Temporal and Spatial Variations of Magnetic Susceptibility of Cretaceous to Neogene Igneous Rocks from the Central and Western Chugoku Province, Japan

Magnetic susceptibility is measured on powdered specimens of about 1100 volcanic rocks and 600 related plutonic rocks of the titled area. The results show systematic variation in magnetic susceptibility with age, geographic location of the rocks, and geologic units. When volcanic and plutonic rocks of the same cycle of activity are compared, their magnetic susceptibility resemble each other. The most prominent temporal change in magnetic susceptibility of igneous rocks occurred between Cretaceous and Paleogene. Cretaceous igneous rocks comprise both the magnetite-series (magnetic susceptibility, higher than 50 x 10-6 emu/g) and ilmenite-series (lower than 50 x 10-6 emu/g), the latter is predominant in volume, while the Paleogene and Neogene igneous activities are characterized by the rocks of only magnetite-series. As for the geographic distribution, the ilmenite-series rocks are developed throughout the studied area, whereas the magnetite-series rocks are restricted to the Sea of Japan side, overlapping over the area of the older ilmenite-series rocks.
The two series of igneous rocks are considered to have resulted from the different oxygen fugacities during evolution of the magma. The oxygen fugacity must have been controlled directly or indirectly by many factors such as differences in source material, the effect of buffering agents such as water and crustal carbon, mode of magma emplacement and tectonic setting. Based on the comparison of the tectonic history of this area and the measured magnetic susceptibility, it is argued that the conspicuous temporal change in magnetic susceptibility between the Cretaceous and Paleogene along the Sea of Japan was probably due to the temporal change in tectonic environment related to the formation of the Sea of Japan. It is also suggested that the classification of igneous rocks based on the magnetic susceptibility provides essential information in considering tectonic environment, for it is known that igneous rocks of the two series exhibit systematic distributions in time and space in the circum-Pacific regions.