Iron-titanium oxide minerals of Cretaceous to Paleogene volcanic rocks in western Chugoku district, Southwest Japan. Special reference to manganese content of ilmenites.

Abstract

Fe-Ti oxide minerals of Cretaceous to Paleogene volcanic rocks in the western Chugoku district, Southwest Japan, have been examined mineralogically by means of electron microprobe. The volcanic rocks are divided into the Cretaceous Kanmon, Shunan, Hikimi, Abu and Paleogene Tamagawa Groups. Magnetite predominates in the volcanic rocks of the Kanmon, some of the Shunan, and Tamagawa Groups, while it is generally absent in those of Hikimi and Abu Groups. Chromian spinel has been found from the basaltic andesites of the Kanmon and Tamagawa Groups. The magnetic susceptibility of whole rock well reflects the above-described difference of the magnetite content, that is, X-value is higher than 50×10-6emu/g for the Kanmon, some of the Shunan and Tamagawa Groups, while it is lower than that for the Hikimi and Abu Groups. Ilmenite is contained in almost all volcanic rocks examined, and its mode of occurrence is classified into three main types; 1) independent type, 2) trellis type, 3) composite type. Manganese least concentrates in the independent type. The difference of the MnO content of ilmenite among the five volcanic groups is far larger than that of the mode of occurrence and it is well distinguished in the FeTiO3-Fe2O3-MnTiO3 triangular diagram. That is, ilmenites from the Paleogene Tamagawa volcanic rocks are higher in MnTiO3 and Fe2O3 molecules than those of the Cretaceous volcanic rocks (e. g., Kanmon, Hikimi and Abu Groups). Ilmenites from the Shunan Group are intermediate between the above-cited two groups of composition. In general, ilmenites from Paleogene volcanic rocks are richer in MnTiO3 and Fe2O3 molecules than Cretaceous ones. Similar tendency in respect to age is observed in ilmenites of plutonic rocks, too. The Mn2+/Fe2+ ratio of ilmenite depends on temperature, Mn/Fe ratio of source magma and fo2. Available data indicate that high M-concentration in ilmenites from Paleogene igneous rocks may be attributable to high fo2, during the formation of ilmenite.