A detailed microprobe study has been carried out on a suite of xenoliths in alkali basalts from the Tariat region in central Mongolia, comprising 16 garnet-spinel lherzolites, 16 garnet and garnet-spinel pyroxenites and over 70 spinel lherzolites. All but one of the garnet-bearing xenoliths are from the Shavaryn-Tsaram volcanic centre. Both lherzolites and pyroxenites show little variation in Mg/(Mg+Fe); the Mg number of orthopyroxene ranges from 0.89 to 0.91 in 30 samples out of 32. The combination of the Ca-in-opx thermometer of Brey and Kohler  and geobarometer of Nickel and Green  yields P-T estimates for these rocks that are consistent with garnet/spinel phase transition boundaries for both lherzolites and magnesian pyroxenites. The P-T data define a locus from about 20.5 kbar at 1100 degrees C to 12 kbar at 850 degrees C; the upper segment of this geotherm shows a good match with PT estimates for lower crustal garnet granulite and plagioclase-bearing pyroxenite xenoliths from the same occurrence. The geotherm has a higher T gradient at low P (12 kbar) than at high P, which may be due to perturbation of the conductive heat flow by magma underplating at the crust-mantle boundary (CMB). T estimates for spinel peridotite xenoliths from Shavaryn-Tsaram and four other volcanic centres inferred to be of similar age and located within 20 km of each other range from 880 degrees to 1060 degrees C. The lowest T values in each site are very similar (similar to 880 degrees C). The low-T peridotites are interpreted to represent the uppermost mantle just below the CMB. Reference of their temperature to the empirical geotherm for this locality indicates that the CMB is located at about 13 kbar (similar to 45 km). Clinopyroxene-rich garnet pyroxenites (T=860-880 degrees C, P similar to 12 kbar) and some feldspar-bearing rocks appear to be cumulates underplated at the CMB. The relatively high crustal thicknesses indicated by our results are generally consistent with scarce geophysical data for the region. Tariat spinel peridotites are typically fertile (low Mg numbers, rich in Na, Ca, Al). The combination of the fertile composition and elevated temperature may explain the lower P-wave velocities in the uppermost mantle as compared to the cold, depleted upper mantle beneath the Siberian platform to the north.