Cordilleran I-type granitoids with a mantle signature are characteristic products of oceanic-plate subduction at continental margins. I-type granitoids with a crustal signature occur in a variety of tectonic settings, including that of subduction. S-type granites with crustal signatures are characteristic of collisional settings, but also occur in the same range of settings as crustal I-types. The role of the tectonic setting is subordinate to that of the composition of the source region in determining the typology of crustal granites, which is a function of the proportion of mantle-derived to crustal material mobilized during magma genesis. The production of melts with similar proportions of mantle to crustal components is triggered by a variety of tectonic processes in different tectonic settings. Crustal heterogeneity is probably the main factor contributing to the diversity of crustal granites.