Lower crustal melting via magma underplating: Elemental and Sr–Nd–Pb isotopic constraints from late Mesozoic intermediate–felsic volcanic rocks in the northeastern North China Block

Ar–Ar dating, major and trace element analyses, and Sr–Nd–Pb isotope results of two groups of Lower Cretaceous (erupted at 126 and 119 Ma, respectively) intermediate–felsic lava from the northeastern North China Block (NCB) suggest their derivation from melting of mixtures between the heterogeneous lower crust and underplated basalts. Both groups exhibit high‐K calc‐alkaline to shoshonitic affinities, characterized by light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and variable high field strength element (HFSE, e.g. Nb, Ta and Ti) depletion, and moderately radiogenic Sr and unradiogenic Nd and Pb isotopic compositions. Compared with Group 2, Group 1 rocks have relatively higher K₂O and Al₂O₃/(CaO + K₂O + Na₂O) in molar ratio, higher HFSE concentrations and lower Nb/Ta ratios, and higher Sr–Nd–Pb isotope ratios. Group 1 rocks were derived from a mixture of an enriched mantle‐derived magma and a lower crust that has developed radiogenic Sr and unradiogenic Nd and Pb isotopic compositions, whereas the Group 2 magmas were melts of another mixture between the same mantle‐derived component and another type of lower crust having even lower Sr, Nd, and Pb isotopic ratios. Shift in source region from Group 1 to Group 2 coincided with a change in melting conditions: hydrous melting of both the underplated basalt and the lower crust produced the earlier high‐Nb and low‐Nb/Ta melts with little or no residual Ti‐rich phases; while the younger low‐Nb and high‐Nb/Ta magmas were melted under a water‐deficient system, in which Ti‐rich phases were retained in the source. Generation of the two groups of intermediate–felsic volcanic rocks was genetically linked with the contemporaneous magma underplating event as a result of lithospheric thinning in the eastern NCB.     

青藏高原北羌塘半岛湖新生代粗面玄武岩橄榄石电子探针和激光探针分析

利用电子探针和激光探针剥蚀系统（LA-ICP-MS），对北羌塘新第三纪粗面玄武岩中的橄榄石主元素和微量、稀土元素进行了系统分析。结果表明，本区橄榄石Fo平均为88，属贵橄榄石种属。相对富集Ni、Co和重稀土，而强烈亏损轻稀土及Rb、Sr、Ba、Zr等大离子亲石元素。其稀土元素配分型配分型式与粗面玄武岩全岩稀土配分型式呈互补状态。     

吉林东部延吉地区中生代火山岩地球化学特征

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2002年6月13日重庆区域大暴雨分析

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