U-Pb zircon ages of syenitic and granitic rocks in the Ashizuri igneous complex, southwestern Shikoku: Constraint for the origin of forearc alkaline magmatism

摘要

We report two first U-Pb zircon ages of the felsic rocks in the Ashizuri igneous complex, southwestern Shikoku, using the LA-ICP-MS. Samples are a syenite (ASH54) and a biotite granite (ASH5). Weighted means of the ~(238)U-~(206)Pb ages of concordant data of ASH54 and ASH5 are 13.12 ± 0.09 Ma (2σ) and 12.95 ± 0.06 Ma (2σ), respectively. It is concluded that the felsic member of the Ashizuri complex was formed almost simultaneously at ca. 13 Ma after the major activity of the Outer Zone Granitic Rocks. Presence of alkali dolerite dike cutting the syenite indicates that the alkali basaltic magma intruded after the subduction of Shikoku Basin of the Philippine Sea plate beneath the southwest Japan arc. Alkali basaltic magma of Ashizuri complex may have been derived from a depth possibly deeper than that of subducted Shikoku Basin of the Philippine Sea Plate.