Major and trace element chemistry and Dili 180/160, 87Sr/86Sr and 143Nd/144Nd ratios of rocks from the spreading center of the Okinawa Trough, a marginal back-arc basin

Abstract

Basalts (OKTB) from the Okinawa Trough (OKT), which is a now opening marginal back-are 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/ i'Sr/ and W3Nd/144Nd ratios are almost identical in the dacite and basalts, and the l0OfnO ratio in the former is only slightly higher than in the latter, suggesting that the dacites are ultimately cognate with basalts. Concentrations of Al203, total iron, MgO, CaO, Na20, Ni, Cr, and V in OKTB are within the range of variation in normal type mid-ocean ridge basalts (N-MORE) whereas OKTB have significantly higher K20, Rb and Sr contents and Dili ,s0/160 and 81Sr1865r ratios and a lower 143Nd/l44Nd 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 intra- back-arc basins, that the H20-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 “Sr/Sr and lower 143Nd/l44Nd ratios, reflecting the higher contribution of continent-derived sedimentary components to the magma source. Oxygen, Sr and Nd isotopic characteristics of the racks of the OKT are fitted well with paired two-com-ponent mixing models: one for the source-mixing and another for the crustal contamination. © 1991, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.