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The voluminous Eocene postcollisional potassium‐rich magmatism in central Tibet is crucial in understanding the deep geodynamic process and related uplift history of the Tibetan Plateau. Here, we present an integrated study of whole‐rock Sr‐Nd‐Mg‐Li isotopes and zircon U‐Pb ages on two suites of lavas from Duocaima and Nariniya in the northern Qiangtang block. The Duocaima high‐K calc‐alkaline trachyandesites were formed at ~35 Ma and are enriched in light rare earth elements and large ion lithophile elements. They have slightly lower δ26Mg (−0.41‰ to −0.33‰) and δ7Li (+1.0‰ to +2.6‰) relative to the mantle. Together with their high 87Sr/86Sr(i) (0.7066–0.7067) and low ɛNd(t) (−1.96 to −1.61) values, they most likely originated from the lithospheric mantle previously metasomatized by melts from subducted carbonate‐bearing sediments. Model calculation suggests that ~7–11% sediments were added into the mantle via melting. By contrast, the contemporaneous Nariniya adakitic trachytes have higher δ26Mg (−0.13‰ to −0.02‰) and δ7Li (+3.3‰ to +5.4‰) values, as well as higher Pb/Ce and Ba/La ratios, which most likely resulted from the thickened lower crust that was metasomatized by fluids released from downgoing Songpan‐Ganzi oceanic slab. Our combined isotopic study suggests the existence of a remnant oceanic slab beneath central Tibetan Plateau during the late Eocene and implies that the initial uplift of central Tibet was before ~38 Ma. Key Points: Highly variable Mg and Li isotopes in potassium‐rich volcanic rocks from Qiangtang resulted from subducted slab inputsAn ancient remnant oceanic slab still existed beneath central Tibet during the late EoceneThe initial uplift of central Tibet was before ~38 Ma [ABSTRACT FROM AUTHOR]