The studied massifs in Western Transbaikalia (Arsentyev and Orongoi) are composed of pyroxenites, high-Ti subalkalic gabbro, gabbro-diorites, monzodiorites, anorthosites, and syenites. The Orongoi massif also includes small olivinite and plagioperidotite bodies. The gabbroids have high contents of Sr, Ba, Nb, Ta, Zr, and Hf, which is typical of intraplate basites. The REE pattern shows the rock enrichment in LREE (La/YbN = 5.35–25.82). The 87Sr/86Sr values vary from 0.7050 to 0.7054, and εNd, from 1.44 to –1.18. The presence of radiogenic Nd in the gabbroids and their enrichment in 87Sr suggest their formation as a result of the melting of the EM-II-type lithospheric mantle.