Permalink : http://hdl.handle.net/2115/36709

Fe-Ti oxide minerals in the Horoman plutonic complex of the Hidaka metamorphic belt are mostly of ilmenite (α-phase: ilmenite-hematite solid solution) and the occurrence of β-phase (magnetite-ulvöspinel solid solution) has not been reported so far. The ilmenite from Horoman are chemically analyzed in order to estimate the crystallization condition of the dioritic host rocks. Although the Mn/Fe2+ ratio of the host rocks remains constant during the differentiation, the Mn/Fe2+ ratio and R2O3 content of the ilmenite increase successively as the differentiation proceeds. This change in composition must be largely attributed to a decrease of temperature and an increase of oxygen activity in the magma from which the ilmenite crystallized. This observation implies that the Horoman diorite was formed under the 'oxidizing' condition, which means that the decreasing rate of fo2 was lower than that of the standard buffer curve and the crystallization might have proceeded along the course intersecting QFM buffer curve. With these points as background, the crystallization condition of the plutonic rocks in the Hidaka metamorphic belt is briefly discussed with respect to fo2, refering to the isotopic data presented by Sasaki and Ishihara (1979) and Shibata and Ishihara (1979).