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 k.y. 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 andesitic recharge magmas, began 219 ± 45 k.y. prior to eruption, and led to the generation of ∼5000 km3 of eruptible crystal-rich (∼45 vol%) dacite. Age-equivalent, but compositionally different, zircons in an andesite enclave from late-erupted Fish Canyon Tuff tie the growth and thermal evolution of the upper-crustal reservoir to a lower-crustal magma processing zone. Our results demonstrate that the combination of high-precision dating and trace element analyses of accessory zircons can reveal invaluable information about the chemical and thermal histories of silicic magmatic systems and provides critical input parameters for fluid dynamic modeling.

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