2023 Volume 17 Pages 29-44
Chichibu tonalitic body, which is distributed in the northwestern Saitama Prefecture, is composed of magnetite series (Main body MT) and ilmenite series (Main body IL) as part of the main body, and the ilmenite series (Satellite IL) as the satellite body. Rocks of both series are tonalite to granodiorite, but the Main body IL and Satellite IL are richer in quartz and mafic minerals than the Main body MT, and Mg/(Fe+Mn+Mg) values of biotite and amphibole contained in the former are smaller than those contained in the latter. The Main body IL is richer in SiO2 than the Main body MT, and has different compositions of MnO, Cr, Pb, Sr, Th, Y, Zn, and Zr. Furthermore, the Main body IL has a slightly high A.S.I. (alumina saturation index), but no reaction with pelitic sedimentary rocks can be confirmed. In addition, there is a clear difference in the composition of the rare earth element patterns of the dark inclusions in both series. These differences in petrological features suggest that the Chichibu tonalitic body may have been formed from multiple magmas, but this has not been confirmed, and further investigation is required.
