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 parts per thousand O-18 and averaged as 8.9 parts per thousand O-18 (n = 3), which is much smaller than the same Miocene granitoids of the Okueyama (avg. 10.1 parts per thousand) and Takakumayama (11.6 parts per thousand) granitic bodies, which are associated with tin mineralization. Among the alkaline granitoids, quartz syenites also have values as low as 7.14-8.70 parts per thousand, with an average of 8.0 parts per thousand (n = 3), and monzonite and gabbro vary from 6.14 to 7.86 parts per thousand, with an average of 7.0 parts per thousand (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.
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