Abstract:
Voluminous andesite and dacite lavas of Daisen volcano, SW Japan, contain features suggesting the reverse of normal fractionation (anti-fractionation), in the sense that magma genesis progressed from dacite to andesite, accompanied by rises in temperature. A positive correlation exists between phenocryst content (0–40?vol.?%) and wt % SiO2 (61–67%). Phenocryst-rich dacites contain hornblende and plagioclase that are generally unaltered, clear, and euhedral. However, phenocryst-poor rocks contain sieve-textured plagioclase, resorbed plagioclase, and opacite in which hornblendes are pseudomorphed. Some Daisen rocks contain two coexisting pyroxenes. Many orthopyroxene phenocrysts from two-pyroxene lavas have high-Ca overgrowth rims (up to 50?µm), a feature consistent with crystallization from a higher-temperature magma than the core. Rim compositions are similar from phenocryst to phenocryst in individual samples. Temperatures of 800–900°C are obtained from the cores, whereas temperatures of 1000–1100°C are indicated for the rims. Lavas ranging from aphyric andesite (?61?wt?% SiO2) to phenocryst-rich dacite (?67?wt?% SiO2) have similar 87Sr/86Sr (0·7045–0·7052) and 143Nd/144Nd (0·5127– 0·5128). Isotopic variability within Daisen volcano is likely to be mantle-derived, reflecting isotopic variability within the magma source region associated with a single mantle diapir. The Daisen andesites and dacites have the same trace element signatures as the associated basalts and were probably derived from primary magmas at the same general depth (?60?km). Our interpretation is that mantle-derived hydrous magnesian andesite, generated in the same mantle diapir as coexisting basalt magma, may be parental to the hydrous calc-alkaline magmas in Daisen volcano. We suggest a two-stage process, involving mid-crustal solidification of bodies of this calc-alkaline magma followed by varying degrees of partial melting from body to body, to produce the magmatic trends and phenocryst zoning patterns observed. The heat required for this melting, according to our model, was supplied by the intermittent rise of subjacent basaltic magma.