Lithium And Neodymium Isotopic Composition Of Intermediate Lavas In Eastern Manus Basin:the Indication To The Magma Source Region

Manus basin is an active back-arc extensional zone,which is affected by the double subduction of the Solomon Sea plate and the Pacific Ocean plate.The subduction processes involved and corresponding magmatismare extremely complex.Therefore,revealing the material source and evolution process of Manus magmatic activity through geochemical methods,which is of great significance for understanding the crust-mantle material cycle of subduction zone and the evolution of back-arc basin.The samples used in this paper were taken from the"Science"research ship-201501 voyage,and the volcanic rock samples from the eastern Manus back-arc basin were obtained with the help of underwater robots.The lithology of the rocks mainly consisted of basaltic andesite and dacite.The analysis testing of the main elements,trace elements,lithium isotopesand neodymium isotopes of the whole rock,combined with the previous data and the geological characteristics of the study area,to focus on the geochemical behavior of lithium elements and isotopes in the subduction process,which lays a foundation for an comprehensive understanding of the material cycle in the subduction zone.According to the main element composition analysis of the volcanic rocks in eastern Manus basin,it shows that the volcanic rocks are mainly formed by fractional crystallization.The crystallized minerals may include olivine,pyroxene,plagioclase,ilmenite and apatite.The trace elements compositon of the samples reflect that the volcanic rocks are enriched light rare earth enrichment,large ion lithophile elements and fluid mobile elements,while are depleted high field strength elements,showing that the magma source of volcanic lavas in eastern Manusi basin are obviously influenced by the subduction components.The Li isotope datashows that the lithium isotopic composition of volcanic rocks in Manus basin is relatively uniform and the variationsare small?0.2-6.1‰?.With the evolution of magma crystallization and differentiation,the change of Li isotope ratio is not obvious.In addition,the variation of Li isotope compositions in East Manus Basinare shown to be affected by slab fluids,so Li isotope can be used as a good tracerindex of fluid and magmatic evolution in the subduction zone.The ¹⁴³Nd/¹⁴⁴Nd of the sample are between 0.512994 and 0.513027,which is similar to the MORB of Manus Spreading Center and New Britain arc lava.According to the relationship of Li and Nd isotopes,the volcanic rocks in eastern Manus basin mainly come from depleted mantle,altered oceanic crust and sefloor sediment.