PETROLOGY OF THE ORIKABE GRANITIC BODY, KITAKAMI MOUNTAINLAND

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  • 北上山地,折壁花崗岩質岩体の岩石学
  • キタカミ サンチ , セツヘキ カシガンシツガンタイ ノ ガンセキガク

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The Orikabe granitic body, Kitakami mountainland, was studied from the petrochemical point of view. Chemical analyses were done on sixteen rocks including a pegmatite, ten biotites, seven hornblendes, three augites, and an olivine (Table 1 to 5). Variation trends on the SiO2-oxides diagrams are smooth excluding K2O (Fig. 1). All analysed biotites are high in TiO2 and low in Al2O3, all being titaniferous. Al2O3 contents of common hornblendes are relatively low and K2O/Na2O ratios are high.<br> Three rock types consisting of the Orikabe body, Tokusenjo, Orikabe, and Sasamori, are derived from the same magma judging from the petrochemical and mineralogical evidences. Initially the Orikabe magma was dioritic and of the Tokusenjo type in composition. Gabbroic rocks of the Tokusenjo type was derived from the cumulous phase. As the result of crystallization the composition of the magma changed to the Orikabe type. During the crystallization the thermal trough on the system Q-Or-Ab-H2O shifted to Q-Ab by the increasing of water vapor pressure. Consequently the composition of the liquid changed to the new trough. This is the Sasamori type magma.<br> The differentiation mentioned above was done in the magma chamber, and three rock types intruded successively. Judging from the chemical composition of pegmatite the magma chamber was located at about 14km in depth.

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