Genesis of high-magnesium andesites and associated basalts from Saga-Futagoyama, northwest Kyushu, southwest Japan

摘要

High-magnesium andesites associated with basalts erupted after the opening of the Sea of Japan are present at Saga-Futagoyama in northwest Kyushu, southwest Japan. High Mg/(Mg + Fe) [=0.84] of orthopyroxene phenocrysts and bulk rock Mg-Fe-Ni compositions suggest that these high-magnesium andesites were originally primitive melts insignificantly modified in crustal magma chambers. K_D~(Ca-Na) [= (Ca/Na)_(pl)/(Ca/Na)_(bulk rock)] ranges from 1.21 to 0.97 and suggests that the high-magnesium andesite magmas would originally have contained H_2O less than 1.8 wt.%. Nb/La does not show a negative correlation with respect to SiO_2. These lines of evidence indicate that hydrous components derived from the subducting slab would not have played a significant role in the genesis of the high-magnesium andesite magmas. Instead, the normative olivine - quartz - [CaTs + Jd] compositions and a negative correlation between Sr/Nd and SiO_2 indicate that the basalt-high-magnesium andesite association would have been formed by multi-stage partial melting of relatively anhydrous source at pressure ranging from 1.5 to 0.5 GPa.