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 v...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.展开更多
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）.
文摘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.
