Petrology of Hime-Shima volcanic group

Abstract

Petrography, whole rock chemistry and phenocryst mineralogy revealed that compositional variation among Hime-shima volcanic rocks are mainly explained by magma mixing of aphyric rhyolite and plagioclase-, hornblende-bearing dacite.
All the major and trace element data plot near straight lines on the Harker diagrams. Least-squares calculations using major element contents show that some volcanic rocks are products of fractional crystallization from less silicic magma. But the variation of the trace elements cannot be explained by the same process if available distribution coefficients are used in the calculation of hornblende and plagioclase fractionation. Conspicuous resorption texture is observed among the plagioclase in the lavas of intermediate compositions, whereas hornblende is euhedral in all lavas. Positive correlation between the whole rock chemistry and the volume ratio of hornblende phenocrysts suggests that aphyric rhyolite magmas (75 wt. % SiO₂) mixed with a plagioclase-, hornblende-bearing dacite magma (66 wt. % SiO₂), and produced biotite-bearing hornblende dacite (68-73 wt. % SiO₂). The contrasting textures of plagioclase and hornblende will be ex-plained by their different liquidus temperatures: With increasing H₂O content (above 1 wt. % H₂0), the plagioclase liquidus drops sharply, whereas hornblende liquidus stays nearly constant.
Hime-shima volcanic group is composed of seven volcanoes which represent individual magma reservoirs, because volcanic rocks erupted from each volcano are distinguished by petro-graphy and chemistry. Field evidences show that the least silicic magma was the first to ascend and emplace. Several batches of silicic magmas ascended through this least-silicic magma reservoir and mixed with it in varying mixing ratios to form intermediate magmas.