ABSTRACT

We have analyzed major, minor, and rare earth elements (REE) in basalts, dolerites, gabbros and an amphibolite from a fore-arc seamount “Ogasawara Paleoland (OPL)” in the Izu-Ogasawara region, western Pacific. Barium and REE compositions of the OPL mafic rocks suggest that they were formed in a mid-ocean ridge. This interpretation is supported by the similarity of the TiO₂–Al₂O₃ systematics of the OPL mafic rocks to those of mid-ocean ridge basalts. To explain the occurrence of MORB-type mafic rocks in a fore-arc region, two possibilities are considered; (1) accreted oceanic crust of subducting slab, or (2) trapped oceanic crust remained in the fore-arc region when the subduction had initiated.