Permalink : http://hdl.handle.net/2115/36777

Major element compositions of 192 Pliocene and Quaternary volcanic rocks from 28 volcanic centers in southwestern Hokkaido (SW Hokkaido), which is situated at the northern part of the northeastern Japan arc, are determined. Using this data with previously reported analyses for volcanic rocks from northern Honshu (N-Honshu) and SW-Hokkaido, spatial variations in SiO2 normalized major element values (SiO2=60 wt%) at an arc junction are examined. The SiO2 normalized values for many major elements, TiO2, Al2O3, FeO, CaO, Na2O, K2O and P2O5, increase or decrease systematically westward. A N-S trending zonation of volcanoes, based on the SiO2-normalized value of the volcanic rocks, can be recognized from N-Honshu to SW-Hokkaido. The zonation in SW-Hokkaido is not parallel to the present trench axis which bends from a N-S to a NE-SW direction off the coast of SW-Hokkaido. The region comprising Yotei, Konbu and Niseko-Waisuhorun volcanoes displays a local anomaly with respect to TiO2, Na2O and P2O5 values.