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Andesites and Dacites from Daisen Volcano, Japan: Partial-to-Total Remelting of an Andesite Magma Body |
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| Authors: | TAMURA Y; YUHARA M; ISHII T; IRINO N; SHUKUNO H |
| Institution: | 1 IFREE, JAMSTEC, YOKOSUKA 237-0061, JAPAN 2 FACULTY OF SCIENCES, FUKUOKA UNIVERSITY, FUKUOKA 814-0180, JAPAN 3 OCEAN RESEARCH INSTITUTE, UNIVERSITY OF TOKYO, TOKYO 164-8639, JAPAN |
| 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 progressedfrom dacite to andesite, accompanied by rises in temperature.A positive correlation exists between phenocryst content (0–40vol. %) and wt % SiO2 (61–67%). Phenocryst-rich dacitescontain hornblende and plagioclase that are generally unaltered,clear, and euhedral. However, phenocryst-poor rocks containsieve-textured plagioclase, resorbed plagioclase, and opacitein which hornblendes are pseudomorphed. Some Daisen rocks containtwo coexisting pyroxenes. Many orthopyroxene phenocrysts fromtwo-pyroxene lavas have high-Ca overgrowth rims (up to 50 µm),a feature consistent with crystallization from a higher-temperaturemagma than the core. Rim compositions are similar from phenocrystto phenocryst in individual samples. Temperatures of 800–900°Care obtained from the cores, whereas temperatures of 1000–1100°Care indicated for the rims. Lavas ranging from aphyric andesite( |
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