北祁连俯冲-增生杂岩带中低温榴辉岩的地球化学特征

Geochemical characteristics of low-temperature eclogites from the subduction-accretionary complex in the North Qilian Mountain

北祁连俯冲-增生杂岩带中的低温榴辉岩以透镜体形式产于蓝片岩和多硅白云母片岩中。根据稀土元素、微量元素及Sr-Nd同位素分析研究，可将本区低温榴辉岩分为两类：Ⅰ类榴辉岩以轻重稀土分异不明显和具有Eu正异常为特征；Ⅱ类榴辉岩的轻稀土富集，有轻微的Eu负异常。Sm-Nd同位素研究显示，Ⅰ类榴辉岩样品的εNd（t）值为2．5～6．9，平均值为4．5；Ⅱ类榴辉岩ε（t）=～3．3～1．4。这些研究成果表明，Ⅰ类榴辉岩的原岩来源于长期亏损的地幔源区，可能形成于大洋环境；而Ⅱ类榴辉岩的原岩在形成过程中很明显混入了陆壳物质，据此推断其原岩形成于大陆边缘或洋陆过渡环境。

The low-temperature eclogites from the subduction-accretionary complex in the North Qilian Mountain occur as lenses within blueschists and phengite-schists. Based on REE and trace element analyses as well as Sr-Nd isotopic data, the authors divided the eclogites into two types. Type I eclogites are characterized by nearly flat REE normalized patterns and a distinctly positive Eu anomaly, suggesting cumulate protoliths, while Type Ⅱ eclogites have slightly Eu anomalies and an enriched LREE pattern. The values of εNd（ t ） from Type Ⅰ eclogite group vary between 2.5 and 6.9, indicating depleted mantle sources. The values of eNd（ t ） from Type Ⅱ eclogites vary between - 3.3 and 1.4. These data suggest that the protoliths of Type Ⅰ eclogites originated in an oceanic environment. Type Ⅱ eclogites show a possible mixing with a continental crust contaminant during the emplacement of the protoliths, suggesting the probable derivation from a continental/oceanic transitional zone or a continental margin.