Abstract

The Myoken-zan igneous complex is a magnetite-series granitoid complex occurring among the ilmenite-series granitoids of the San'yo belt, Japan. The complex is composed of seven rock units that vary in composition from tonalite to monzogranite. Petrographically, the calcic amphiboles and magnetite from the complex can be divided in two groups, conveniently called amphiboles-1 and -2, and magnetite-1 and -2, respectively. With increasing Si contents, Mg/(Mg+Fe+Mn) in amphiboles-1 commonly increases, but that in amphiboles-2 remains unchanged. Magnetite-1 and -2 are characterized by thin (<1 mu m) and thick (>3 mu m) lamellae of ilmenite, respectively. Three rock units contain a pair of amphiboles-1+magnetite-1, and the rest (four units) contain a pair of amphiboles-2+magnetite-2. Exceptions to the rule are rare. Geothermometric and geobarometric calculations for the Fe-Ti oxides and the mafic silicates indicate that two separate T-f(O 2 ) paths produced the magnetite of the Myoken-zan igneous complex. The first path developed in the late-magmatic to subsolidus stages and led to the production of magnetite-1 over a temperature range of about 750 degrees to 650 degrees C, while oxygen fugacities in the melts remained above those of the Ni-NiO buffer. The second path developed at the subsolidus stage and led to the production of magnetite-2 by a reaction between mafic silicates and the water-rich fluid exsolved from the magmas. Oxygen fugacities in the fluid phase varied from those of the Ni-NiO buffer to those of the FMQ buffer, with falling temperature from about 670 degrees to 570DGC.

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