To constrain Tianchi Volcano basalt petrogenesis, this study focuses on tholeiitic basalts from the CZK06 drill core on the northern slope. Using elemental geochemistry and Mg isotope analyses, we investigate magma evolution, petrogenesis, and mantle source properties. The tholeiitic basalts formed during the Pliocene-Early Pleistocene shield-forming stage, recording three stages of basaltic volcanism (Phases I to III). Classified as sodium-series basalts, they exhibit geochemical affinities with EM1-type OIB. Their δ²⁶Mg values (−0.420‰ to −0.150‰) show a substantially wider range than N-MORB. Their geochemical compositions are primarily controlled by source region characteristics and partial melting degree, with minor additional influences from fractional crystallization and crustal contamination. Fractional crystallization intensity shows a progressive increase from Phase I to III. Integrated with geochemical tracing studies of Changbaishan basalts, we propose that the tholeiitic basalts are derived predominantly from the partial melting of carbonatized pyroxenite, which originated from subducted ancient clay-rich altered oceanic crust. The carbonate melts driving the carbonatization were generated by low-pressure melting of recent oceanic sediments, transported by the deeply subducted carbonate-rich Pacific Plate within the Mantle Transition Zone. The tholeiitic magma formed in the Low-Velocity Zone at depths of 160–180 km beneath the lithospheric mantle.