We conducted the petrological study intended to the Back-Arc Basin Basalt (BABB) in the North Lau basin. Samples from the Fonualei Rift and Spreading Center (FRSC) and the Northeast Lau Spreading Center (NELSC) are collected and subdivided to the sFRSC, nFRSC, sNELSC and nNELSC (using reference compilation for nNELSC) in northward-directional order to clarify geochemical changes. In the spider diagram for trace element, there is successive change from the Tonga arc (TA) through sFRSC to nNELSC, while the Samoa plume has different pattern. This tendency presents in the diagram for Rare-earth elements (REE) as well. First, the mantle fertility represented by Yb and Nb ratios is enhances from south to north, having more distinct variations in NELSC compare with FRSC. Degree of melting calculated by fractionation-corrected Na and Fe contents has no significant range in accordance with the assumption that the fertility variations are caused by the fertile melt input, not a degree of melting. NELSC showed evident influence of the plume has similar isotopic ratios of Nd, Sr and Pb with the island arc now shutoff on the East Lau basin, indicating the fertile melt derived from subducting Pacific slab. Second, influence of subduction-induced fluid represented by Ba/Th and Th/Nb plots indicate that NELSC is less affected by both shallow and deep subduction fluids than FRSC, the subduction input from the northern boundary of two plates therefore does not contribute to North Lau basin distinctively. Third, we plot La/Sm versus Ba/Nb to show the contributions of two main factors, plume (fertile melt input) and fluid (subduction-induced input). The plots show that the plume dominates in norther part and the fluid do in southern part relatively. So, we conclude that NELSC located in the northernmost part of Lau basin is mainly affected by fertile plume, yet further studies are necessary to whether the derivation of this plume is the Samoa plume or not.
Located in the southwest Pacific ocean, The seamounts, from TA07 to TA26 seamount in the Tonga arc, are located from 20 °S to 25 °S. At 25 °S, the Tonga trench is intersected by the Louisville ridge, a ~4,300-km-long chain of seamounts and the Osbourn trough, a paleo-spreading center. For this reason, magma which created these seamounts may have various origin. Based on this, the seamounts composed of the Tonga arc are divided into two groups; including group 1(those affected by Louisville ridge, correspond to TA07~24); and group 2(those affected by the Louisville ridge and the Osbourn trough, correspond to TA25~26). These seamounts are mostly stratovolcanoes with caldera. Rocks recovered by dredging have been identified as pumice, dacite, andesite, basaltic andesites and basalts(most abundant). Major element concentrations are constant, trace element concentrations are enriched LILE, depleted HFSE compared with MORB. Tectonic model for Tonga arc is proposed, comprising: (1) At 115 Ma, Osbourn trough was active, Manihiki and Hikurangi plateau was rifted, (2) At 87-91 Ma, it ceased spreading. (3) After then, because of hot spot activity, Louisville seamount chain have been formed. (4) The Tonga-Kermadec arc has been formed since 25 Ma. (5) At ~6-5 Ma, Louisville ridge is adjacent to Tonga trench. (6) Since 3 Ma, Louisville ridge subduction was occured along the northern Tonga trench. The Tonga arc is affected by subduction components divided into the shallow and deep subduction components. Related to subduction components, variables include mantle source, AOC(Altered oceanic crust), PS(pelagic sediment), LSC(Louisville seamount chain) and OS(Osbourn Trough). Three variables, including mantle source, AOC and PS, are background value. In case of the shallow subduction component, influence of the Louisville ridge decreases southward. The effect of the Osbourn trough is only definitely distinctive in group 2. The deep subduction component reduces southward since the effect of Louisville ridge decreases southward and the effect of the Osbourn Trough is insignificant.