Comparative Study of Chemistry of Rocks from the Kirishima, and the Daisen Volcanic Belts in Kyushu, Southwest Japan

Abstract

Eruptions of lava domes and thick lava-flows of hornblende andesites characterize the Daisen volcanic belt in Kyushu, whereas eruptions of voluminous pyroclastics and thin lava-flows of pyroxene andesites do the Kirishima volcanic belt. The hornblende andesites contain more abundant phenocrysts and have higher Fe³⁺/Fe²⁺ ratios than the pyroxene andesites. Sixty-two samples of rocks from the two volcanic belts were analyzed with X-ray fluorescence spectrometer. The hornblende andesites are lower in Fe/Mg, K/Ba and Rb/K ratios, poorer in Zr, Y, Rb and Cu contents, and richer in Nb, Sr and Ba contents than the pyroxene andesites. Along the Daisen volcanic belt, K/Rb and Ba/K ratios, and Sr and Ba contents in rocks increase eastwards. On the other hand, Fe/Mg ratio and Ti, K, P, Nb, Zr, Y, Sr, Rb, Ga and Ba contents in rocks increase northwards along the Kirishima volcanic belt. Although fractionation of hornblende and magnetite from hornblende andesite magma gives a possible explanation to the difference in major-oxide, Y and V compositions, the difference in other trace-element compositions between the hornblende and the pyroxene andesites may reflect differences in source materials. Chemical variations along the Kirishima volcanic belt may have originated either from northward increase in the degree of fractionation at higher pressures or from northward decrease in the degree of partial melting of source materials.