Geology and Petrology of the Nishi and Fudenzaki Formations of the Aguni Group, Aguni Island, the Central Ryukyu Arc : Petrogenetic Relationship between the Andesites and Dacites

摘要

The late Miocene volcanic rocks of the Aguni Group range from basalt to rhyolite, and have a pronounced compositional gap between 57 and 69 wt.% SiO_2. Sr isotopic ratios for a dacite lava from the Nishi Formation (0.70463) and a gabbroic fragment from the Fudenzaki Formation (0.70455) are similar to those of the basalt-andesite suite (0.70452-0.70466). Both major- and trace-element variations from andesite to dacite are explainable in terms of fractional crystallization involving removal of the observed phenocryst phases in the evolved volcanic rocks. The Rare Earth Elements (REE) patterns of gabbroic fragments appear to coincide with the pattern of cumulate expected from fractional crystallization. A shallow composition-temperature slope of cotectic involving amphibole-bearing gabbroic assemblages is responsible for the development of bimodality between the andesites and dacites.