Petrogenesis of basalts along the eastern Woodlark spreading center, equatorial western Pacific

Seafloor spreading in the Woodlark Basin is taking place on pre-existing arc crust that was produced by the subduction of the Indo-Australian Plate into the Pocklington Trough (now inactive) to the south during the Paleogene. The Woodlark Basin has a unique tectonic setting characterized by two surrounding subduction zones. To the east, a spreading ridge is also currently being subducted beneath the Solomon Arc. Moreover, long-term subduction of the Pacific Plate occurred in this area, which was halted by the collision of the Ontong-Java Plateau with the Vitiaz Trench at ca. 10 Ma. Any one of these subduction zones could have influenced the mantle beneath the Woodlark Basin. In this study, basalts from the eastern Woodlark Basin spreading center (EWLB; eastern Woodlark Basin basalts) were analyzed for major and trace element compositions and Sr-Nd-Pb isotopic compositions to investigate the melting processes and mantle heterogeneity in this unusual tectonic setting. Our results show that the EWLB can be classified into three types based on major and trace elements, and Sr-Nd-Pb isotopic characteristics: normal EWLB (N-EWLB), very depleted EWLB (VD-EWLB), and ultra-depleted EWLB (UD-EWLB). N-EWLB are similar to normal mid-ocean ridge basalts (N-MORB) and comprise most of the EWLB. The EWLB formed from local mantle, which is similar to depleted MORB mantle. VD-EWLB are more depleted than N-EWLB and have a weak subduction fingerprint. These rocks are characterized by increasing Nb/La with increasing Sm/La, which is a trend that is not produced by peridotite melting. As such, VD-EWLB may have formed by melting of a source containing residual eclogite that had previously undergone low-degree partial melting during subduction, leaving residual rutile in the source. UD-EWLB are extremely depleted relative to global MORB, have elevated H2O/Ce and Ba/Nb ratios similar to back-arc basin basalts (BABB), and lower concentrations of H2O and Ba than N-MORB. We propose that UD-EWLB was derived from sub-arc residual mantle that was enriched by fluid and then experienced melt depletion. The subduction fingerprints in the VD- and UD-EWLB are not related to the current ridge subduction or earlier, long-term subduction of the Pacific Plate in the northeast of the basin, as they are geochemically distinct from the Solomon Arc, which was strongly influenced by both these subduction systems. Instead, we suggest that the subduction fingerprint of the VD- and UD-EWLB was produced during Paleogene subduction of the Indo-Australian Plate to the south.