Abstract: The Quaternary basalts in Datong, Shanxi Province contain small mantle xenoliths which are generally only 2～3cm in size. These mantle xenoliths comprise mainly spinel lherzolite. The textures of the xenoliths are variable from protogranular to porphyroclastic. The dislocation substructures of olivine from the xenoliths are dominated mainly by dislocation walls representative of high temperature dislocation creep. In addition, some dislocation substructures typical of lower temperature plastic deformation and high strain rate events such as dislocation tangles and slip band can also be observed. All these indicate that at least two deformation events had occurred within the upper mantle of this area. The equilibrium temperature and pressure of these xenoliths are within the range of 900～1050℃ and 1.1～1.7Gpa, respectively, corresponding to a range of depth of 40～60km. The differential stress estimated by using microstructural piezometer is in the range of 21～31MPa, showing a tendency of decreasing with depth, while the estimated strain rate and equivalent viscosity are within the range of 10¹⁵～10¹⁷s¹ and 10²¹～10²³ Pa·s, respectively. All these results have shown that an upper mantle diapir had continued to occur during the Quaternary in this area.

Key words: Upper mantle, Microstructures, Quaternary, Shanxi Province

摘要： 山西大同第四纪玄武岩中含有少量小型的幔源包体,主要为尖晶石二辉橄榄岩。包体的变形显微构造反映了上地幔条件下至少有两次变形事件;包体的平衡温度为900～1050℃,平衡压力为1.1～1.7GPa,对应的来源深度为40～60km。由包体的变形显微构造推导的上地幔流变学参数表明,该地区上地幔符合大陆拉张带上地幔的特征。在第四纪时期,该地区上地幔仍然继续其底辟上涌过程。

关键词: 上地幔, 显微构造, 第四纪, 山西