瀬戸内海西部産瀬戸内火山岩の微量元素の特徴

抄録

The Setouchi volcanic rocks of Middle Miocene age, which were erupted in the present Seto Inland Sea (Setonaikai) region of southwest Japan, are characterized by the preponderance of magnesian andesites. The magnesian andesites from western Setonaikai are higher in Rb but lower in Sr than most alkali basalts from the Japanese islands. Incompatible element concentrations in the Setouchi magnesian andesites are generally high, compared to those in magnesian andesites and tholeiites from the Izu-Mariana and northeast Japan arcs. The MORB-normalized incompatible element pattern for the western Setouchi magnesian andesites is similar to that for the nearby Yabakei tholeiites, though Rb, K and to a lesser extent, Ba and Zr are enriched above the level of the Yabakei tholeiites. Similarly, the pattern for the Chichijima boninites lies parallel to, but at a lower level than the Hahajima tholeiite pattern, except for Rb and K exhibiting a marked enrichment. The incompatible element characteristics of the western Setouchi magnesian andesites result possibly from addition of Rb, K and to a lesser extent, Ba and Zr accompanying introduction of water into a mantle source with the same incompatible element characteristics as that of the Yabakei tholeiites.