LAYERED GRANITIC BATHOLITHS OF THE HIROSHIMA GRANITE, SOUTHWESTERN JAPAN

Hiroshima granite is one of the largest granitic batholiths in the Japan Arc, extending about 300×60 km along the Inner zone of the southwestern Japan Arc. The radiometric ages of the Hiroshima granite range from 90 to 70 Ma, and according to their petrography the granite is classified mostly into I-type and ilmenite-series. The results of detailed field surveys have indicated that the Hiroshima granite consists of several tabular plutons 10 to 500 m thick (e.g., Hayashi, 1995 Bull. Fac. School Edu. Hiroshima Univ., Part II, 17, 95-150; Takahashi, 1993 Jour. Japan. Assoc. Min. Petrol. Econ. Geol., 88, 20-27). The tabular plutons are piled up horizontally, and are inclined gently to the south on a regional scale. The deeper limit of the lowest layer pluton cannot be observed at present erosion levels. Intrusive boundaries between each tabular pluton are generally distinctive; mixing or mingling phenomena between two plutons are uncommon. Rhyolitic pyroclastic rocks and thermally metamorphosed pre-Cretaceous sedimentary rocks often occur on the plutons as roof-pendants. Intrusive boundaries between the roof-pendants and plutons are also sharp, and xenoliths of the roof-pendants or wall rocks are rare within the plutons. The chemical compositions of the plutons tend to become more felsic with younger age, and more felsic plutons tend to occur in shallower layers. The grain size of the plutons also tends to become finer toward shallower layers. Ryoke granite, which has deeper rock-facies than the Hiroshima granite, is intruded by or thrusts upon the Hiroshima granite along the southern margins (e.g., Suzuki et al., 1995 Abst. Annual Meeting Geol. Soc. Japan, 268). The geological and petrological evidence described above indicate that the Hiroshima granite was intruded intermittently along low-angle thrust faults dipping to the south. The crust southern to the thrusts shows relative upheaval to the northern crust due to fault movement, and consequently the deeper facies of the Ryoke granite are juxtaposed with the Hiroshima granite. The formation of thrust faults would be due to large-scale crustal shortening associated with back-arc lifting during the Cretaceous period.