As one of the world’s largest extensional structures, the South Tibetan Detachment System (STDS) is spatially and temporally associated with Himalayan leucogranites and spodumene pegmatites. This study investigates variably deformed leucogranites near the Qiongjiagang lithium deposit in southern Tibet (Everest/Himalaya region) using field observations, mineral crystallographic preferred orientations, rheological parameters, and zircon geochronology. Key findings include the following: (1) Leucogranites within the STDS record systematically variable strain, with weakly deformed bodies preserving primary magmatic fabrics and hosting abundant spodumene pegmatites at higher structural levels. (2) Zircons within strongly deformed leucogranites exhibit significant fluid alteration. Weakly modified zircons yield ages of ca. 24 Ma, while hydrothermally altered zircons date to ca. 18 Ma. (3) Quantitative deformation analysis reveals that strongly deformed leucogranites record deformation temperatures of 450−590 °C, differential stresses of ∼36.5−75.2 MPa, and strain rates of ∼10−13 s−1 to 10−12 s−1, whereas weakly deformed leucogranites record higher temperatures (∼720 °C) and strain rates (∼10−10 s−1) under comparable stress conditions. (4) Comparative rheological analysis reveals that spodumene pegmatite emplacement requires specific mechanical conditions—a differential stress threshold of 26 MPa and strain rates of ∼10−10 s−1. Through a case study of the Qiongjiagang lithium deposit, this work elucidates the role of the STDS in leucogranite emplacement and evolution and provides quantitative constraints on the rheological conditions that facilitate the formation of spodumene pegmatites within the STDS.