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

Recent studies indicate that the Mesozoic litho- spheric thinning in North China was diachronous with that in west to the Taihangshan gravity lineament being later than in the eastern part of the North China Cra- ton1―3]. During the Cenozoic, lithospher…

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

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 ɛ _Nd ranging from −6.9 to −10.6 and ⁸⁷Sr/⁸⁶Sr from 0.7044 to 0.7047. By contrast, another LREE-enriched sample has a positive ɛ _Nd (+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.