We present the results of study of garnet-bearing paragneisses, migmatites, and vein granites of migmatite–gneiss complex of the Irkut terrane (Sharyzhalgai uplift, southwestern Siberian craton), major- and trace-element zoning of the garnet, and the age and trace-element composition of zircon. The migmatite–gneiss complex of the Irkut terrane formed in the Paleoproterozoic, at 1.85–1.86 Ga. The maximum temperatures of metamorphism and partial melting evaluated with the Ti-in-zircon and Zr-in-rutile thermometers are 790–830 and 830–860 °C, respectively. Three generations of garnet have been recognized according to texture and zoning: metamorphic, peritectic, and magmatic. Metamorphic generation forms the core of garnet in paragneisses and migmatites, contains numerous fine inclusions, and has the highest contents of grossular (Grs) component, HREE, and Y decreasing from core to rim, which agrees with the Rayleigh fractionation of compatible components. Peritectic garnet with large inclusions of quartz and feldspar is predominant in diatexites and in gneisses in contact with leucosome. It has almost constant low contents of Grs, HREE, and Y slightly increasing in the rim. Garnet in vein melanocratic granites is similar in composition zoning to garnet in diatexites. Magmatic garnet in leucocratic granites is characterized by a decrease in the contents of Grs, HREE, and Y from core to rim and by a strong negative Eu anomaly. It seems to have resulted from the dissolution/precipitation of peritectic garnet in the melt. According to mineral and chemical compositions, the gneisses and migmatites are subdivided into high- and medium-alumina series produced through the metamorphism and melting of two source rocks, pelitic and graywacke, which is confirmed by their different REE patterns. The FeO and MgO enrichment of the granites relative to the melts experimentally produced of pelites and graywackes, the inherited contents of HREE in the diatexites and granites from paragneisses, and the presence of peritectic garnet in melanocratic granites evidence that the formation of granite was not accompanied by the effective segregation of garnet from the melt.