Geochemistry Characteristics,magma Source Components And Geodynamic Significance Of Late Cenozoic Basalts In Northern Hainan Island

Hainan Island is located on the southeastern margin of the Eurasian continent.Since the late Cenozoic period,violent volcanic eruptions and massive basaltic effusive events,as well as frequent earthquakes have occurred in this area.The late Cenozoic volcanic activity is mainly distributed in Northern part of Hainan Island.Volcanism in the area is considered to be related to the subduction of the Indo-Austrilian plate and the Pacific plate,and may also be affected by the extension of the South China Sea basin.Existing studies proposed the upwelling of the mantle plume played a crucial role in the genesis of Hainan basalt.The mantle source for Northern Hainan Island Basalt consists of different components.Previous studies believed that in addition to the depleted mantle（DMM）and EM2,the source may also include EM1 and FOZO components.Although numerous researchers have different views,this issue is still on debate.Moreover,the origin for EM mantle reservoir beneath Hainan Island and the melting mechanism of recycled marterials（i.e.subducted sediment）are still unclear.Based on the above situation,this study analysis the major,trace elements,Sr-Nd-Pb-Hf isotopes,and combines stable Mg isotopes and existing geophysical evidence to determine the origin for mantle source beneath Hainan Island.Addtionally,we explain the origin for EM mantle reservoir and the melting of recycled sediments.Finally we obtained the geodynamic process of Hainan Island basalts formation.Hainan basalt can be divided into two types in the field investigation,one is brown scoria-like basalts,and the other is dark massive lava.In the microscopic observation,Hainan basalts have porphyritic structure.Phenocrysts include olivine and a small amount of pyroxene,plagioclase.The matrix is characteristic by intergranular structure and aphanitic structure,and the matrix components are mainly composed of plagioclase,olivine,pyroxene,magnetite and aphanitic minerals.The vesicular content of scoria-like basalts is higher than that of massive lava.The SiO₂ content of Hainan basalts have a range from 44.81%wt%to 52.06wt%,and the ALK（K₂O+Na₂O）content is 4.63-5.05 wt%.Most samples in the TAS diagram are sub-alkaline basalt,and a few samples are alkaline basalt.According to CIPW calculation results,most Hainan basalts are olivine tholeiitic basalt and quartz tholeiitic basalt,and only few samples are alkaline olivine basalt.In the chondrite-normalized REE pattern,the LREE/HREE ratio of Hainan basalts is 6.03-11.35,and the La _N/Yb _Nratio is 8.76-20.14,exhibit the enrichment of LREEs and the depletion of HREEs.In Primitive-normalized trace element pattern.Hainan basalts show the enrichment of LILEs and depletion of HFSEs.The Hainan basalts exhibit depleted Sr-Nd isotopic characteristics and Dupal-Pb isotopic anomaly,which is similar to the basalts from South China Sea basin and Southeast Asia,indicating that Hainan basalts would be related to the subduction of the Indo-Austrilian plate.Hainan basalts have similar Sr-Nd-Pb-Hf isotopic compositions with Somoan mantle plume basalts,indicating that the Hainan basalts can be explained by the mixing between DMM（depleted mantle）and EM2 component.In addition,theδ²⁶Mg values of Hainan basalts are ranging from-0.40‰±0.06‰to-0.28‰±0.05‰,exhibits lowerδ²⁶Mg values than normal mantle（-0.25‰±0.04‰）.Nb concentration of exhbits positive correlation with easy-mobile elements（i.e.Ba,Th,U,La,Sr,Pb and Nd）,indicating the post-magmatic alteration on Hainan basalts is negligible.The Nb/La ratios of Hainan basalts are higher than that of continental crust（0.40）,and the Th/Ta ratios are lower than that of continental crust（8）.Moreover,the Nb/U values are higher than that of the continental crust,and the Ce/Pb values are also higher than that of the continental crust（6.15）.There are no correlation between SiO₂and ⁸⁷Sr/⁸⁶Sr,MgO and ⁸⁷Sr/⁸⁶Sr,suggests Hainan basalts cannot be subject to conspicuous crustal contamination.Hainan basalts underwent the fractional crystallization of olivine（MgO>7.5%）and obsolete fractional crystallization of clinopyroxene,plagioclase and Fe-Ti oxides（MgO<7.5%）.SiO₂ and MgO do not show positive correlation withδ²⁶Mg,suggests the Mg isotopic composition is not affected by fractional crystallization of olivine,and LOI does not show a positive correlation withδ²⁶Mg,manifests post-magmatic alteration does not affect the Mg isotopic composition of basalt.In addition,Nb/Sm and Nb/Zr ratios do not show negative correlations withδ²⁶Mg suggests that partial melting did not affect the Mg isotopic composition of basalts,while Ba/Th and Nb/Th ratios showed negative