2 T-F(O-2) PATHS IN THE MYOKEN-ZAN MAGNETITE-BEARING GRANITIC COMPLEX, SANYO BELT, SOUTHWESTERN JAPAN

The Myoken-zan igneous complex is a magnetite-series granitoid complex occurring among the ilmenite-series granitoids of the San'yo belt, Ja pan. The complex is composed of seven rock units that vary in composit ion from tonalite to monzogranite. Petrographically, the calcic amphib oles and magnetite from the complex can be divided in two groups, conv eniently called amphiboles-1 and -2, and magnetite-1 and -2, respectiv ely. With increasing Si contents, Mg/(Mg+Fe+Mn) in amphiboles-1 common ly increases, but that in amphiboles-2 remains unchanged. Magnetite-1 and -2 are characterized by thin (<1 mu m) and thick (>3 mu m) lamella e of ilmenite, respectively. Three rock units contain a pair of amphib oles-1 + magnetite-1, and the rest (four units) contain a pair of amph iboles-2 + magnetite-2. Exceptions to the rule are rare. Geothermometr ic and geobarometric calculations for the Fe-Ti oxides and the mafic s ilicates indicate that two separate T-f(O-2) paths produced the magnet ite of the Myoken-zan igneous complex. The first path developed in the late-magmatic to subsolidus stages and led to the production of magne tite-1 over a temperature rang of about 750 degrees to 650 degrees C, while oxygen fugacities in the melts remained above those of the Ni-Ni O buffer. The second path developed at the subsolidus stage and led to the production of magnetite-2 by a reaction between mafic silicates a nd 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 th e FMQ buffer, with falling temperature from about 670 degrees to 570 d egrees C.