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Granite genesis is crucial to understanding the evolution of continental crust, yet many concerns about granite genesis remain not well answered, such as whether I-type granite contains metasedimentary components, what controls granite compositional diversity, and how granitic plutons are constructed. To explore these issues, we conducted a detailed study on the two-mica plagiogranite, tonalite, and biotite plagiogranite units of the Wujinxia composite pluton in the eastern Qilian orogen, NE Tibetan Plateau. These units comprise two-mica plagiogranite, tonalite (with diorite enclave), and biotite plagiogranite. Zircon U–Pb data reveal that three granitic units formed at ~ 487 Ma, ~ 464 Ma, and ~ 430 Ma, respectively. Magmatic and xenocrystic garnet were identified from the tonalite and biotite plagiogranite, respectively. The two-mica plagiogranite, tonalite, and biotite plagiogranite all belong to low-K series rocks (K2O/Na2O = 0.10–0.26), and were derived from deep crustal sources mainly consisting of juvenile mafic rocks, with involvement of minor metasedimentary rocks in the magma sources of the two-mica plagiogranite and tonalite. The diorite enclave within the tonalite was probably derived from an enriched mantle-derived basaltic magma. Mineral compositions, thermobarometric calculations, and whole-rock geochemical data indicate that the low-K intrusive units of the Wujinxia composite pluton resulted from multiple magmatic systems at different depths. The results suggest that I-type granites can contain metasedimentary components by partial melting of a mixed crustal source, and high-Mn content helps the preservation of high-Ca garnet within such rocks. For a composite pluton spanning a large compositional variation, its compositional diversity is jointly controlled by magma source composition, melting condition and thermal evolution of individual magma pulses, and the resulted assembly style during pluton construction. [ABSTRACT FROM AUTHOR]