<p>The North China Craton (NCC) has experienced significant lithospheric modification due to multiple subduction events. This study investigates the petrogenesis and geochemical characteristics of two types of pyroxenite xenoliths hosted in the Siziwangqi (Siziwang Banner) Cenozoic basalts in the northwestern NCC. The websterites exhibit reaction textures and mineral compositions indicative of peridotite-melt interaction, with formation depths comparable to those of Siziwangqi peridotites. Clinopyroxenes in these websterites display high Ti/Eu ratios and low (La/Yb)_N ratios, along with ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.703 2 to 0.705 4. These features suggest metasomatism by hydrous silicate melts derived from the partial melting of altered Paleo-Asian oceanic crust. In contrast, the clinopyroxenites exhibit interstitial textures and convex-upward rare earth element (REE) patterns, consistent with low-pressure cumulates formed in the crust-mantle transition zone. Their parental magmas resemble Early Cretaceous basalts, likely originating from an enriched mantle source modified by subduction-related terrigenous sediments. Clinopyroxenes in these clinopyroxenites show radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.707 7–0.708 4), low Ti/Eu and high (La/Yb)_N characteristics, which suggest the post-formation metasomatism by sediment-derived carbonatitic hydrous melts. The distinct geochemical signatures of the two pyroxenite types record the complex interaction between subducted materials and overlying lithosphere, emphasizing the significant role of subduction-related metasomatism in the lithospheric modification beneath the northwestern NCC.</p>