Oxygen Isotopic Compositions of the Cape Ashizuri Granitoids in Southwest Japan

Oxygen isotopic composition was determined on representative samples of the Cape Ashizuri plutonic rocks, in order to estimate the genetic background of the biotite granite and alkaline granitoids. The biotite granite (70.1–76.1% SiO₂) ranged from 8.61 to 9.30‰ δ¹⁸O and averaged as 8.9‰ δ¹⁸O (n = 3), which is much smaller than the same Miocene granitoids of the Okueyama (avg. 10.1‰) and Takakumayama (11.6‰) granitic bodies, which are associated with tin mineralization. Among the alkaline granitoids, quartz syenites also have values as low as 7.14–8.70‰, with an average of 8.0‰ (n = 3), and monzonite and gabbro vary from 6.14 to 7.86‰, with an average of 7.0‰ (n = 3). These alkaline granitoids may be lower crustal in origin. The gabbroids containing 12.5% MgO at the maximum with low Sr initial ratio, are considered to be derived from the upper mantle through the fore‐arc tectonic break‐up on the subducting slab.