<p>Apatite is a robust recorder of open-system magmatic evolution in granitoids, as it may preserve subtle geochemical signals of magma compositional changes. Here, we integrate textural, compositional, and O-isotopic analyses of magmatic apatite from Jurassic granodiorites and biotite granites in the Cathaysia Block, Southeast China, to better constrain the nature and evolution of open-system magmatic processes and refine the petrogenesis of these granitic rocks. In granodiorites, O-isotope disequilibrium between apatite and zircon documents open-system magmatism: low-δ¹⁸O apatite (Group 1; 5.7–7.1‰; Δδ¹⁸O_{zircon–apatite} = +1.31‰) indicates greater mantle-derived input, whereas high-δ¹⁸O apatite (Group 2; 8.0–9.0‰; Δδ¹⁸O_{zircon–apatite} = -0.84‰) reflects enhanced supracrustal assimilation. In biotite granites, apatite and zircon are in near isotopic equilibrium (Group 3; 6.3–9.0‰; Δδ¹⁸O_{zircon–apatite} = -0.08–0.09‰ ), and δ¹⁸O differences among samples likely reflect variable proportions of mantle- and crust-derived components. Apatite core–rim zoning provides additional evidence for complex magma interaction: apatite in granodiorites shows abrupt core-to-rim decreases in REE, Y, and SiO₂, consistent with mafic recharge, while apatite in biotite granite displays opposite trends, reflecting hybridization with congenetic, more evolved felsic melts. Apatite trace-element systematics further track mineral–melt differentiation during magma evolution: decreasing Sr coupled with increasingly negative Eu anomalies reflects plagioclase fractional crystallization, whereas pronounced LREE depletion in apatite indicates early allanite crystallization. Together, these in situ textural, geochemical, and O-isotopic data highlight apatite as a powerful tracer of magma sources, open-system evolution, and differentiation processes in granitoids.</p>