Volcanisms of the Backarc Echelon Seamounts along the Enpo Seamount Chain in the Northern Izu-Ogasawara Arc

Echelon seamount chains trending ENE-WSW exist in the backarc region of the northern Izu-Ogasawara Arc. The Kan'ei, Manji, Enpo and Genroku seamount chains (from north to south) constitute four especially well documented and investigated example. These seamount chains formed between 17 Ma and 3 Ma (Ishizuka, 1999). The eastern sections of each seamount chain are over-printed by many small knolls formed by intra arc rifting volcanism after 2.5 Ma (e.g. Hochsteadter et al., 2000; Morita, 1999). We report results of detail petrological analysis of basalts from two different seamounts and andesites from four different seamounts of the Enpo seamount chain. The Ar-Ar age of these volcanic rocks range from 5.8 Ma to 3.9 Ma (Ishizuka, 1999). Bulk chemistry of trace element and composition of chrome spinel included in olivine phenocrysts indicate that there are two kinds of primitive basalts in the Enpo seamount chain. These were produced from different mantle sources each other. One type of basalt has 'enriched' composition similar to enriched mid-ocean ridge basalt (E-MORB). Trace element signatures indicate that the other type of basalt is produced by 'subduction-related' magmatism. It is defined that there are two mantle souses, enriched (E-MORB-like) and subduction related, for the volcanic rocks constituting the Enpo seamount chain. Enriched basalts exhibit reverse zoning and resorptive rims of olivine crystals, which may indicate magma mixing. We use mineralogical and geochemical studies, to conclude that most andesites from this region are produced mainly by fractionation of 'subduction related' basalts, and that fractionation occurs along with magma mixing and/or interaction with crustal materials. Andesites, which cannot be explained by fractionation, require either different mechanisms for magma genesis and/or the presence of a different primary basalt with a different petrological characteristics. Volcanism on the Enpo seamount chain is characterized by complex relationships between enriched (E-MORB-like) basalt, subduction-related basalt, fractionation-related andesites, and other apparently unrelated andesites.