To form a granite pluton, the felsic melt produced by partial melting of the middle and lower continental crust must separate from its source and residuum. This can happen in three ways: (1) simple melt segregation, where only the melt fraction moves; (2) magma mobility, in which all the melt and residuum move together; and (3) magma mobility with melt segregation, in which the melt and residuum move together as a magma, but become separated during flow. The first mechanism applies to metatexite migmatites and the other two to diatexite migmatites, but the primary driving forces for each are deviatoric stresses related to regional-scale deformation. Neither of the first two mechanisms generates parental granite magmas. In the first mechanism segregation is so effective that the resulting magmas are too depleted in FeOT, MgO, Rb, Zr, Th and the REEs, and in the second no segregation occurs. Only the third mechanism produces magmas with compositions comparable with parental granites, and occurs at a large enough scale in the highest grade parts of migmatite terranes, to be considered representative of the segregation processes occurring in the source regions of granites.