2018 年 63 巻 2 号 p. 19-32
Four peaks (Osanbe, Mesanbe, Kosanbe, and Magosanbe) form the major body of Sanbe Volcano. A possible scenario of formation of such discrete peaks could be creation of a single lava dome followed by eruption and erosion, as an explosion-center-like depression is located in the center of four peaks. However, the following lines of evidence may suggest that four lava domes formed individually. Firstly, rocks constituting each peak exhibit distinctive petrographic features in terms of the amount of quartz phenocrysts and compositions of plagioclase phenocrysts. Secondly, Mesanbe rocks tend to possess SiO2 contents lower than other rocks and show unique chemical trends. Lastly, rocks from Osanbe and Mesanbe exhibit systematically higher, K, Rb, Sr, Ba, and Zr concentrations than those from Kosanbe and Magosanbe. This geochemical characteristics together with rather constant concentration of Y for Sanbe lavas may lead to the conclusion that four types of magmas are produced by anatexis of the lower crust in the presence of garnet, plagioclase, and amphibole by different degrees of melting.