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Oceanic Arc Systems |
Department of Geology, The University, Southampton, SO9 5NH, UK
Hahajima represents an Eocene forearc volcanic centre produced during the initiation of subduction within an intra-oceanic domain of the western Pacific. The Eocene-Oligocene Izu-Bonin arc-forearc is envisaged as a broad region of volcanism generated in an extensional tectonic regime. The forearc is likely to have consisted of a series of small rift basins, punctuated and bordered by coherent volcanic edifices. Hahajima represents one such edifice at the southern termination of the Chichijima-Mukojima rift basin.
Major element, trace element and isotopic data are presented for representative volcanic rocks from Hahajima. The volcanic sequence comprises porphyritic basalt and andesite breccias and flows intercalated with tuffaceous sandstones. All lavas contain high proportions of plagioclase phenocrysts (20–36%) and comparatively low mafic mineral contents. The andesite suite is typified by the presence of magnetite and orthopyroxene phenocrysts and pseudomorphs after olivine. Basalts are dominated by an assemblage of plagioclase, clinopyroxene and olivine. Nodules and xenocrysts typical of the andesites are found in the basalts and vice versa, suggesting an intimate relationship between the two magmas during volcanism.
The basaltic suite follows a tholeiitic fractionation path (iron enrichment) while the andesites are more representative of calc-alkaline magma evolution. These trends are found to be clearer when corrections are made for the excess proportions of plagioclase in the whole rock. Trace element differences between the suites can not be reconciled by a straightforward crystallization model, but can be related to a similar parental magma if the shallow level magma evolution involves a combination of open and closed system fractionation processes.
Comparisons of volcanic suites within and between each site sampled along the Izu-Bonin forearc indicate that each eruptive centre has an individual trace element signature. This suggests that variations within each site can be explained by melting and/or shallow-level fractionation processes, while differences between localities are related to heterogeneities in the composition, trace element and isotopic enrichment, and thermal characteristics of the source. The Hahajima volcanic rocks contrast with the more recent arc magmas of the Izu-Bonin system in that they share some features, such as high-Mg and elevated Zr/Y, with the contemporaneous boninitic lavas.
