Many I-type granitoid magmas are generated through partial melting of older metaigneous rocks, and the compositions of such melts are broadly calc-alkaline and metaluminous. These melts are granitic to tonalitic, and result from thermal extremes in their lower crustal source regions. Data on the experimental partial melting of common crustal rocks suggest that high-K, I-type granitoid magmas can be derived only from the partial melting of hydrous, calc-alkaline to high-K calc-alkaline, mafic to intermediate metamorphic rocks in the crust. Because of their low K2O contents, metabasaltic rocks of all kinds are unsuitable as sources, and models that propose mixing of mantle-derived basaltic magmas and crustal melts are also inadequate. There is no requirement that I- type calc-alkaline magmatism be related in any way to subduction processes.