A note on the chemical composition of the lavas from the Daisetsu-Tokachi volcanic chain

Abstract

The Daisetsu-Tokachi volcanic chain lying on the central highland of Hokkaido extends in NE-SW direction about 80km. It belongs to the inner subzone of the Kurile volcanic zone. This volcanic chain comprises vast pyroclastic plateaux of Pleistocene (or later Pliocene-Pleistocene) welded tuffs and Holocene stratocones, lava-domes and cindercones. The rocks of the volcanic chain vary in composition from basalt to rhyolite (46.79-71.57% in silica content), most of which are comprised in calc-alkali rock series (or hypersthenic rock series) and are characterized by common bearing of hornblende and biotite. The general chemical character of the rocks is the richness in alkalies (alkali-lime index=62) and in K₂O/Na₂O and Fe₂O₃/FeO ratios and poorness in FeO+Fe₂O₃/MgO ratio, which shows quite different from that of the pyroxene andesitedacite suite (calc-alkali rock series) of the Shiretoko-Akan volcanic chain belonging to the outer subzone of the same volcanic zone. The marked difference between the two rock suites of the outer and inner subzones owes its origin to the difference in character of their parental basaltic magmas; i.e. alkali-por tholeiite magma in the Shiretoko-Akan volcanic chain, and weak alkalic tholeite magma in the Daisetsu-Tokachi volcanic chain and alkali olivine basalt magma in the Japan Sea coast, respectively. Most hornblende (and biotite)-bearing rocks are considered to be derived through sialic contamination of the latter two magmas.