ABSTRACT

Basalts (OKTB) from the Okinawa Trough (OKT), which is a now opening marginal back-arc basin, are chemically quite similar to those from the intra-oceanic back-arc basins such as the Mariana Trough. Both olivine-normative and quartz-normative basalts occur, and major and trace element concentrations show a systematic variation, probably derived by differentiation of the magma. D/H, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios are almost identical in the dacite and basalts, and the ¹⁸O/¹⁶O ratio in the former is only slightly higher than in the latter, suggesting that the dacites are ultimately cognate with basalts. Concentrations of Al₂O₃, total iron, MgO, CaO, Na₂O, Ni, Cr, and V in OKTB are within the range of variation in normal type mid-ocean ridge basalts (N-MORB), whereas OKTB have significantly higher K₂O, Rb and Sr contents and D/H, ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr ratios and a lower ¹⁴³Nd/¹⁴⁴Nd ratio than N-MORB have. These chemical and isotopic features of OKTB can be well explained by the concept, which has been proposed by many authors for magma genesis in the intraoceanic back-arc basins, that the H₂O-rich primary magma is generated from normal-type mantle peridotite modified by component from the subducted slab. Compared with those from intra-oceanic back-arc basins, the rocks from the OKT have distinctly higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd ratios, reflecting the higher contribution of continent-derived sedimentary components to the magma source. Oxygen, Sr and Nd isotopic characteristics of the rocks of the OKT are fitted well with paired two-component mixing models: one for the source-mixing and another for the crustal contamination.