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The most voluminous silicic volcanic eruptions in the geological past were associated with caldera collapses above giant silicic magma reservoirs. The thermal evolution of these sub-caldera magma reservoirs controls the volume of eruptible magma and eruptive style. Here we combine high-precision zircon U-Pb geochronology, trace element analyses of the same mineral grains, and mass balance modeling of zircon trace element compositions allowing us to track the thermal and chemical evolution of the Oligocene Fish Canyon Tuff magma reservoir (Colorado, United States) as a function of absolute time. Systematic compositional variations in U-Pb dated zircons record∼ 440 ky of magma evolution. An early phase of volumetric growth was followed by a period of cooling and crystallization, during which the Fish Canyon magma approached complete solidification. Subsequent remelting, due to underplated …