correlations withδ²⁶Mg values,indicating that the source heterogeneity could have an effect onδ²⁶Mg valuesThere is no negative correlation between TiO₂ and Nb/Ta,showing the lowδ²⁶Mg values of Hainan basalts are not caused by the accumulation of ilmenite in their mantle source.Another alternative explanation suggests that the lowδ²⁶Mg value could originate from subducted carbonate-bearing sediments,which is evidenced by the similar Ca/Al ratios to those basalts from South China.The Sr-Nd isotopic composition of Hainan basalts is lower than that of the mantle xenolith from South China Block,and the Pb isotopic composition is also significantly higher than that of mantle xenolith,indicating the lithospheric mantle cannot be the source for Hainan basalts.In theε_Nd-Ce/Pb diagram and ⁸⁷Sr/⁸⁶Sr-Nd/Pb diagram,Hainan basalts have an evolution trend towards subducted sediments,suggesting recycled sediments is existing in the source.In ²⁰⁶Pb/²⁰⁴Pb-⁸⁷Sr/⁸⁶Sr diagram,²⁰⁶Pb/²⁰⁴Pb-¹⁴³Nd/¹⁴⁴Nd diagram,²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb diagram and ²⁰⁸Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb diagram,the Hainan basalts are plotted on the mixing curve between DMM and recycled sediments,it shows the Hainan basalts can be explained by the binary mixing between depleted mantle and subducted sediments.Additionally,the Sr-Nd-Mg isotopic compositions of Northern Hainan Island basalts can be calculated by three end-members（including MORB,silicious sediments and carbonate）mixing model.The K/U and Ba/Th ratios of extraction melts from carbonate-bearing sediments at different pressures were calculated.The results show that low-pressure carbonated silicate melts（6-8GPa）have higher K/U and Ba/Th ratios than primitive mantle,while high-pressure carbonate silicate melts（18–20GPa）have lower K/U and Ba/Th ratios than primitive mantle.In trace element pattern,few alkaline basalts share similar trace-element patterns with nephelinites from eastern China,showing negative spike in K,Rb,Pb and Ti,while most tholeiitic basalts do not show similar trace element characteristics to nephelinites from eastern China.On the contrary,these tholeiitic basalts show positive spike of K element,suggesting the carbonated silicate melts may originate from upper mantle rather than mantle transition zone.The Hainan alkaline basalt has lower K/U and Ba/Th ratios than the primitive mantle,while tholeiitic basalts have higher K/U and Ba/Th ratios than primitive mantle,and display highly correlated compositions with low-pressure carbonated silicate melts.The K/U-Ba/Th diagram suggests that few Hainan alkaline basalts can be produced by the binary mixing between high-pressure carbonated silicate melt and primitive mantle peridotite,while Hainan tholeiitic basalt can be generated by the binary mixing between low-pressure carbonated silicate melt and primitive mantle peridotite.The Hainan basalts have higher⁸⁷Sr/⁸⁶Sr andδ²⁶Mg values than the initial carbonated silicate melt,and lower ⁸⁷Sr/⁸⁶Sr andδ²⁶Mg values than reacted melt（generated by the reaction between initial carbonated silicate melt and primitive mantle peridotite）.⁸⁷Sr/⁸⁶Sr-δ²⁶Mg diagram shows that Hainan basalts also can be generated by the mixture between initial carbonated silicate melt and reacted melt.Geophysical evidence indicates that Hainan basalts is closely related to the subduction of the Indo-Australian plate and Pacific plate.Subducted slabs can either be stagnant in the mantle transition zone or entering the lower mantle.Existing studies suggest that subducted sediments with lowδ²⁶Mg signature can enter the lower mantle through rock-rock reactions.With the subduction of the Indo-Austrilian plate and the Pacific plate,the subducted slab will enter the lower mantle.Subsequently,the mantle plume originating from the core-mantle boundary will carry the subducted slab fragment to the upper mantle.When the melting pressure are between 18GPa and20GPa,the residual lithofacies of the subducted sediments is carbonate garnetite.In this case,extraction melts from recycled sediments will react with the surrounding mantle peridotite to form less alkaline basaltic melts（low K/U and Ba/Th ratios）and residual pyroxenite（peridotite）.When the melting pressure is less than 9GPa,the residual lithofacies of the subducted sediments is carbonated eclogite.In this case,extraction melts from recycled sediments will react with the surrounding mantle peridotite to form most tholeiitic basaltic melts（high K/U and Ba/Th ratios）and residual pyroxenite（peridotite）.An alternatively way to explain the origin for Hainan basalts is that initial carbonated silicate melt mixed with the existing reacted melt,then continually serve as the magma source of Hainan basalts.