Granitic rocks and mafic enclaves from the Bernasconi Hills pluton in the Peninsular Ranges Batholith, California, USA provide constraint on the extent and spatial lengthscale of mafic-felsic interaction in upper crustal granitic plutons. Further detailed mapping of mineralogical variations in the pluton and among enclaves highlights late-stage magmatic processes such as segregation of evolved residual melt and reactions between solidified enclaves and granitic melts. Collectively, the field, mineralogical and geochemical systematics of the Bernasconi Hills pluton show that mafic-felsic mixing between host granitic magma and partially solidified mafic enclaves was limited spatially, on the order of meters, and only proceeded to an early, incipient stage. Phosphorus-zirconium systematics indicate that the pluton developed through a combination of crystal fractionation and incipient mixing, in contrast to volcanic rocks, where magmatic evolution is primarily driven by crystal fractionation. Further examination of global geochemical systematics of subaerial arc lavas show a correlation between SiO2 and crustal thickness as expressed by elevation. Arc lavas become more compositionally evolved with increasing elevation/crustal thickness, tend towards increasingly iron-depleted, or calc-alkaline, compositions and show evidence for garnet fractionation at depth in thick arcs. Coupled thermal and thermodynamic compositional modeling indicates that crustal thickness controls arc lava compositions by modifying the thermal lifespan and segregation times of melt in crustal magma chambers.