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 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.

Modification of the lithospheric mantle by melt derived from recycled continental crust evidenced by wehrlite xenoliths in Early Cretaceous high-Mg diorites from western Shandong, China

This paper reports petrographic, mineral chemical, olivine oxygen isotopic, and whole-rock geochemical data for wehrlite xenoliths from the Early Cretaceous Tietonggou high-Mg diorites in western Shandong Province, in the eastern part of the North China Craton (NCC), and describes the origin of these wehrlites and the processes that affected the deep lithospheric mantle in this area.Wehrlite xenoliths are rounded and vary in size between 3 cm × 4 cm 5 cm and 3 cm 2 cm 1 cm.Olivine within these xenoliths occurs as an isolated residual phase within clinopyroxene, has Fo contents between 89 and 91, and contains between 1414 and 3629 ppm Ni, similar to the values of olivine from peridotite xenoliths in the Cenozoic basalts of eastern China, but lower than the values of olivine from harzburgite xenoliths in the Early Cretaceous high-Mg diorites in western Shandong.In situ oxygen isotope analysis yielded 18 O values of olivine from (6.03±0.33)‰ to (6.82±0.35)‰, averaging (6.5±0.4)‰; this is higher than typical mantle-derived olivine ((5.2±0.3)‰).Compared with clinopyroxenes from peridotite xenoliths in the Late Cretaceous and Cenozoic basalts, clinopyroxenes in the wehrlites contain relatively low concentrations of Na 2 O, TiO 2 , and Al 2 O 3 , high concentrations of CaO, and higher Mg # (91.2-94.1) and Ti/Eu ratios (2082-2845), being similar in composition to clinopyroxenes within harzburgite xenoliths in the Early Cretaceous high-Mg diorites.Clinopyroxenes from wehrlite xenoliths are characterized by low total REE abundance, enrichment in light REEs, and depletion in high field strength elements such as Nb, Ta, Zr, and Hf.Moreover, the 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 187 Os/ 188 Os (125 Ma) ratios of these wehrlites vary from 0.70596 to 0.70737, 0.512181 to 0.512416, and 0.12661 to 0.57650, respectively.These data suggest that these wehrlite xenoliths were formed by modification of the lithospheric mantle by melts derived from recycled continental crust.