Genesis of magmas producing pumice flow and fall deposits of towada caldera, Japan

Abstract

Towada caldera, lying near the northern end of Honsyu, Japan was constructed by eruptions of lavas and pyroclastic materials in three separate periods. At the ends of the first and second periods, great amounts of pumice were erupted in the form of pumice flow and fall respectively. Each pumice cruption was followed by collapse of the center of the cones resulting in double calderas. The lavas of these three periods and the pumice of the first and second periods were chemically analysed. The result was plotted in several different types of variation diagrams. The points for the lavas and pumice lie generally on smooth curves, indicating that the magmas which caused the pumice cruptions belong to the same general differentiation series as do the lavas. If SiO2/FeO+Fe2O, is plotted against sodification index (MgO x 100/MgO+FeO+Fe2O, +Na2O+K2O), points for the lavas lie on a straight line, whereas those for the pumice lie on another straight line branching from the former at some point in the middle stage of differentiation. The rate of increase of this ratio in the pumice is greater than in the lavas, implying that less SiO2 and more iron were subtracted from the magmas producing the pumice than from those producing the lavas. This was probably caused by crystallization of a greater amount of magnetite in the former magmas possibly due to higher oxygen partial pressure which may be in turn related to higher water content. It is not necessary to postulate melting of the crust in order to generate magmas of the pumice eruptions of the central type.