ORIGIN OF REDOX CONTRAST OF GRANITIC MAGMA, DISCUSSION OF MAGNETITE- AND ILMENITE-SERIES FOR 36 YEARS

Magnetite- and ilmenite-series is a petrographic classification of granitic rocks based on redox states (in practice, magnetite contents), proposed by Ishihara (1977 Mining Geol., 27, 293-305). The regional arrangement of magnetite- and ilmenite-series granitic terranes and their close connection with metallogenic provinces indicate that the two series is basically derived from the nature of magmatic sources. The magnetite- and ilmenite-series has been applied practically for mineral exploration, because magnetite contents of granitic rocks sensitively reflects oxygen and sulfur fugacities of magma, and those can be measured simply with a magnetic susceptibility meter. In the early stage discussion, however, most petrologists believed that the redox states of granitic magma depend basically on the crystallization conditions of late-magmatic stages (e.g., Czamanske et al., 1981 JGR, 86.B11, 10431-10469), and thus, they tended not to regard magnetite- and ilmenite-series as a petrogenetic classification such as I- and S-type. The criticism would be based on progressive oxidation during crystallization, which is a common phenomenon in magnetite-bearing granites. This issue was essentially solved by Takagi and Tsukimura (1997 Econ Geol., 92, 81-86); they showed thermodynamically that SO₂ gas in magma acts as an oxidizing agent in late-magmatic to subsolidus stages. Thus, magnetite- and ilmenite-series can be regarded as the classification by the composition of magmatic volatiles. In 1980’s to 90’s, numerous geochemical and geochronological data of magnetite- and ilmenite-series granitic rocks were accumulated, and temporal and spatial changes of the two series magmatism have been elucidated clearly. On the basis of the previous studies, Takagi (2004 AJS, 304, 169-202) demonstrated that redox states of arc granitic magma are basically controlled by the properties of subducting materials, and the properties would depend on the environment of the earth’s surface. The discussion of magnetite- and ilmenite-series are developing from local petrology to tectonics in the global scale (e.g., Scholl and von Huene, 2007 GSA Spec. Paper, 200, 9-32).