Fractionated Ilmenite-series Granites in Southwest Japan: Source Magma for REE-Sn-W Mineralizations

Abstract. Various leucocratic biotite granites, low-temperature I-type, from the middle zone of the Sanyo ilmenite-series granitic terrane were studied chemically. These granites are locally associated with REE-Sn-W mineralizations, and were compared with unmineralized granites and batholithic Ryoke granites in three areas of the Chubu, Kinki and Chugoku Districts. They are unique in the region because they have extremely low ferromagnesian components but high Rb/Sr and 10000Ga/Al ratios. These granites are divided petrographically into the main phase, finer-grained marginal phase and younger sheets and dikelets. These rocks have increasing of HREE+Y and Nb+Ta contents in this order, which is also followed by decreasing zircon saturation temperature from 780 to 725C. Together with the mode of occurrence of these granites, the leucogranitic magmas are considered to have formed by in-situ fractionation of the host granitic magmas near the top of the magma chambers. The concentration of HREE, Y, Nb and Ta in these Sanyo Belt leucogranites is principally controlled by magmatic fractionation.