Alkali mapping of the japanese quaternary volcanic rocks

Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K₂O and Na₂O concentrations of their ejecta normalized to SiO₂ = 60% K₂O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na₂O values show no geographical trend. Relationships between the SiO₂-normalized K₂O and the depth of the deep seismic zone (K-h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K₂O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K₂O increases ca. 2.5 to 3.0% per 100 km depth of the seismic zone. Groups of volcanic centers near the junction between the Northeast Honshu and Izu-Mariana arcs give characteristically lower K₂O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K₂O value.