The discovery of peridotite xenoliths and megacrysts in Jining, Inner Mongolia

Located in Inner Mongolia, the northern edge of the North China craton, Jining Cenozoic basalts belong to the generalized “Hannuoba Basalts”. A lot of peridotite xenoliths as well as some pyroxene, feldspar and titanomagnetite megacrysts were discovered for the first time in the alkali basalt southeast of Sanyitang. Most of these xenoliths are spinel lherzolites with the assemblage of olivine + orthopyroxene + clinopyroxene + spinel. The pyroxene megacrysts are mostly diopside and augite in composition. Feldspar megacrysts are mainly anorthoclase and minor plagioclase. The equilibrium temperature represented by these spinel lherzolite xenoliths is about 950℃ at 1.5 GPa, which is almost the same as that of the Hannuoba xenoliths and reflects the temperature of spinel phase in the lithospheric mantle beneath the northern edge of the North China craton. The crystallization temperature and pressure of clinopyroxene megacrysts are higher than those of lherzolite xenoliths, implying an origin deeper than that of the peridotite xenoliths. Petrography and mineral chemistry of these xenoliths indicate that they are fragments of the upper mantle captured by basaltic magmas along the ascending way. The high Mg# (89.5～91.7) and high NiO (0.29%～0.55%) contents of olivines demonstrate that Jining peridotite xenoliths are derived from the infusible lithospheric mantle.