Mass recycling from subduction to magmatic extrusion shapes our habitable environment and Earth’s interior. Subducted igneous crust may form pyroxenites before participating magmatism, but the deep journey of associated carbonates remains unclear. Here we report new Mg-isotope data for ~89 to 81 Ma basaltic rocks in Langshan area, central Asia (δ²⁶Mg = -0.391 to -0.513 ‰) with a synthesis for post-110 Ma basalts across eastern Asian continent. The merged low-δ²⁶Mg basaltic province normally interpreted as derivations from carbonated sources paradoxically displays geochemical signatures (low Ca/Al and high K₂O contents) resembling partial melts of uncarbonated sources. Negative correlations of δ²⁶Mg vs TiO₂ and FCKANTMS, the proxy of pyroxenitic melts, and adiabatic melting modeling suggest presence of Mg-isotopically light source pyroxenites transformed from decarbonated altered oceanic crust. This may explain ubiquitous pyroxenitic contributions in many low-δ²⁶Mg basaltic suites and has significant implication for deep carbon cycling.