This paper describes the petrography of magnetite-series and ilmenite-series granitoids in the San'yo and San'in belts, and estimates the redox paths of the granitoid magmas during cooling.
In magnetite-series granitoids in these areas, the magnetite commonly occurs as secondary minerals after decomposition of magmatic mafic silicates such as pyroxenes, hornblende and biotite. The magnetite is divided into two types, differentiated by their coexisting minerals. The first-type of magnetite crystallizes concurrently with hornblende and biotie after decomposition of pyroxenes and brown hornblende at the late magmatic stage, and continues to crystallize during subsolidus conditions. The second-type of magnetite crystallizes with actinolite, cummingtonite and chlorite during subsolidus stages of crystallization.
Petrography, combined with Fe-Ti oxide geothermo-barometers for the magnetite-series granitoids, suggests that the magnetite crystallizes due to an increase in the oxidation state of the magma (and/or fluid phase) above the NNO-buffer during cooling. However, the redox states of the ilmenite-series granitoid magmas are kept below the FMQ-buffer, throughout the solidifying process of the magma. The chemistry of the fluid phase determines whether the magma follows the magnetite-series or ilmenite-series trend.

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