Post-collisional ultrapotassic volcanic rocks (UPVRs) in the Tethyan domain archive crucial information about the composition and evolution of the lithospheric mantle, offering valuable insights into deep geodynamic processes. The Tibetan Plateau, particularly the Lhasa Terrane in its southern part, remains a subject of debate regarding the genesis and geodynamic mechanisms of the UPVRs. This study investigates geochronology, mineralogy, and whole-rock geochemistry of the UPVRs in the Majiang area of Lhasa Terrane, southern Tibet. Zircon UPb dating reveals that these rocks formed at ca. 21 Ma. Petrologically, the Majiang volcanic rocks are classified as trachyandesites, while geochemically they qualify as ultrapotassic rocks (characterized by K₂O > 3 wt%, MgO > 3 wt% and K₂O/Na₂O > 2). They exhibit a broad range of SiO₂ contents (45.85–57.94 wt%) and high Mg# values (57.7–72.3). Elevated trace element concentrations, marked by enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), suggest a mantle source influenced by fluid/melt interactions. The presence of phlogopite phenocrysts in the Majiang UPVRs suggests high magma temperatures (1116 ± 15 °C) and low crystallization pressures (1.3 ± 0.3 GPa). Trace element geochemical modeling indicates that the Majiang UPVRs likely originate from a garnet stable mantle domain and experience rapid magma ascent after their formation. The Majiang UPVRs exhibit high Th/U (8.97–19.08) and Th/La (2.27–2.45) ratios, low Hf/Sm (0.52–0.64) ratios, and enriched SrNd isotopes (⁸⁷Sr/⁸⁶Sr_i = 0.717286–0.721658, ε_Nd(t) = −9.55 - -9.28). These geochemical signatures suggest derivation from an enriched lithospheric mantle metasomatized by oceanic sediments. Integration of published data reveals that the UPVRs in both the eastern and western Lhasa Terrane share close geochemical affinities, suggesting a common mantle source. However, limited UPVR exposures in the eastern Lhasa Terrane and slightly more depleted SrNd isotopes in the Majiang UPVRs compared to those in the western Lhasa Terrane imply distinct Miocene geodynamic processes. Lithospheric convective thinning caused by post-collisional extension and mantle upwelling likely occurred more extensively in the eastern Lhasa Terrane. This process may have led to the gradual replacement of ancient lithospheric mantle by juvenile material in the east, contrasting with the more localized effects in the western Lhasa Terrane.