REDOX PATHS OF GRANITES DEDUCED FROM THEIR TEXTURE AND MINERALS

The texture and constituent minerals of granites are a useful indicator for estimating redox paths of magmas during cooling. The fO₂ buffering systems of magmas are divided into gas-buffer and rock-buffer, and the former includes SO₂-H₂S and CO₂-CO(CH₄) buffers and the latter includes the buffer reactions involving redox of Fe such as fayalite-magnetite-quartz (FMQ). In the logfO₂-T diagram, the inclination of SO₂-H₂S buffer lines are gentler than that of common rock-buffer lines, and the two lines intersect between the temperature range from 850 to 700°C (Takagi and Tsukimura, 1997 Econ Geol., 92, 81-86). SO₂ gas acts as a strong oxidizing agent for rocks in the temperature range below 850°C. The temperature of the intersection becomes higher with rising oxidation states of magma. In contrast, CO₂-CO(CH₄) buffer lines are almost parallel to common rock-buffer lines and located the lower fO₂ side than FMQ. Therefore, we can suppose the three redox paths as follows. PATH-1: following SO₂-H₂S buffer throughout magmatic and subsolidus stages, PATH-2: following SO₂-H₂S buffer in the early magmatic stage and rock-buffer in the late magmatic to subsolidus stages, and PATH-3: following CO₂-CO(CH₄) buffer throughout magmatic to subsolidus stages. In the magmas following PATH-1, early (primary) magnetite, euhedral titanite, zoned hornblende (increasing #mg outward), anhydrite and sulfides are present. In the magmas following PATH-2, secondary magnetite, ilmenite or anhedral titanite, zoned hornblende (increasing #mg outward), non-magmatic silicates (e.g., actinolite, chlorite) and sulfides with no anhydrite are present. In the magmas following PATH-3, no magnetite, zoned hornblende (constant or decreasing #mg outward), and pyrrhotite are present. The granites following rock-buffer throughout magmatic to subsolidus stages are rare, because sulfur is commonly abundant enough to exert SO₂-H₂S buffer system in highly oxidized magmas (e.g., Hattori, 1993 Geology 21, 1083-1086). Even in PATH-2, when the SO₂/H₂S ratios are very low (PATH-2b), the texture and minerals are similar to those of PATH-3. The redox paths are basically controlled by the volatiles in magmas, and the magmas following PATH-1 and -2 become magnetite-series, and PATH-2b and -3 ilmenite-series granites.