Miocene tholeiites from Mt. Kunimi-dake ( ?? 15 Ma) on the back-arc side of central Japan carry plagioclase (Pl)+olivine (O1)±clinopyroxene (Cpx)±orthopyroxene (Opx) as phenocrysts. The Cpx phenocrysts are augitic and show zoning patterns with increase in Fe and decrease in Ca from the core to rim. Their groundmass consists of Pl+Cpx (augite+pigeonite)+opaque minerals+devitrification products±quartz. Pyroxene thermometers suggest ?? 1, 100°C for their crystallization temperature. The apparent partition coefficient of potassium (K) between Pl and melt at initial stages of groundmass crystallization suggests that K has not been moved by secondary processes in a majority of the tholeiites studied.
The studied tholeiites are broadly of island arc tholeiite type in chemical composition and as low in K₂O content (<0.6%) as basalts from the Quaternary volcanic front of NE Japan, but richer in TiO₂, P₂O₅ and Sr than the latter. The Kunimi-dake tholeiites are extremely high in P/Zr compared with not only arc tholeiites but also average compositions of mid-ocean ridge and ocean island basalts.

View full abstract

Cenozoic volcanism in the Takikawa district of the central Hokkaido, located at the junction of the Kuril and the northeastern Japan arcs, is unique in arc-trench system. It consists of Takikawa monogenetic volcano group (TMV) and four polygenetic volcanoes (Ofuyu-dake, Etai-dake, Shokanbetsu and Irumukeppu volcanoes). TMV are widely distributed in the district and mainly made up of alkali olivine basalt and olivine tholeiitic basalt. Whereas, voluminous polygenetic volcanoes associated with the TMV are mainly composed of calc-alkaline andesite and dacite.
New data of K-Ar ages for the rocks show that the volcanic activity of TMV continued from late Miocene to early Pleistocene, with coeval polygenetic volcanism during Pliocene. The presence of TMV and the geological evidence for the development of sedimentary basin suggest that the junction area of the two arcs has been under extensional stress field during late Miocene and Pliocene.
The SiO₂ contents of nearly primitive basalts (FeO/MgO<1.3) in the Takikawa district exhibit an ellipse shaped zonation in space. More silica-saturated basalts are distributed in the center of the district. This feature is not concordant with across-arc compositional zonation in arc volcanics. Major element chemistry and their spatial zonation of these rocks from Takikawa district suggest that the magmatism should not be explained by the steady state subduction-related one but by mantle upwelling under extensional stress field in the arc-arc junction.

View full abstract

Paleozoic rocks in the Yaguki area were thermally metamorphosed and mineralized by the Cretaceous granodiorite. Graphitization of carbonaceous material in pelitic and carbonate rocks was investigated by X-ray powder diffraction method.
L_c(002) values of carbonaceous material in pelitic rocks increase toward the granodiorite pluton, and reaches 800 Å near the contact with granodiorite. The ratio between L_a(110) and L_c(002) isapproximately 1.7 below L_c(002) of 600A and increases markedly above 600 Å. Lattice dimension 4. of carbonaceos material decrease with increasing L_c(002), and reaches 3.35A at L_c(002) value of 300 Å. Carbonaceous materials in limestone near skarn orebodies have lower L_c(002) values than those of pelitic rocks at the same distance from granodiorite pluton. The fact indicates that the graphitization of carbonaceous material was obstructed by the fluid that formed the skarn ore deposit

View full abstract