Tectonic relationship between the Median Tectonic Line and the Tanakura Tectonic Line viewed from isotopic ages and Sr isotopes of granitic rocks in the northern Kanto Mountains

Initial 87Sr/86Sr ratios have been determined for Permian and Early Cretaceous allochthonous granitic rocks and Late Cretaceous ones from the northern marginal areas of the Kanto Mountains, and for granitic rocks from the Yamizo Mountains and granitic clasts from the Usuginu conglomerate in Northeast Japan. The Kawaiyama, Kanezawa and Kinshozan quartzdiorites, which are dated as 250-277 Ma, give 87Sr/86Sr initial ratios of 0.7038-0.7043. These ages and initial ratios correspond with those for the Usuginu conglomerate. The Yotsumatayama quartzdiorite and tonalite of the Yamagata body with ages of 105-112 Ma, give an initial ratio of 0.7052, which is identical to those for granitic rocks in the Abukuma Belt. The initial ratios for the Namee granite and tonalite from Hiki Hills, whose ages are 62-66 Ma, are 0.7091 and 0.7079 respectively, and are correlated to those for the Ryoke granitic rocks. The granitic rocks along the Tanakura Tectonic Line in the Yamizo Mountains are grouped into the Ryoke and Abukuma types based on age and initial ratio. Ages and Sr isotopic data of granitic rocks from the Kanto Mountains suggest that the Okitano-Iwayama Line and the Naranashi Fault are the eastern extension of the Median Tectonic Line. In addition, the components of the Abukuma-South Kitakami Belt are now shown to be distributed in the northern marginal areas of the Kanto Mountains between the Ryoke and Sambagawa Belts, and thus the Median Tectonic Line can be extended to the Tanakura Tectonic Line. (author)