Li–Cs–Ta (LCT)-type granitic pegmatites commonly occur adjacent to granitic bodies. For many pegmatite fields, it is not obvious whether the pegmatitic melt originated from an evolved granitic magma or from low-degree partial melting of metasedimentary rocks. The Ke’eryin granitic pegmatites in the Songpan–Ganze orogenic belt, China, which hosts large Li₂O reserves around large-volume granitic intrusions, including biotite granite (BG), two-mica granite (TG), and muscovite granite (MG), present an excellent location to investigate the petrogenesis of granitic pegmatites and associated Li mineralization. Our results suggest that these granites were generated from a common magma source and emplaced in pulses, coupled with fractional crystallization. These granites and associated pegmatites show discordant trends in the bulk-rock Zr/Hf and Nb/Ta ratios and apatite Y/Ho and Sr/Y ratios, which reflect an evolution from granitic magma to flux-rich pegmatite melts. Pegmatitic melts might have derived from TG magma during evolution from BG to TG and MG magma. Initial ⁸⁷Sr/⁸⁶Sr ratios of BG apatite (0.7161–0.7188) and low bulk-rock Fe₂O₃/FeO ratios (0.04–0.22) imply that the Xikang Group at depth might have undergone high-degree partial melting to produce the original granitic magma. This melting of metasedimentary rocks, resulting in a large-volume magma with low flux and rare-metal contents, was followed by protracted fractionation during multiple pulses of magma emplacement. This process resulted in the formation of flux- and rare metal-rich pegmatite melts from granitic magma. This mechanism may be applicable to many LCT-type pegmatites associated with large granitic complexes worldwide.