Alkali mapping of the japanese quaternary volcanic rocks

Abstract

Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K 2O and Na 2O concentrations of their ejecta normalized to SiO 2 = 60% K 2O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na 2O values show no geographical trend. Relationships between the SiO 2-normalized K 2O and the depth of the deep seismic zone (K- h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K 2O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K 2O 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 K 2O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K 2O value.