Abstract
Petrological and geochemical study of volcanic rocks from Aguni-jima island, central Ryukyus has revealed their island-arc type characteristics. The volcanic rocks belong to Higashi Formation of early Pliocene age, and are divided into lower and upper units. Samples of lower unit are basaltic, whereas those of upper unit are andesitic. Trace element patterns are similar to those of island-arc volcanic rocks, and the rock suite may belong to the high-alumina basalt series. Major, trace and isotopic compositions show that the lower unit contains volcanic rocks of two different origins; one related to those of upper unit andesites, and the other unrelated ones. The basalt-andesite suite is possibly the result of low-pressure fractional crystallization of plagioclase, augite, magnetite, ±olivine and ±hypersthene.
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