四国北西部の中央構造線に沿う酸性火山岩の起源とK-Ar年代  [in Japanese] K-Ar ages and the origin of acidic volcanic rocks along the Median Tectonic Line at northwest Shikoku, Japan  [in Japanese]

In northwest Shikoku, igneous activity of Miocene age have been known as 1) Setouchi high magnesian andesite suite, and 2) Ishizuchi calcalkaline suite. Both igneous suite have been accompanied with acidic volcanic rocks as rhyodacite to rhyolite. These acidic rocks are classified into: 1) Acidic volcanic rocks as the later product of fractional crystallization of high magnesian andesite magma, 2) Acidic volcanic rocks as initial volcanism of the Ishizuchi cauldron. Along the Median Tectonic Line in northwest Shikoku, several acidic volcanic rocks of Miocene age are distributed. They are considered to be one of the consisting members of Ishizuchi supergroup, calcalkaline volcanic suite. Petrogenetic relationship between these acidic rocks and Ishizuchi calcalkaline rocks have not been clarified. This study determined K-Ar age of the acidic volcanic rocks and also focussed on geochemistry to consider the origin of these rocks. The K-Ar age of these rocks are from 13.9 to 14.3 and 14.1 Ma on average, which is 1 Ma younger than that previously reported for an andesitic intrusion along MTL. SrI (^<87>Sr/^<86>Sr ratio at 14 Ma) of the acidic volcanic rocks are divided into three groups as follows: 1) Taisanji type; 0.70967, 2) Myojin-yama type; 0.70688. Major element chemistry and Sr isotope ratios suggest that: 1) Taisan-ji type magma having extremely high SrI and K_2O contents may be produced by the partial melting of per-aluminous rocks similar to pelitic hornfels xenolith with high SrI included in Takatsukiyama granitoid, and 2) Myojin-yama type one has the same origin as the per-aluminous granitoid of the SW Japan outer zone because they have similar ^<87>Sr/^<86>Sr and ^<87>Rb/^<86>Sr isotope ratios to those of Takatsukiyama granitoid and also have the same metamorphic xenoliths such as basic to acidic gneisses.