The Jinlongdingzi volcanic eruption occurred 1500 years before present. The mant le lherzolite xenoliths contained in the volc anic products are the ones found only in the latest volcanic products in China. These mantle xenoliths are characterized by the pervasive occurrence of pargasite. The textures and dislocation of substructures of olivine of these xenoliths revealt hat they have been subjected to a certain degree of deformation under the upper mantle con dition. The equilibrium temperatures of these xenoliths are concentrated within the range of 800～950℃, and the equilibrium pressures within the range of 1.0～ 1. 5GPa. The upper mantle geotherm constructed on the basis of the equilibrium temp erature and pressure of these xenoliths is slightly lower than that from mantle xenolith s in the Neogene basalts, but is closer to that from Quaternary basalts. The differential stress o f these mantle xenoliths is estimated to be in the range of 30～45 MPa, while th e strain rate is from 10-19s-1～10-15s-1. The occurrence of pargasite reflects the participation of the upper mantle fluids and the metasomatism. It s eems that the basaltic magma from the Jinlongdingzi eruption directly came from th e top of upper mantle at depths of 35～50km.