the Kinshozan Quartz Diorite provides the opportunity to re‐examine the model. The Kinshozan Quartz Diorite and other Permian granites in the south of the Median Tectonic Line of Japan were constituents of the Paleo‐Ryoke Belt. The geochemical characteristics of the granitic rocks in the Paleo‐Ryoke Belt indicated that the granitic rocks were formed in a primitive island arc environment, and the new trace element data also support this interpretation. Examination of the available data and results of the present study suggests the late Paleozoic granitic activity in Japan as follows. At about 310–290 Ma, arc magmatism generated adakitic granites and other granites in the South Kitakami Belt. Quartz diorite and tonalites of primitive characteristic, such as the Kinshozan Quartz Diorite and granites in the Maizuru Belt appear to have been formed at the immature island arc, and accreted to the Japanese Islands at the end of Paleozoic or early Mesozoic era. During 260–240 Ma, granitic activity took place in the Hida and Maizuru Belts as a part of the Asian continent. 関東山地、金勝山石英閃緑岩のSHRIMP U‐Pbジルコン年代:日本列島の後期古生代の花崗岩活動における意義 抄録 関東山地に露出する金勝山石英閃緑岩のSHRIMP U‐Pbジルコン年代測定を行った。試料は寄居地域兜川から採取した。金勝山石英閃緑岩は日本のペルム紀花崗岩類の典型例として議論されることが多く、その角閃石K‐Ar年代から、250 Maの花崗岩類として理解されてきた。得られた206Pb/238U 年代は281.5 ± 1.8 Maで、角閃石K‐Ar年代251 ± 8 Maと比較すると30 m.y.古い。得られた年代と、近年公表された日本の他の地域の後期古生代花崗岩類の年代及び堆積岩中の砕屑性ジルコンの年代の比較から、後期古生代の花崗岩活動の推移を再検討した。310‐280Maの間、島弧火成活動によるアダカイト質花崗岩類や、未成熟な島弧において形成されたと考えられる金勝山石英閃緑岩のような花崗岩類が形成されたと考えられる。 The new result of SHRIMP U‐Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan, provides 281.5 ± 1.8 Ma. The age is 30 m.y. older than the available age of the Kinshoz