A SUPERVOLCANO AND ITS SIDEKICKS: A 100 KA ERUPTIVE CHRONOLOGY OF THE FISH CANYON TUFF AND ASSOCIATED UNITS OF THE LA GARITA MAGMATIC SYSTEM

Establishing eruptive chronologies of major volcanic systems is challenging and mostly limited by the precision of modern geochronometers. For example, prior work on the La Garita caldera (San Jan volcanic field, southwest CO), which produced the Fish Canyon Tuff, failed to resolve temporal differences between pre- and post-collapse eruptive units. Here, we report new ⁴⁰Ar/³⁹Ar geochronologic data on sanidine supporting a ca. 100 ka eruptive history of the La Garita caldera, and resolving the timing of the precursor Pagosa Peak Dacite, the Fish Canyon Tuff, and the postcaldera Dacite of Nutras Creek. We minimize uncertainty in one important variable of ⁴⁰Ar/³⁹Ar geochronology, the neutron flux, by rotating samples during irradiation. Our ⁴⁰Ar/³⁹Ar results are consistent with geologic relationships: the Pagosa Peak Dacite underlies the Fish Canyon Tuff; the Dacite of Nutras Creek overlies it. Bayesian statistical analysis, which is robust in the presence of outliers, reveals an extended ca. 60 ka eruptive history of the Pagosa Peak Dacite prior to eruption of the Fish Canyon Tuff, and that the Dacite of Nutras Creek likely post-dates the Fish Canyon Tuff by ca. 20 ka. The improved precision demonstrated using these methods underscores the potential of unraveling other closely-spaced geologic events.