秋田県太平山複合花崗岩質岩体の岩石学的研究 Petrology of the Taiheizan composite granitic rocks, Akita Prefecture, Northeast Japan
Cretaceous Taiheizan composite granitic pluton occurs in the Taiheizan mountains, central Akita prefecture, Northeast Japan. The rocks are divided into the East and West masses by the Kyowa-Hitachinai mylonite zone, which resulted in right lateral movement. The East mass is composed of mainly weakly foliated medium to coarse-grained hornblende biotite granodiorite and tonalite. They contain magnetite, but these magnetic susceptibilities average 8.08 × 10<sup>−</sup><sup>3</sup> SI units, which are relatively lower than those of typical magnetite-series Kitakami granitic rocks. On the other hand, the West mass is composed of mainly foliated granodiorite and tonalite, which belong to the ilmenite-series due to the absence of magnetite. Both rocks have different chemical features. The magnetite-series East rocks have lower contents of Al<sub>2</sub>O<sub>3</sub>, Na<sub>2</sub>O and higher contents of MgO, K<sub>2</sub>O than the ilmenite-series West rocks. Trace element contents show that the East rocks have higher contents in Cr, Ni, V, but lower in Zr. However, the chemistries of both rocks resemble those of Abukuma granitic rocks as compared with those of Kitakami rocks. The reported K-Ar ages of the East and West masses also show the same ages from Abukuma rocks.<br> Therefore, I conclude that the Taiheizan granitic rocks belong to the Abukuma belt, and the Kyowa-Hitachinai mylonite may coincide with the boundary of low magnetic susceptibility magnetite series rocks and ilmenite series rocks in the Abukuma granitic rocks.
岩石鉱物科学 35(6), 253-269, 2006-11-30