| The Manus Basin is situated at the northeast section of the Bismarck sea in the Pacific,and it is one of the typical active back-arc basins in the western Pacific.This back-arc basin has experienced a complex tectonic history,and suffered two episodes of subduction involving the Pacific Plate and the Solomon Plate.In this study,we analyzed the petrography,mineralogy,whole-rock major and trace elements,and Sr-Nd-Pb-Hf isotopic compositions of intermediate-acid lavas from East Rift,Manus Basin.This study is helpful to understand the behavior of upper mantle,magmatic evolution process and crust-mantle interaction in back-arc basin,and to reveal the material exchange mechanism between the subduction plate and mantle wedge.Compositionally,our lava samples in the ER can be divided into basaltic andesite and dacite.The phenocrysts in the basaltic andesite samples are consist of olivine,clinopyroxene,and plagioclase.And the dacite samples have phenocryst assemblages including plagioclase,clinopyroxene and orthopyroxene.The thermobarometer calculation results show that,pyroxene and plagioclase phenocrysts in ER lavas are mostly formed in the pressure range of 1-3 kbar,indicating that a magma chamber may exist within approximately 3-10 km.Therefore,the magmatic process in ER is proposed that;in the early age,due to the large magma supply,the magma raised rapidly and stayed in the magma chamber for a short time,after a a small degree of fractional crystallization,the magma eventually ejected at a fast speed and formed basalt-basaltic andesites.And in the late stage of the ER lavas,the magma experienced strong fractional crystallization due to the low supply rate of magma and long stay of magma in the chamber,and finally formed acidic lavas.All samples fall within the medium-K calc-alkaline rock series,and the spider diagram normalized to primitive mantle shows significant enrichment in Pb,U and large ion lithophile elements(LILEs),and relative depletion in high field strength elements(HFSEs).This pattern is similar to that of island arc lava rather than that of typical back-arc basin lava.Most of ER lavas have“Dupal anomaly”and plot into Indian-type mantle in?Ndvs.?Hfdiagram,while all samples belong to Pacific-type mantle in the 208Pb/204Pb vs.206Pb/204Pb plot;this opposite result could be cause by the subduction components.Our calculation shows that,85-95%Pb in the mantle source of ER lavas came from subduction components,and the amount of involved Nd and Hf are less than Pb.Therefore,we argue that the mantle type of ER lavas is close to Indian-type mantle which reflected by its less affected Nd-Hf isotopic compositions.The mantle source for Manus Basin lavas is high extent depleted,and the mantle source for ER lavas is more depleted than that of lavas for MSC and ETZ,indicating the mantle source for ER lavas had already depleted by some prior event(s).The primitive magma of ER lavas generated from highly partial melting of spinel peridotite(30-80km).In the magmatic evolution process,fractional crystallization played an important role,and no assimilation and magma mixing occurred.In this paper,we made a detailed geochemical comparison of the altered oceanic crust(AOC)and sediments of the Pacific plate and the Solomon plate.Combining with our data,we argue that the Solomon Plate is a more likely source of the subduction components than the Pacific Plate,and the subduction components consist of AOC and sediments.The analyses also indicate that the AOC materials entered the mantle source through fluids,while the sedimentary materials were transferred as melts.Results of endmember modeling of Pb isotopes reveal that the subduction components beneath the East Rift area consist of approximately 60-65%AOC fluids and 35-40%sediment melts and that the source contains approximately 1-5%subduction components.Furthermore,the results of modeling of?Ndvs.Th/Nd show that,the basalt source contained approximately 1.2-1.8%sediment melts,which is consistent with the mixing results of the Pb isotopes. |
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