Abstract S-type granites show strong positive or negative correlations for several elements and element ratios plotted against maficity (FeO + MgO), and the maficity of the granites is usually higher than that of experimental melts from metasedimentary sources. These compositional variations appear over a large range of scales within the same pluton and may reflect a strong influence of source-controlled processes in the composition of granites, i.e., the entrainment of a mafic mineral fraction to the melt, prior to co-segregation of the minerals and melt as magmas. This paper presents a thermodynamically constrained study of the magma major element compositions produced by the entrainment of a fraction of the mineral assemblage which coexisted with melt in the source, either the peritectic assemblage, the non-peritectic assemblage or mixtures of both, in several metasedimentary source compositions from 650 to 950 °C at 0.8 GPa. The compositions of the modelled magmas have been compared to a large dataset of S-type granites. Several potential factors that may control the composition of the magmas have been considered: melt loss, batch and fractional melting, spatially restricted equilibration of plagioclase during partial melting and the nature and amount of the entrained mineral assemblage. The results of the investigation show that, irrespective of the melting process, melts are too felsic to account for the more mafic granite compositions. When the entrainment of only the peritectic mineral fraction is considered, the modelled magmas are able to reproduce the geochemical trends defined by the granites with increasing maficity for most major elements. However, Ca-rich plagioclase must be present in the entrained mineral assemblage to reproduce the Ca content of granites. Ca-rich plagioclase is produced only when plagioclase equilibration is strongly spatially restricted, with a core of unreacted plagioclase not participating in the melting reactions and a rim of plagioclase reacting in equilibrium with the melting system. The amount of plagioclase participating in the melting reaction controls the Ca content of granites: the higher the amount of plagioclase involved, the higher the amount of Ca-rich plagioclase produced and the higher the Ca content of magmas. When the entrainment of both the peritectic and non-peritectic fractions is considered, the match between the composition of the modelled magmas and the granites is improved, but only if the peritectic fraction dominates over the non-peritectic fraction and spatially restricted equilibration of plagioclase occurs, with the residual Ca-rich plagioclase behaving in suspension within the melt. Modelling of the consequences of varying the relative proportions of the minerals in the peritectic assemblage shows that the geochemical trends defined by the granites are only achieved if the stoichiometric proportions of these minerals are maintained. Magmas produced at approximately the same temperature from several metasedimentary sources display variable compositions at the same maficity, but the trends produced by increasing amounts of entrained minerals are generally parallel, as well as parallel to the S-type granite trends. In summary, this study supports a source control on the composition of S-type granites, and indicates that this control is a consequence of entrainment of variable amounts of the stoichiometric peritectic mineral assemblage, including Ca-rich plagioclase. This process allows very similar compositional trends to be produced within magmas derived from metasedimentary sources of different compositions.