東北本州弧第四紀火山岩類の研究  [in Japanese] Study of the Quaternary volcanic rocks in the Northeast Honshu Arc, Japan  [in Japanese]

Recent studies on Quaternary volcanic rocks from the Northeast Honshu Arc, Japan, are reviewed. This mature arc is divided into four volcanic zones, Aoso-Osore, Sekiryo, Moriyoshi and Chokai, from the trench to back arc side. In the Aoso-Osore zone, low-K calc-alkaline rocks with subordinate low-K tholeiites erupted. Medium-K calc-alkaline rocks are associated with, and often follow low-K tholeiites at the volcanoes of the Sekiryo zone. The Moriyoshi zone is characterized by medium-K calc-alkaline rocks with subordinate medium-K tholeiites. The Chokai zone contains medium-K calc-alkaline rocks on the eastern side, and high-K calc-alkaline rocks on the western side. These are associated with high-alumina basalts and alkali basalts, respectively. The Sekiryo volcanic zone is an axial zone of arc volcanism in which the eruptive materials are much more voluminous than in the other three zones. Among the four zones, the temperature of magmas is the highest at the Sekiryo zone. Crystal fractionation of phenocrysts from mantle-derived basalts may be the common origin for andesites and more felsic magmas. The low-K calc-alkaline andesitic to dacitic rocks in the Aoso-Osore zone may be derived from the low-K tholeiites by crystal fractionation. In contrast, trace elemental and isotopic data show that the associated medium-K calc-alkaline and low-K tholeiitic rocks in the Sekiryo zone were derived from different degrees of partial melting and/or different mantle sources. Heterogeneity in the magma reservoir at the latest stage of stratovolcano formation is inferred from the ubiquity of magma mixing textures at that stage, especially, in the Moriyoshi and Chokai zones. Quaternary basaltic rocks from the Northeast Honshu arc are enriched in K, Ba, Rb and Sr and depleted in Nb and Ti relative to the REE. The isotopic data and the back-calculated source compositions show chemical heterogeneity within the mantle wedge. The lateral variation of elemental abundances and across-arc profile of magrnatic temperature in the Northeast Honshu arc is closely related to the three dimensional velocity structure of the upper mantle. A model is proposed to describe island arc magmatism in the Northeast Honshu arc.