In recent years, several Nb-rare-earth-element (REE) deposits have been discovered in trachyte from the Pingli-Zhenping area of the North Daba Mountain. Of these, the Zhujiayuan Nb (-REE) deposit is particularly rich in monazite. In this study, electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to better understand the genesis of the deposit and constrain the timing of Nb-REE mineralization. The results show that most monazite in the deposit is closely related to various alteration minerals (biotite, sericite, albite, quartz, pyrite, and calcite), and monazite often has a lower ThO₂ content (0.09–1.32 wt%), indicating its hydrothermal origin. Monazite and the dominant Nb-bearing mineral (columbite) have a close paragenetic relationship, indicating that Nb and REE were mineralized in the same period. LA-ICP-MS U-Pb dating on hydrothermal monazite grains shows a weighted average ²⁰⁶Pb/²³⁸U age of 237.2 ± 7.9 Ma, which represents the timing of REE mineralization. The origin of monazite, in combination with the alteration mineralogy and ore-forming age, suggests that fluid exsolved from the Early Mesozoic alkaline felsic melt induced widespread hydrothermal alteration and the formation of the Zhujiayuan Nb-REE deposit.