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

Abstract

Seafloor spreading in the Woodlark Basin is taking place on pre-existing arc crust that was produced by the sub- duction of the Indo-Australian Plate into the Pocklington Trough (now inactive) to the south during the Paleo- gene. 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 ofthe Pacific Plateoccurredinthisarea,which was halted by thecollision of theOntong？Java Plateau withtheVitiazTrench atca.10 Ma.Any oneofthesesubductionzonescouldhaveinfluencedthemantlebeneath the Woodlark Basin. In this study, basalts from the eastern Woodlark Basin spreading center (EWLB; eastern WoodlarkBasinbasalts) wereanalyzedfor majorand trace element compositions and Sr-Nd-Pb isotopiccompo- sitionsto investigate themelting processes andmantleheterogeneity inthis unusualtectonic setting. Our results show that theEWLB canbeclassifiedintothreetypes basedonmajorand 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 de- pleted 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 globalMORB,haveelevatedH 2 O/CeandBa/Nb ratios similartoback-arcbasinbasalts (BABB),andlower concen- trations of H 2 O and Ba than N-MORB. We propose that UD-EWLB was derived from sub-arc residual mantle that wasenrichedbyfluidandthenexperiencedmeltdepletion.ThesubductionfingerprintsintheVD-andUD-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.