The Petrogenesis And Geodynamic Mechanism Of Late Cenozoic Basalts In North Hainan Island

We carried out a detailed petrographic,major elements and trace element geochemistry,Sr-Nd-Pb-Hf isotopes,Pb and Sr isotopes geochemistry of melt inclusions in olivine and plagioclase Sr isotope geochemical analysis of the late Cenozoic basalts in northern Hainan Island,and combining the Leizhou peninsula,South China Sea basin,Indo-Chinese Peninsula Cenozoic basalts data from other researchers,in order to investigate the influence of the crustal contamination,the source lithology,and the nature of the mantle source.Hainan Island basalts with positive Nb and Ta anomaly,high Nb/U ratios,and Th/La and Ta/Nb ratios between primitive mantle and OIB.Moreover,the relationships between ⁸⁷Sr/⁸⁶Sr,?Nd and SiO₂,?Nd and MgO/SiO₂ suggest that effect of continental crust contamination on Hainan Island basalts was weak.By restoring MgO>7.5wt%original of basaltic magma composition,we found original basalt magma from Hainan Island-Leizhou Peninsula has high Zn/Fe,Zn/Mn and Fe/Mn ratios,and low content of CaO,TiO₂ with high FeO*and low Na₂O/TiO₂,CaO/Al₂O₃ratio,with obvious difference in peridotite melt,which is consistent with the characteristics of pyroxenite melt,thus indicating the parent lithology in basaltic source should be pyroxenite.Recycling of oceanic crust and sediments into mantle plays an important role in the chemical evolution of Earth's crust-mantle system.Previous studies suggest potential dynamic linkages between the plate subduction and Hainan plume generation,so recycled materials have great effects on mantle source of Hainan basalts.However,whether there exist recycled sediments in source is still unknown.In this paper,we will report,for the first time,the Hf isotope data of the Cenozoic basalts in Hainan Island,and combine the new Sr,Nd,and Pb isotope data together with the previous data to constrain on the composition and age of the recycled materials.Compared with Mantle Array,?_Hf values of Hainan basalts are lower relative to given?_Nd.Moreover,the basalts have the characteristics of Eu/Eu*>1 and Sr/Sr*>1,but neither of them has correlation with Al₂O₃ and MgO,indicating that the positive anomalies of Eu and Sr are the characteristics of the source.These serval lines of evidence suggest that recycled oceanic crust plays a significant role in mantle source.The Ce/Pb ratios of Hainan basalts are positively correlated with?_Nd and?_Hf,and Nd/Pb ratios are negatively correlated with ⁸⁷Sr/⁸⁶Sr,pointing to the subducted oceanic sediments end member.In addition,the Ce/Ce*ratios with 0.92-0.98,indicate that there are sediments formed under marine oxidation in the source area.According to the evolution model of the?_Hf-?_Nd and Pb isotope,we conclude that the recycled oceanic crust and sediments in the source region are relatively young,and its age should be less than 1.0 Ga.The high-density eclogite derived from a mixture of recycled oceanic crust and sediments drives the material to sink into lower mantle and even as deep on the core–mantle boundary,reacting with the ambient peridotite to produce the solid pyroxenite in the rising plume.With Hainan plume rising,pyroxenite bodies melt continually to generate magma for Hainan basalts.By analyzing the Pb and Sr isotopes of the melt inclusions in olivine,the results show that the isotopic composition of the Hainan Plume mantle source is heterogenous.Melt inclusions data show that EM2,DMM and FOZO may be important mantle endmembers for Hainan plume source,and EM1,HIMU components are very limited.In the process of magma mixing,these limited EM1 and HIMU components were diluted by other major components.The EM2-like components are mixed with different parts of the mantle to form a relatively dispersed Pb-Sr isotopic signature shown by melt inclusions data.