Nature Of Mantle Lithosphere Beneath The Western North China Craton:Constraints From The Mantle Xenoliths In Cenozoic Basalts From Yangyuan, Hebei Province

Mantle xenolths in Cenozoic basalts in Yangyuan, Hebei province, which is located to the west of the Daxing'anling-Taihangshan gravity lineament (DTGL), have been studied with aims of characterising the nature of the lithospheric mantle in this region. The Yangyuan perdotites are mostly spinel lherzoites and subordinately spinel harzburgites. Most of them show a protogranular texture and only a small number of samples display a porphyroclastic texture. Geothermometric calculation shows that the Yangyuan peridotites are equilibrated at 880-1067 ℃. The negative correlation between CaO, Al2O3, heavey earth elements and MgO, and positive correlation between NiO and MgO are consistent with the conclusion that the Yangyuan peridotites represent residues of mantle melting. Trace element compositions further suggest that they experienced multiple stages of mantle metasomatism.The Yangyuan pyroxenite xenoliths all are websterites in which clinopyroxene is more abundant than orthopyroxene. Two types of pyroxenite xenoliths are noted, namely green and black pyroxenites. Green pyroxenites have higher Mg#, lower Al2O3 and TiO2 contents than the black suite. The black pyroxenites are enriched in REE but depleted in high field strength elements. Texture, major and trace element compositions suggest that the Yangyuan pyroxenites represent magmatic cumulates at high pressure. Their EMI-type Sr-Nd isotopic composition is different from that of the host basalts, ruling out the genetic relationship between them. It instead may have resulted from ancient magmatic events.Os isotopic data available in literature suggest that the lithospheric mantle beneath both sides of the gravity lineament have different formation ages; the mantle beneath western North China is Proterozoic in age, weheras that beneath eastern North China mostly has modern age. However, the peridotite xenoliths collecetd from these two regions do not show fundmental difference in major element compositions. This suggests that Proterozoic and Phanerozoic mantle have similar composition, rather than the contrasting composition as previously expected. It is important to note that mantle-derived xenoliths from either sides of the DTGL show contrasting Sr-Nd isotopic compositions, implyingthat the DTGL separats two mantle domains. This is consistent with the notion that the lithospheric mantle under western North China is older than that beneath eastern North China.