1982 Volume 32 Issue 175 Pages 391-404
The magnetite-series and ilmenite-series granitoids situate in the San-in and Sanyoh belts respectively, which are bounded by the Ikuno-Tsuyama line, in the eastern Chuhgoku district (KANAYA and ISHIHARA, 1973). Some ilmenite-series granitic bodies, however, such as Wadayama, Hikihara, Mochigase, Chizu and Suriba-chiyama, situate in the northern side of the boundary, while some magnetite-series bodies, such as Siso, My-ohkenzan, Yanahara, Takakurayama and Sugiyama, in the southern side (KANAYA and ISHIHARA, 1973; ISHI-HARA et al., 1981).
TERASHIMA et al. (1980) re-examined the airborne magnetic and gravitational data in the Ikuno area(M.I.T.I., 1971-1974), and clarified that the Wadayama body of the ilmenite-series situates in the area of remarkable, low gravitational anomaly but of no magnetic anomaly, while the Siso body of the magnetite-series, in the area of no gravitational anomaly but of remarkable magnetic anomaly, in the area of Paleozoic system such as Maizuru group and Tamba group.
The author tried to re-examine the airborne magnetic and gravitational data in the Tsuyama area (M.I.T.I.; 1979), and confirmed that the ilmenite-series granitoids, which intruded into Paleozoicsystem, situate only in the area of the low gravitational anomaly, while the magnetite-series granitoids, in the area showing no remarkable gravitational anomaly. This phenomena indicates that the former is of lesser density than the Paleozoic system, and the latter is of almost equal density to Paleozoic system.
The author also clarified the possible distribution of the ilmenite-series and magnetite-series granitoids in the Ikuno-Tsuyama area, and considered that the latter intruded into the area of the former to make a composite body, and to give some thermal affection to the radiometric age of the former. This presumption could be powerful to explain reasonably the unordinary radiometric ages of K-Ar method from the Myohkenzan, Yanahara, Tenguyama, Sugiyama bodies of the magnetite-series and the Suribachiyama, Chizu, Mochigase bodies of the ilmenite-series.
