ABSTRACT

This paper describes petrology and chemistry of kimberlites from four localities in North China. The distribution pattern of kimberlite intrusions is controlled not only by large-scale but also by local-scale structures. The U-Pb ages of the kimberlites in Region A range from 77 to 88m.y. and the kimberlites in Region C are emplaced between the Cretaceous and the Eocene. Based on texture and mineralogy, the kimberlites are classified mostly as basaltic kimberlite, which is composed of olivine, phlogopite, pyrope and spinel in a groundmass of serpentine and calcite. Their chemical composition and Si/Mg ratio show that most of selected samples except for Regions C and D are relatively uncontaminated kimberlite. The kimberlites contain lower Al₂O₃ and Na₂O + K₂O than those from other areas of the world. The unaltered relic olivines contain 92% forsterite component. The phlogopites have a Mg/(Mg + Fe) ratio ranging from 84 to 93%. The garnets are enriched in Mg, and their most abundant component is pyrope (62–70%). Garnets contain certain amounts of Cr₂O₃. The highest Cr₂O₃ content of garnet is ca. 8wt.%. The chromites have varied values in Cr/(Cr + Al) and high Ti contents. The available evidence both petrological and geochemical reveals that the kimberlitic magma was derived by partial melting of upper mantle garnet lherzolite under volatile-rich conditions, particularly rich in H₂O and CO₂.