He and Ar isotope geochemistry of pyroxene megacrysts and mantle xenoliths in Cenozoic basalt from the Changle–Linqu area in western Shandong  

作　　者：Fei Su Yan Xiao Huaiyu He Benxun Su Ying Wang Rixiang Zhu 

机构地区：[1]Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences [2]University of Chinese Academy of Sciences [3]State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences

出　　处：《Chinese Science Bulletin》2014年第4期396-411,共16页

基　　金：supported by the National Basic Research Program of China(2009CB825001);the National Natural Science Foundation of China(90714010)

摘　　要：We performed a combined study of He–Ar isotopes and element geochemistry for pyroxene megacrysts and mantle xenoliths from Cenozoic basalt in the Changle–Linqu area in western Shandong.The results are used to trace the evolution of subcontinental lithospheric mantle after destruction of the North China Craton.The3He/4He ratios of pyroxene megacrysts,websterite,and dunites are 7.0–7.7 Ra,7.4 Ra,and 7.0–7.7 Ra,respectively.They are related to the origin of host basalts,and are slightly lower than that of mid-ocean ridge basalts(MORB).Their40Ar/36Ar ratios are much lower than that of MORB and close to that of air.The lherzolites and wehrlites represent the fragments of the newly accreted lithospheric mantle,and their REE and trace elements indicate that they experienced melt metasomatism and partial melting.The3He/4He ratios of their olivine are slightly lower than that of MORB,but3He/4He ratios of their clinopyroxene are low(2.3–7.1 Ra)and display an inverse correlation with(La/Yb)N.The40Ar/36Ar ratios of these clinopyroxene are much lower than that of MORB and close to the air ratio.Combining existing studies of petrology,Sr–Nd–Pb isotopes,O isotopes,trace elements,and Mg isotopes,we infer that the juvenile lithospheric mantle in the Changle–Linqu area was metasomatized by oceanic crust-derived melts,which transfer the supracrustal Ar isotope signatures to the mantle sources.The low4He abundance and low3He/4He ratios of clinopyroxene in the lherzolites and wehrlites are ascribed to metasomatism by crustal melts from the subducted Pacific plate.We performed a combined study of He-Ar isotopes and element geochemistry for pyroxene mega- crysts and mantle xenoliths from Cenozoic basalt in the Changle-Linqu area in western Shandong. The results are used to trace the evolution of subcontinental lithospheric mantle after destruction of the North China Craton. The 3He/4He ratios of pyroxene megacrysts, websterite, and dunites are 7.0-7.7 Ra, 7.4 Ra, and 7.0-7.7 Ra, respec- tively. They are related to the origin of host basalts, and are slightly lower than that of mid-ocean ridge basalts （MORB）. Their 40Ar/36Ar ratios are much lower than that of MORB and close to that of air. The lherzolites and wehrlites represent the fragments of the newly accreted lithospheric mantle, and their REE and trace elements indicate that they experienced melt metasomatism and partial melting. The 3He/4He ratios of their olivine are slightly lower than that of MORB, but 3He/4He ratios of their clinopyroxene are low （2.3-7.1 Ra） and display an inverse correlation with （La/Yb）y. The 40Ar/36Ar ratios of these clinopyroxene are much lower than that of MORB and close to the air ratio. Combining existing studies of petrology, Sr-Nd-Pb isotopes, O isotopes, trace elements,and Mg isotopes, we infer that the juvenile lithospheric mantle in the Changle-Linqu area was metasomatized by oceanic crust-derived melts, which transfer the supracrustal Ar isotope signatures to the mantle sources. The low 4He abundance and low 3He/4He ratios of clinopyroxene in the lherzolites and wehrlites are ascribed to metasomatism by crustal melts from the subducted Pacific plate.

关 键 词：同位素地球化学 洋中脊玄武岩 辉石巨晶 地幔捕虏体 氩同位素 新生代 西部 山东 

分 类 号：P542.5[天文地球—构造地质学] P597[天文地球—地质学]