Abstract
The Goyosan pluton in the Kitakami Mountains belongs to Kitakami zone V by Katada (1974) and has been divided into 3 types, e.g. Yoshihama, Kuroiwa and Okuboyama types. I newly found gabbroic rocks and redefined the Okuboyama type by a geological survey of the Goyosan pluton. In contrast with the previous results, the Okuboyama type granite is distributed in E-W direction with nearly constant width. The Okuboyama type granite has a quench texture near contact to the Yoshihama type granite. This shows that Okuboyama type granite intruded into the Yoshihama type tranite. However, magnetic susceptibility increases from the Yoshihama type granite to Okuboyama type granite near the intrusion boundary the Okuboyama and the Yoshihama type granite. This observation suggests that the Yoshihama type granite did not completely solidificate at the time of the intrusion of the Okuboyama type granite. Although both the Yoshihama and the Okuboyama type granite are composed of granodiorite and tonalite, there are the differences in modal compositions, for instance, the Okuboyama type is richer in feldspar than the Yoshihama type. As corresponding to the differences in the modal compositions, the Okuboyama type is also characterized by high Na2O, Al2O3 and P2O5 concentrations, and low K2O, total Fe2O3 concentrations, in comparison with the Yoshihama type. Therefore, chemical trend of the mentioned above elements of the Okuboyama type on variation diagrams against SiO2 is different from these of the Yoshihama type. Furthermore, the rocks of Okuboyama type are characterized by high Sr content and low Rb, Zr, Y contents. These features are similar to those of the plutons in the Kitakami zone II, and suggest that the Okuboyama type can be recognized as high-Sr series (adakitic rocks) in the sense of Tsuchiya and Kanisawa (1994). A model based on slab melting may account for high Sr/Y ratio of adakitic rocks and closely distribution of adakitic rocks and non-adakitic rocks.
