Magnesium And Zinc Isotope Study Of Shandong Lamprophyres And Its Geological Significance

The deep carbon cycle has become the international research hotspot in recent years,and stable metal isotopes have been widely used to research this project.Previous studies conducted detailed Mg-Zn isotope research on the Late Cretaceous and Cenozoic?<110Ma?basalts in eastern China and found that there was a large-scale of light Mg and heavy Zn isotopic anomalies area in the upper mantle of eastern China.The carbonates carried by the Pacific plate subduction,entering the mantle transition zone and mixing with the convective upper mantle might be the cause of these anomalies.Previous studies have shown that the Pacific plate began to subduct towards theNorth China Craton during the Mesozoic,and affected the composition of the mantle.However,the Cenozoic basalt can only represent the characteristic of the asthenosphere mantle.Thus,the properties of the lithosphere mantle before the Late Cretaceous is unclear.Generally,lamprophyre is one of the main rock types formed by partial melting of the lithosphere mantle.Therefore,studying the late Cretaceous lamprophyre is expected to prove the time when carbonates began to affect the mantle,the depth of its stayed in the mantle,and the effects on the mantle.Then we can complement the deep carbon cycle research in eastern China.In this thesis,the bulk element,microelement and Mg-Zn-Sr-Nd isotopes of Jiaodong Mesozoic lamprophyre?about 125 Ma?from Pingdu,Linglong,Haiyang,Qixia and Muping were studied in detail,and we found that:1)The Sr isotopes of these lamprophyre are from 0.7079 to 0.7101,and the value of?Nd?t?is between-12.7 and-19.3.The Sr-Nd isotopic characteristics are between EM?and EM??.2)The lamprophyre has low?²⁶Mg composition?the lowest value is-0.59‰?,whichhint that the source area of these lamprophyre is affected by a certain degree of recycled carbonate.3)The Zn isotope composition of the Mesozoic lamprophyre in Jiaodong?0.15‰?0.31‰?are not exactly the same as those of the Cenozoic basalt,and have similar Zn isotope values to the mid-ocean ridge basalt.In summary,combined with the composition of Mg-Zn isotopes,this paper considersthat during the Mesozoic,the recycled carbonate carried by the subducting plate mainly affected the composition of the lithosphere mantle in the form of carbonated mudstone.Therefore,the subducted carbon can be stored in the lithosphere mantle and re-released into the atmosphere following the destruction of the North China Craton.