Cenozoic (∼40–30 Ma) high-Mg potassic volcanic rocks are abundant in the Jinshajiang–Ailaoshan magmatic belt that stretches from eastern Tibet to western Yunnan. The petrogenesis and compositional variation of the mantle source of these rocks are poorly known. In this paper, we obtained U-Pb zircon ages and whole-rock major and trace element and Sr-Nd-Pb-Hf isotopic data for these volcanic rocks from the Nangqian basin in the Eastern Qiangtang block, eastern Tibet. High-Mg volcanic rocks in Nangqian are potassic trachybasalts, shoshonites and latites and contain 6.49–10.54 wt% MgO, 48.04–56.89 wt% SiO2 and 3.36–4.14 wt% K2O, with K2O/Na2O ratios of 0.94–1.24. Zircon U-Pb dating shows that they are slightly older (40–38 Ma) than their counterparts (37–32 Ma) from the Dali district, western Yunnan. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and have significantly negative Nb-Ta-Ti anomalies. Isotopically, they have primitive mantle-like 87Sr/86Sr ratios (0.7050–0.7055) and εNd(t) values of −1.06 to +0.10 and restricted ranges of 206Pb/204Pb (18.63–18.71), 207Pb/204Pb (15.52–15.57) and 208Pb/204Pb ratios (38.42–38.59). Zircon εHf(t) values vary from +4.1 to +9.4 and are thus decoupled from their whole-rock Nd isotope ratios. Previous studies have shown that Cenozoic high-Mg potassic volcanic rocks in the Dali district have more enriched Sr-Nd-Pb-Hf isotopic compositions, higher LILE/HFSE (high field strength elements) and LILE/LREE ratios but less enrichment in LREE than the counterparts from the Nangqian basin. Geochemical differences between high-Mg volcanic rocks from these two locations can be explained by different mantle sources caused by different metasomatic processes. Underneath the Nangqian basin, a juvenile, isotopically “depleted”, and garnet-bearing lithospheric mantle was metasomatized by Paleozoic and Cenozoic subduction, whereas an ancient (probably as old as Neoproterozoic), fertilized, and spinel facies-dominated lithospheric mantle was metasomatized by Neoproterozoic subduction underneath the Dali district.