A detailed microprobe study has been carried out on several xenolith suites along the axial part of the Baikal rift system, including Oka (Sayan), Tunka, Hamar-Daban, and Udokan volcanic fields. Rare garnet-bearing xenoliths were firstly detected in Sayan and Udokan areas. Mantle xenoliths from considered volcanic fields generally represent the uppermost mantle domain with abundant depleted nodules of harzburgites and absence of intensive hydrous metasomatism. Most depleted xenoliths were derived from Miocene melanephelinites of Udokan volcanic field and most fertile compositions are characteristic for Sayan area (Oka volcanic field). The T-P estimations suggest that South Baikal mantle are hottest in the rift zone. Constructed geotherms for Oka xenoliths together with data for Bartoy xenoliths correspond to reference Southeast Australia geotherm or have even hotter profile. Geophysical data show that zone of anomalous mantle beneath the Baikal rift system with lithosphere thickness less than 50-60km extend from Udokan volcanic field to South Baikal and next to Hangai volcanic field in the Central Mongolia. T-P estimations for Udokan and Taryat (Hangai) xenoliths are very similar and correspond to the lower-T geotherms than those of Sayan and Bartoy localities. These data support the ideas about existence of "South Baikal hot spot", maximal mantle heating and location of hottest plume head beneath Hamar-Daban area of the Baikal rift system. We detected that petrologic crust-mantle boundary is different from seismic Moho according to possible location of low-temperature spinel peridotite xenoliths near 32-35km depth. Crust-mantle transition zone in the South Baikal area is interpreted to be complex pyroxenite-peridotite section located between 32 and 45-50km.