Petrochemical studies on the welded tuffs in Hokkaido, Japan (II)

Petrochemical studies on the welded tuffs from Kimobetsu, Toya, Tokachi and Daisetsu districts

Abstract

The authors analyzed chemically 16 specimens of the welded tuffs in Hokkaido; those are 4 from Kimobetsu, 3 from Toya, 4 from Tokachi, 4 from Daisetsu and 1 from Shikotsu districts. Si0₂ contents range from 54.31 to 72.12 percents, therefore there are rhyolitic, dacitic and andesitic welded tuffs. They are mostly plotted in the field of Kuno's hypersthenic rock series in the MgO-(FeO+Fe₂O₃)-(Na₂O+K₂0) diagram, proving to have been derived from the magma contaminated by sialic materials. Among them, the Tokachi and Daisetsu welded tuffs are clearly richer in alkalies, especially K₂0 corresponding to Si0₂ than the others.
Such characteristics shown in the chemical composition of the welded tuffs is similar to that of lavas from the same inner subzone of volcanic zone along the Kurile arc. Original magma from which the Tokachi and Daisetsu welded tuffs were derived must have had the composition of the high alumina basalt proposed newly by Kuno, which is rich in alumina and higher in alkalies than the tholeiitic basalt magma. While the other welded tuffs which belong to the Nasu zone situated at the outer side of volcanoes along the north-eastern Japan arc considered to have been derived originally from the tholeiitic magma.