Strontianite-rich carbonatite, containing over 30 vol% carbonate minerals predominantly composed of strontianite (SrCO3), is identified in the Zhengjialiangzi ore segment of the Muluozhai rare earth element (REE) deposit, western Sichuan Province, China. It exhibits a unique mineral assemblage dominated by strontianite, fluorite, bastn?site, barite, calcite and dolomite, distinguishing it from conventional calcio-, magnesio-, ferro-, or natro-carbonatites. The rock shows extreme enrichment in REEs (ΣREE = 47335–64367 ppm), with strong LREE/HREE fractionation [(La/Yb)N = 1151–2119] and notably high concentrations of high-value critical REEs (e.g., Pr, Nd, Tb, Dy), 5–10 times greater than those in local calcite-dominated carbonatites. Trace element patterns indicate significant enrichment in REEs, Sr, and Ba, along with depletion in high-field-strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf). In-situ Sr isotopes of strontianite [(87Sr/86Sr)i = 0.706190–0.707305] indicate an enriched mantle source (EMI–EMII). Sr enrichment is attributed to initial mantle source enrichment and extensive fractional crystallization, possibly accompanied by minor wall-rock assimilation. We propose that the strontianite-rich carbonatite formed from a highly evolved, Sr- and REEs-rich carbonatitic magma that intruded into shallow structural breccias, followed by rapid cooling. Its formation is associated with a continuous melt-fluid evolutionary process that is characteristic of carbonatitic systems.