The rates at which large volumes of eruptible, silicic (>65 wt% SiO2) magma (magma chambers) are assembled, as well as their longevity in the upper crust, remain controversial. This controversy is due, in part, to a missing record of granitoid plutonic complexes that represent large fossil upper crustal magma chambers. We present new geologic mapping and high-precision U-Pb zircon geochronology from the Eocene Golden Horn batholith in Washington State, USA. These data reveal that the batholith was constructed as a series of sills over 739 ± 34 k.y. Topographic relief of >2 km permits volume estimates for 4 of the sills, the largest of which, a >424 km3 rapakivi granite, was emplaced over 26 ± 25 k.y. at a rate of ∼0.0125 km3/yr. This rate exceeds those needed to build large, silicic magma chambers in thermal models, and we suggest that that this unit may represent the first fossil magma chamber of this type recognized in the geologic record.