中国东南沿海中-新生代玄武岩微量元素和Nd-Sr-Pb同位素研究

Trace element and Nd-Sr-Pb isotope studies of Mesozoic and Cenozoic basalts in coastal area of SE China.

本文对中国东南沿海不含幔源包体的中生代玄武岩和含幔源包体的新生代玄武岩进行了微量元素和Nd-Sr-Pb同位素对比研究。中生代玄武岩呈Ta、Nb和Hf负异常,低Ce/Pb、Nb/U比值和高La/Nb比值,与岛弧火山岩和陆壳岩石的微量元素特征相类似,说明在岩浆生成和上升过程中,幔源组分受到了陆壳组分的混染。新生代玄武岩呈Ta、Nb正异常和Pb负异常,高Ce/Pb、Nb/U比值和低La/Nb比值,与海岛玄武岩(OIB)相类似,Nd-Sr同位素成分与夏威夷玄武岩类似,因而它们未受明显的陆壳混染。143Nd/144Nd与206Pb/204Pb之间的负相关关系和87Sr/86Sr与206Pb/204Pb之间的正相关关系说明本区新生代玄武岩起源于中等亏损程度的软流圈地幔,并与EMII富集地幔组分发生了混合。

Trace element and Nd-Sr-Pb isotopic compositions of mantle xenolith-free Mesozoic basalts and mantle xenolith-bearing Cenozoic basalts in the coastal area of SE China are studied to provide an insight into the nature of their mantle sources and processes. The Mesozoic basalts have negative Ta, Nb and Hf anomalies, low Ce/Pb, Nb/U and high La/Nb ratios, similar to those for island arc volcanics and continental crustal rocks, suggesting a mixing of chemical components of mantle and the continental crust during the generation and ascending of the magma. In contrast, the Cenozoic basalts have positive Ta, Nb anomalies and negative Pb anomalies, high Ce/Pb, Nb/U and low La/Nb ratios, similar to those for oceanic island basalts (OIB); in addition, their Nd-Sr isotopic compositions are similar to those for Hawaiian basalts, so they haven' t been significantly contaminated by crustal components. Furthermore, the negative relationship between 143Nd/144Nd and 206Pb/204Pb, and the positive relationship between 87Sr/86Sr and 206Pb/204Pb suggest that the Cenozoic basalts in the studied area were generated from the intermediate-depleted asthenospheric mantle and EMII component.