Origin of silicic magmas and the compositional gap at Sumisu submarine caldera, Izu-Bonin arc, Japan

摘要

Sumisu caldera volcano, Izu-Bonin arc, Japan, is dominated by the bimodal assemblage of basalt-basaltic andesite ( < 55 wt.% SiO_2) and dacite-rhyolite ( > 66 wt.% SiO_2). Andesites (56-61 wt.% SiO_2) are minor in volume and a compositional gap of ～6 wt.% SiO_2 exists between the andesites and the dacites. Most andesites contain sieve-textured plagioclase, and some dacites have resorbed plagioclase containing many glass inclusions. Some orthopyroxene phenocrysts in andesites and dacites have overgrowth rims with a high Ca content, a feature consistent with crystallization from a higher-temperature magma than their cores. Dacites, rhyolites and glasses in the matrix and plagioclases possess compositions similar to those of partial melts produced by dehydration-melting of calc-alkaline andesite (Tanzawa tonalite) at low pressure (3 kbar). Phenocryst assemblages of these andesites, dacites and rhyolites resemble restite phase assemblages of the dehydration melting. Prolonged fractionation or partial melting of basalts, in contrast, is untenable based on petrography and fractionation calculations. We suggest that the silicic magmas at Sumisu caldera volcano have been produced by dehydration melting of solidified calc-alkaline andesite (protolith) in the upper to middle crust, which segregated efficiently from restite crystals. In rare situations, reheating and en-mass remobilization permitted the same andesites to erupt as andesites (56-61 wt.% SiO_2). Thus, Sumisu has a relatively narrow temperature difference (900-1100℃) associated with a large compositional range (56— 74 wt.% SiO_2).