The formation of sediment-derived granitoids often occurs at relatively low temperatures during incongruent melting or muscovite dehydration. However, the petrogenesis of high- to ultrahigh-temperature S-type granites remains enigmatic. Volatile compositions can be powerful tracers of fluid origins and can provide insights into granite petrogenesis. We present data on the chlorine (Cl), fluorine (F), and water (H₂O) contents, along with hydrogen and oxygen isotopic compositions, of apatite from the Darongshan-Shiwandashan granitic belt in South China. The petrogenesis and geodynamic setting responsible for the formation of this largest Mesozoic granitic complex in South China are highly debated. Various interpretations include those related to the collision between South China and Indochina in the southwest, subduction and tearing of the Paleo-Pacific oceanic slab in the southeast, and heating by the Emeishan mantle plume. The Cl/F ratio of melt, calculated from apatite data, decreases from southwest to northeast, which suggests halogen inputs from a northeastward-subducting slab. The δ¹⁸O value of apatite is relatively uniform across the granite suites, ranging from 9.7‰ to 12.1‰, and indicates a common metasedimentary source. The δD value of apatite varies widely, with the lowest hydrogen isotopic ratios (−238.6‰ ± 27.2‰) being comparable to those in the mantle transition zone, which possibly indicates H₂O contributions from a deep mantle that was transferred into the upper crust. Based on these results, we propose a unified model involving the subduction of the Paleo-Tethyan oceanic crust and subsequent slab tearing, which favors the underplating of basaltic magmas from both the mantle wedge and deep mantle at the mantle-crust transition. These mafic magmas released fluids and heat into the overlying continental crust, inducing partial melting of metasedimentary rocks and formation of the high- to ultrahigh-temperature Darongshan-Shiwandashan granitic belt. Apatite is a powerful recorder of magmatic, metamorphic, and metasomatic processes occurring in complex geodynamic scenarios.