Anomalous trace element compositions of Middle to Late Jurassic detrital zircon separated from Sierra Nevada forearc and intra-arc strata reveal processes of differentiation occurring within the deep arc lithosphere. REE-Sc-Nb-Ti-Hf-U-Th covariations define three populations of atypically REE-rich grains that we interpret as crystallizing from (1) differentiates produced by olivine+clinopyroxene+plagioclase+garnet±ilmenite fractionation; (2) mixing between mafic arc magmas and partial melts of Proterozoic Mojave province crust; and (3) compositionally transient, low Gd/Yb magmas generated by hornblende resorption during decompression. We interpret a fourth population of Middle Jurassic to Early Cretaceous zircons having REE contents similar to “typical” arc zircon but with atypically high Gd/Yb ratios as having crystallized from partial melts of recycled arc crust and from deep-arc differentiates that evolved down-temperature through hornblende saturation. We hypothesize that latest Jurassic extension ripped open the arc, facilitating upward migration and eruption of geochemically anomalous zircon-bearing magmas. The anomalous compositions relative to “typical” arc zircon imply that these zircons and their host magmas rarely reach the upper arc crust, where eruption and/or erosion would release their zircon cargo to the clastic system. Focusing on the trace element compositions of zircons of syn-extensional age represents a productive new strategy for learning about deep magmatic reservoirs and early differentiation pathways within the thick lithosphere of continental margin arcs.