Abstract: Garnet is a common mineral in skarn-type polymetallic deposits, and dating such deposits using garnet has become an essential means of constraining their metallogenic ages. However, garnet is typically characterized by low U and high common Pb contents, so the main technical challenges in in-situ garnet U-Pb geochronology are the selection of an appropriate matrix-matched reference material and the accurate correction of common Pb. Based on the principle of maximizing U sensitivity while minimizing elemental fractionation, this study determined the optimal instrumental parameters as follows: a laser spot diameter of 74 μm, laser repetition rate of 6 Hz, and energy density of 4 J/cm², with the addition of 2 mL/min high-purity nitrogen to the carrier gas. Using the garnet reference material PL57 as an external standard and applying the Tera-Wasserburg diagram method for common Pb correction, a well-constrained lower intercept age was obtained. This method was applied to constrain the metallogenic epoch of the Hengxiyuan tin polymetallic deposit associated with the Yuechengling pluton in Guangxi. Using in-situ LA-ICP-MS garnet U-Pb isotope analysis, we obtained Tera-Wasserburg lower-intercept ages of 432 ± 49 Ma (MSWD = 2.7) and 441 ± 15 Ma (MSWD = 0.63) from two skarn samples collected at Hengxiyuan. These ages indicate that the main mineralization at Hengxiyuan deposit occurred during the Caledonian and was intimately linked to the magmatic activity of the Yuechengling granite, thus providing key geochronological constraints supporting a magmatic-hydrothermal origin of the Hengxiyuan deposit. The regional timing of W-Sn mineralization at Hengxiyuan is markedly distinct from the dominantly Yanshanian ages recorded for major deposits elsewhere in the Nanling Range, suggesting significant potential for Caledonian W-Sn mineralization in this region.