He and Ar isotope geochemistry of pyroxene megacrysts and mantle xenoliths in Cenozoic basalt from the Changle–Linqu area in western Shandong
He and Ar isotope geochemistry of pyroxene megacrysts and mantle xenoliths in Cenozoic basalt from the Changle–Linqu area in western Shandong
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DOI：
10.1007/s11434-013-0027-2
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发表时间：
2014-02
影响因子：
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通讯作者：
F. Su;Yan Xiao;Huaiyu He;B. Su;Ying Wang;R. Zhu
中科院分区：
文献类型：
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作者：
F. Su;Yan Xiao;Huaiyu He;B. Su;Ying Wang;R. Zhu
对鲁西昌乐-临朐地区新生代玄武岩中的辉石巨晶和地幔包体进行了He-Ar同位素和元素地球化学的综合研究。利用这些结果追踪了华北板块破裂后陆下岩石圈地幔的演化。辉石巨晶、镁橄榄石和纯橄榄岩的3He/4He比值分别为7.0 - 7.7Ra、7.4Ra和7.0 - 7.7Ra。它们与寄主玄武岩的成因有关，并略低于大洋中脊玄武岩（MORB）。它们的40 Ar/36 Ar比远低于MORB，接近空气。二辉橄榄岩和伟尔岩代表新增生的岩石圈地幔碎片，其稀土元素和微量元素表明它们经历了熔融交代和部分熔融作用。橄榄石的3He/4He值略低于MORB，单斜辉石的3He/4He值较低（2.3-7.1Ra），与（La/Yb）N呈负相关。这些单斜辉石的~（40）Ar/~（36）Ar比值远低于MORB，接近空气比值。结合岩石学、Sr-Nd-Pb同位素、O同位素、微量元素和Mg同位素等研究成果，我们推断长乐-临朐地区幼年岩石圈地幔是被洋壳源熔体交代的，洋壳源熔体将表壳Ar同位素特征传递到地幔源区。二辉橄榄岩和伟尔岩中单斜辉石的低4He丰度和低3He/4He比值归因于俯冲太平洋板块的地壳熔体的交代作用。
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.