A systematic dataset of petrography, mineralogy, geochronology, and geochemistry is reported for the enclave-bearing calc-alkaline I-type granitoids from the Chinese Altai, Central Asian Orogenic Belt (CAOB). Zircon U–Pb dating and geochemical data indicate that the MME and granitoids formed coevally at ~395 Ma in a subduction setting. Geochemical modelling and hybrid testing suggest that the granitoid parental magma was formed by mixing between a mafic and a felsic endmember that can be identified by isotopic compositions. The mafic rocks have (⁸⁷Sr/⁸⁶Sr)_i of 0.7048 – 0.7062, εNd(t) of -0.5 – +2.6, and zircon εHf(t) of +2.3 – +5.4, while the host granitoids have similar Sr isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7054 – 0.7064), but generally lower whole-rock εNd(t) and zircon εHf(t) values (-2.2 – +0.4 and +0.6 – +4.6, respectively). The sharp decrease of An values from cores to rims (e.g., from ca. 80 to 40) of plagioclase phenocrysts points to polybaric crystallization accompanied by degassing, which is supported by the pressure and water content estimations based on amphibole compositions. Petrographic evidence and plagioclase in-situ Sr isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7053 - 0.7071) confirm the interaction of two isotopically different magmas during the mineral crystallization. A model for the formation of the enclave-bearing calc-alkaline plutons in an arc setting is presented: in-depth mantle and crustal melting and efficient magma mixing controlled the principal chemical compositions of the granitoid intrusions, while the later decompression-dominated crystallization, magma mingling and limited mixing in the higher crustal level finally determined the texture, mineral composition, and enclave morphology.