Geochemical Characteristics Of Cenozoic Basalts From The South China Sea And Implications For Magma Evolution

The South China Sea is the largest marginal sea in China.It has almost undergone a complete geological cycle.Understanding its formation and evolution is of great significance for the improvement of the formation theory of marginal ocean basin.The South China Sea is located in the interaction zone of Eurasian plate,Pacific plate and Indo-Australian plate.Meanwhile,Hainan mantle plume was developed in the northern margin of the South China Sea.Therefore,its formation and evolution was affected by plate subduction,mantle plume and other factors,resulting in the complexity of its mantle source.Because the oceanic crust of the South China Sea is covered by thick sediments,it is difficult to obtain oceanic crust rock samples,which limits the understanding of the characteristics of mantle source in the South China Sea from the perspective of petro-geochemistry.In this study,the oceanic crust basalts obtained during International Ocean Discovery Program(IODP)Expedition 349 in the South China Sea are used to deeply understand the characteristics of the mantle source,and further reveal the influences of plate subduction on the formation and evolution of the South China Sea,through the analysis of whole-rock major and trace elements,Sr-Nd isotopes and emerging Mg-Fe-Zn metal stable isotopes and chemical composition of olivine minerals.The oceanic crust basalts studied here were formed during the late period of seafloor expansion,and belong to tholeiite.Among them,the basalts in the East sub-basin(U1431)are mainly N-MORB,and E-MORB appears in some layers;the basalts in the Southwest sub-basin(U1433 and U1434)are mainly E-MORB.The characteristics of major and trace elements show that these basalts have undergone different degrees of crystallization,e.g.,olivine,pyroxene,plagioclase and other minerals.Trace element spider diagrams show that they are enriched in fluid-active elements,such as Rb,Ba,U,etc.,and crustal elements,such as Pb;meanwhile,they have different Ce/Pb(7.56?31.18)and Nb/U(21.53?55.45)ratios from typical mantle;Sr-Nd isotope results show that they have a slightly high 87Sr/86Sr(0.703113?0.704190),but 143Nd/144Nd(0.512963?0.503091)similar to N-MORB;Comparing with olivine chemistry in global MORB,the chemical compositions of olivine in studied basalts are with high Ni contents and low CaO contents,indicate that there are pyroxenitic components in peridotitic mantle source of the South China SeaThe Mg isotopic compositions show that 826Mg of the South China Sea basalts are from-0.24‰ to 0.00‰,which are higher than that of typical MORB(826Mg?-0.25‰)Due to the lack of correlation between ?26Mg and indicators of alteration and magmatic processes,the heavy Mg isotopes of the studied basalts are inherited from their magma source,which are most likely from subducted talc-and/or serpentine-derived fluids with high 826Mg.Through the comparative analysis of the light Mg isotopes characteristics of basalts in the surroundings of the South China Sea,we proposed that the two types basalts with different Mg isotopic compositions are all the products of plate subduction,and the difference in Mg isotopic compositions between them was controlled by minerals with different Mg contents and Mg isotopes during subduction.Specifically,the shallow subduction was mainly affected by heavy Mg isotopic fluids released from talc and/or serpentine,while the deep mantle was mainly affected by light Mg isotopes fluids/melts released from dolomiteThe Fe isotopic compositions(?56Fe)of the South China Sea basalts are from 0.08‰to 0.16‰.After evaluating the effects of alteration and fractional crystallization,we got the Fe isotopic compositions of primitive melts(?56Fe=.02‰?0.10‰),which are similar to that of typical MORB(?56Fe=0.067‰).The simulation calculation of mixed mantle shows that about 10?40%pyroxenitic melts was involved into the peridotite mantle source in the South China Sea,which theoretically should produce basalts with heavier Fe isotopes than typical N-MORB.However,due to the effect of fluid-metasomatism released from serpentinite with low ?56Fe,the Fe isotopic compositions of the magma source were further reduced,and similar to that of typical MORB Pyroxenitic components was involved into the formation of the South China Sea basalts by ridge-plume interaction during the expansion of the South China Sea.Combined with the high Ni contents of olivine,the pyroxenitic components in the peridotitic mantle was the secondary pyroxenite formed by reaction of mantle peridotite and the eclogite or pyroxenite melts originated from subducted oceanic crustIn this study,the influence of plate subduction on the mantle source of basalt in the South China Sea was further illustrated by Zn isotopes.The results show that Zn isotopic compositions(?66Zn)of the South China Sea basalts are from 0.22‰ to 0.31‰,which are similar to that of typical MORB(?66Zn=0.27‰),but higher than that of average upper mantle(?66Zn=0.18‰).Since there is no correaltion between ?66Zn and indicators of alteration,fractional crystallization and subducted fluid-metasomatism,it indicates that alteration,fractional crystallization and plate subduction have a limted effect on the variation of Fe isotopes.The MORB-like Zn isotopes of the South China Sea basalts mainly reflect the mantle melting process.By collecting and sorting out the data of volcanic rocks with different ?66Zn formed in the background of subduction,this study proposed that the behavior of Zn isotopes during subduction was mainly affected by dolomite and other Zn-rich mineralsThis study provides direct evidence that the mantle source in the South China Sea was affected by plate subduction from the perspective of petro-geochemistry,quantitatively simulates the proportion of heterogeneous materials in the basaltic magma source of the South China Sea,and limits the main factors that affect the behavior of zinc isotopes in plate subduction.