Comparative Petrologic Study of Basaltic Rocks from the Kinki and Chubu Districts

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The origins of basaltic rocks from the Kinki and Chubu districts were studied through extensive microprobe analysis of the phenocrysts and the whole-rock geochemistry. The studied rocks are alkalic, olivine tholeiitic and calc-alcalic basalts to andesites erupted in central Japan where the depth of the subducting Pacific plate (subduction zone) ranges from -150 to -400 km. Rocks from volcanoes beneath which the subduction zone is <-250 km deep are typical island-arc basalts and their derivatives in mineralogy and chemistry. On the other hand, despite their distribution in an island arc system, rocks from places beneath which the subduction zone is >-250 km deep show a magmatic affinity unlike the arc type: chemical compositions of the phenocrysts (olivine, plagioclase and clinopyroxene) are characteristic to non-arc settings; and the whole-rock analyses of the basalts do not show marked depletion of Ta, Hf and Ti relative to Th, Ce and Sm. These basaltic rocks formed probably from a so … More urce which has generated the Neogene alkalic volcanic rocks bearing a within-plate alkalic basalt affinity in west Japan, by greater degrees of melting at shallower depths. At places where the depth of the subduction zone is -250 km, olivine tholeiitic rocks with a within-plate basalt affinity erupted at -2 Ma, while rocks with an island-arc basalt affinity have been erupted at <1 Ma.A batch of mantle material, potentially a source of the Neogene alkalic volcanic rocks in west Japan, is likely to have emplaced beneath the Ryohaku Mts. at the back-arc side of the East Japan Volcanic Zone. It is probable that a change in motion of the Philippine Sea plate at -3 Macaused the sudden drift of the non-arc type magma source from the uppermost mantle beneath the Ryohaku Mts. to that beneath the Norikura Volcanic Chain. The K-Ar age data suggest that at -1.5 Ma, the magma source was drifted westward back to the uppermost mantle beneath the Ryohaku Mts. by a steady flow of mantle convection beneath the East Japan Arc System. Less