Old EMI-type enriched mantle under the middle North China Craton as indicated by Sr and Nd isotopes of mantle xenoliths from Yangyuan, Hebei Province  

作　　者：MA Jinlong XU Yigang 

机构地区：[1]Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China [2]Graduate School of Chinese Academy of Sciences, Beijing 100039, China

出　　处：《Chinese Science Bulletin》2006年第11期1343-1349,共7页

基　　金：This work was supported by the Baren Project of Chinese Academy of Sciences and the National Natural Science Foundation of China （Grant No： 49925308 ＆ 40421303）.

摘　　要：Mantle xenoliths hosted in Tertiary alkali basalts in Yangyuan, Hebei Province, located to the west of the Taihangshan gravity lineament, include lherzolites, harzburgites and pyroxenite. In the plot of olivine mode vs Fo, most of the Yangyuan peridotites deviate from the oceanic trend, falling within the fields of Archean and Proterozoic mantles. Some LREE- enriched samples exhibit EMI-type isotopic signature with εNd ranging from –6.9 to –10.6 and 87Sr/86Sr from 0.7044 to 0.7047. By contrast, another LREE-en- riched sample has a positive εNd (+5.7) similar to that for LREE-depleted peridotites. This observation suggests that the upper mantle beneath Yangyuan underwent a multi-stage metasomatism. Given the fact that EMI-type isotopic signature is usually ob- served in the lithosphere mantle underneath ancient cratons, the isotopic composition of the Yangyuan xenoliths provides new evidence for the existence of the old lithosphere mantle beneath western North China.Mantle xenoliths hosted in Tertiary alkali basalts in Yangyuan, Hebei Province, located to the west of the Taihangshan gravity lineament, include Iherzolites, harzburgites and pyroxenite. In the plot of olivine mode vs Fo, most of the Yangyuan peridotites deviate from the oceanic trend, falling within the fields of Archean and Proterozoic mantles. Some LREE- enriched samples exhibit EMI-type isotopic signature with εNd ranging from -6.9 to -10.6 and ^87Sr/^86Sr from 0.7044 to 0.7047. By contrast, another LREE-en- riched sample has a positive εNd （＋5.7） similar to that for LREE-depleted peridotites. This observation suggests that the upper mantle beneath Yangyuan underwent a multi-stage metasomatism. Given the fact that EMI-type isotopic signature is usually observed in the lithosphere mantle underneath ancient cratons, the isotopic composition of the Yangyuan xenoliths provides new evidence for the existence of the old lithosphere mantle beneath western North China.

关 键 词：地幔捕虏岩 锶-钕同位素 EMI 河北 克拉通地台 碱性玄武岩 

分 类 号：P58[天文地球—岩石学]