Origin of the Early Mesozoic Granitic Rocks in the Hida Terrane, Japan, and Its Implication for Evolution of the Continental Crust

TANAKA Shinobu

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

The early Mesozoic granitic rocks show Rb-Sr ages of 173 to 211 Ma and are intruded into three belts, Hida Gneiss Belt, Unazuki Belt and Hida Marginal Belt, being divided into Triassic ones and Jurassic ones. The granitic rocks exhibit spatial variation in chemical compositions of constituent minerals, whole-rock geochemistry and Sr and Nd isotopic data. Especially, based on Sr and Nd isotopic compositions of the granites, the Hida Terrane (Hida Gneiss Belt and Unazuki Belt) can be divided roughly into two zones, Outer Plutonic Zone (lower initial 87Sr86Sr ratios: 0.7045 to 0.7065 and higher initial εNd values: +3.1 to -4.2) and Inner Plutonic Zone (higher initial 87Sr/86Sr ratios: 0.7054 to 0.7105 and lower initial εNd values: +1.6 to -9.3). Gabbroic rocks indicate the same spatial variation in isotopic characteristics of the granitic rocks.
The negative initial εNd values (-3.3 and -4.2) from Triassic Hayatsukigawa mass suggest that its formation was incorporated by significant amounts of older continental crust. The mass is characterized by low K2O/Na2O ratios, Rb and MgO contents and high Sr and Ba contents. The granites belong to the I-type and magnetite-series granitic rocks, which is the same in the Jurassic granitic rocks. The geochemical data indicate that the magma source of the Hayatsukigawa mass rocks is a granulite facies igneous lower crust. The main partial melting in the lower crust may be induced by adiabatic decompression resulting from the rapid uplifting occurred in the Outer Plutonic Zone.
The geochemical criteria imply that the Jurassic granites was generated in a destructive plate margin. From the isotopic data and geochemical characteristics, the granitic rocks in the Outer Plutonic Zone may have been derived from Phanerozoic basic igneous rocks (e.g. basaltic composition) with island-arc or continental margin character. The granitic rocks in the Inner Plutonic Zone can be divided into two groups, Group I and Group II on the basis of their isotopic compositions. The source material of the Group Ⅰ is inferred to have been enriched in 87Sr than that of the granites in the Outer Plutonic Zone. The Group II rocks with higher initial 87Sr/86Sr ratios and lower εNd values appear to have primarily been derived from older (Proterozoic) crustal sources.
Considering the isotopic and geochemical informations of the early Mesozoic granitic rocks and Hida metamorphic rocks, the Inner Plutoni Zone of the Hida Terrane would have been an island arc or a microcontinent which collided with the eastern edge of the Sino-Korean craton during the early Paleozoic.

eng

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718