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The Matsumae plutonic rocks, consisting of ultramafic cumulates to adamellite, are divided into three types in terms of the assemblage of Fe-Ti oxide minerals; Type I is characterized by the presence of magnetite and hemo-ilmenite, Type II by the presence of magnetite and absence of hemo-ilmenite, and Type III by the absence of magnetite. Chemistry of Fe-Ti oxide minerals suggests that origin of these types is ascribed to the dirrerence in oxidation state of magma, i.e. Type I is the highest in fO2, while Type III is the lowest. However, these differences in fO2 do not strictly correlate with the variation of assumed fH2O. Some rocks of Type II may represent an original redox state of the Matsumae plutonic magma. The Type I rocks were probably oxidized mainly in the magma chamber, whereas the Type III rocks may be reduced. Since some of the hornblendite show mixed facies of Types I and II, incorporation of less oxidized magma into the oxidized magma chamber may have occurred in early stages of crystallization. Redox state of the Matsumae plutonic magma is considered to be affected by such magma incorporation.