Field observations from an exposed section of deep continental crust, the Athabasca granulite terrane (AGT), Saskatchewan, Canada, provide a view of granite genesis and a mechanism for deep-seated contamination of felsic and mafic magmas. The 1.9 Ga Chipman mafic dike swarm was emplaced into the Chipman Tonalite (ca. 3.3 Ga) and the megacrystic Fehr granite (ca. 2.6 Ga) at a crustal depth of ∼40 km. The Fehr granite shows evidence for extensive partial melting and generation of granitic leucosome. Mafic dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures. The AGT provides a view of a dynamic, heterogeneous, and locally fertile deep crust. Mantle-derived mafic magma promotes extensive partial melting of fertile granitoids, which in turn filter and entrap later mafic dikes and sills. The result is almost inevitable mingling and hybridization (i.e., contamination) of mafic and felsic end members. This interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa.