Tracing the origin of subduction components beneath the South East rift in the Manus Basin, Papua New Guinea

Abstract

The Manus Basin to the northeast of Papua New Guinea is an actively spreading/rifting back-arc basin in the Bismarck Sea located between the inactive Manus-Kilinailau trench on the Pacific-plate side and the activeNew Britain trench on the Solomon-plate side. Spreading/rifting in the Manus Basin takes place in the last 0.78Myr or so. We present major and trace elements, and Sr？Nd？Pb isotope compositions of rock samples taken from the South East Rift (SER) at the eastern end of the Manus Basin. The strong enrichment of Pb and LILE (large ion lithophile elements) relative toHFSE (high field strength elements) and REE (rare earth elements) in the SER lava is also quite similar to other island arc lavas, suggesting that substantial amount of subduction componentswere present in its sourcemantle. To investigate the origin of the subduction components in SER lavas,we compare the geochemical data of SER lavas to published data fromNewBritain Arc (NBA) and Tabar？Lihir？Tanga？Feni (TLTF) island chain. The volcanisminNBAis related to presently active subduction of the Solomon slab,whereas the TLTF volcanism is located in the forearc area of New Ireland arc which was formed during a former subduction of the Pacific slab. In otherwords, the NBA and TLTF lavaswere influenced by subduction components fromthe present and former subduction, respectively.We argue that the subduction components in SER lava were incorporated in the mantle lithosphere during the active arc volcanism on New Ireland because the amount of the subduction component in SER decreases with increasing in distance from New Ireland. On the other hand, no relationships are found with respect to New Britain. The Sr？Nd？Pb isotopes indicate that SER lavas contain little sediment component and less amount of fluid component derived from altered oceanic crust compared to the TLTF lavas. This is probably due to the fact that SER is located in backarc settings in