Coupled age and trace element geochemical analyses of detrital zircons provide criteria that may be applied to describe average melt compositions through virtually the entire history of a long-lived magmatic arc. Detrital zircon data suggest that the California (western United States) Cordilleran arc was characterized by five mean magmatic states. Three geochemically distinct pulses were characterized by high Th/U and progressively heavy rare earth element (HREE)–depleted melts. The first and last pulses were also characterized by higher than average U/Yb, suggesting that progressive crustal thickening coupled with variable fluid inputs from the subducting slab modulated pulse volumes. Lulls between pulses were states generally characterized by low magmatic volumes and low Th/U and U/Yb, suggesting fluid-poor conditions with minimized crustal involvement in magmatism.