We report high-precision K isotopes, apatite U-Pb ages, whole-rock elements and Sr-Nd isotopes for the Saima nephelite syenite in the North China Craton. Trace-element and Sr-Nd-Hf-O isotope data indicate the presence of subducting sediments in the source region, while K isotopic compositions show a narrow range between -0.54 parts per thousand and -0.28 parts per thousand, with an average of -0.41 +/- 0.06 parts per thousand, identical to the value of the asthenosphere. The nearly identical K isotopic compositions are low probability events compared with the K isotopic compositions of island arc lavas reported previously (-1.55 parts per thousand to +0.2 parts per thousand). Although crustal contamination is consistent with the Sr-Nd-K isotopic data, alternatively we propose that the isotopic data also reconcile with the interaction between cratonic roots and the underlying convective asthenosphere, if this interaction is over prolonged periods of time. Numerical simulations successfully reproduced the observed data, if the metasomatism of the lithospheric mantle root, the source of the Saima alkaline rocks, occurred 500 Ma ago. Our study reveals that the isotopic compositions of fast-diffusion components in a lithospheric mantle metasomatized by ancient subducting melts can be effectively homogenized by convective asthenosphere through diffusion over a long time interval.