Post-collisional magmatic intrusions and their mafic microgranular enclaves (MMEs) can provide significant insights into granite petrogenesis, crust-mantle interaction, and the reworking and growth of the continental crust. A combined petrological and geochemical study was carried out on MMEs and their host Niubiziliang I-type felsic composite pluton (granite and granodiorite) and aluminous A2-type Datonggou biotite granite from the Niubiziliang batholith in the northwestern segment of the North Qaidam Orogenic Belt (NQOB), China. This study presents whole-rock major- and trace elements and Sr–Nd isotopic compositions, electron probe microanalyses of plagioclase, hornblende, and biotite, and zircon U–Pb dating and Hf isotopes. Zircon U–Pb dating demonstrates that the studied rocks formed during the Late Devonian (ca. 382 Ma). The Niubiziliang composite felsic pluton samples yield a metaluminous calc-alkaline to high-K calc-alkaline I-type granitic composition. The MMEs from Niubiziliang composite felsic pluton have low SiO2 (52–64 wt%) and high MgO (2.8–4.9 wt%) concentrations. Both MMEs and host granites are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in P and high field strength elements (HFSEs). All MMEs and host granitoids samples from Niubiziliang have indistinguishable Sr-Nd-Hf isotopic compositions characterized by initial Sr isotope ratios of 0.70545–0.70633 (MMEs), 0.70555–0.70742 (host rock), whole-rock εNd (382 Ma) values of 􀀀 2.95 to +1.6, 􀀀 0.25 to +3.5, and zircon εHf(382Ma) values of +2.2 to +10.2, +1.3 to +10.4, respectively. The Niubiziliang composite felsic pluton is a product of mixing juvenile crust-derived and depleted mantle-derived melts in a 40:60 proportion. MMEs crystallized from the same source as the host granite, and they were formed by self-mixing with the earlier crystallized cumulates when they were affected by the emplacement of subsequent hot magmas. The Datonggou biotite granite has high TFe2O3/(TFe2O3 + MgO), TiO2/MgO, Ga/Al, and Y/Nb ratios, high zircon saturation temperatures (802–846 ◦C), high HFSE concentrations (e.g., Zr, Nb, and Y), moderate (Na2O + K2O) concentrations, and low Eu and Sr concentrations. Interstitial biotite has high Fe2O3 but low MgO concentrations, similar to those of aluminous A-type granite. The above-mentioned characteristics imply that the Datonggou biotite granite is an aluminous A2-type granite. Considering their high initial Sr isotopic composition (0.72218–0.72652), low εNd (382 Ma) (􀀀 6.73 to 􀀀 6.31), and comparatively old two-stage Nd model ages (1640–1674 Ma), we suggest the biotite granite was generated by partial melting of Ordovician-Silurian granitoids with tonalitic compositions at HT-LP conditions. The formation of coeval I- and aluminous A2-type granitoids in this region was probably related to the lithospheric mantle delamination and associated asthenospheric upwelling at ca. 390 Ma. The I-type granitoids and MMEs from the Niubiziliang area represent a net addition of juvenile mantle to the crust and, given the wide distribution of contemporaneous I-type granitoids, the post-collisional magmatism has significantly contributed to the growth of the continental crust in the NQOB.