T113 EARTHTIMEs and EarthRates: Exploring the Tempo and Mode of Earth-System Processes and Evolution (Posters)

Monday, 5 November 2018
9:00 AM - 6:30 PM
Halls J-K

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

Paper number: T113-6
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

  • Leah Morgan Presenting Author
    Geology, Geophysics, and Geochemistry Science Center, Denver, CO
  • Sam Johnstone
    Geology, Geophysics, and Geochemistry Science Center, Denver, CO
  • Amy Gilmer
    Geology, Geophysics, and Geochemistry Science Center, Denver, CO
  • Michael Cosca
    Geology, Geophysics, and Geochemistry Science Center, Denver, CO
  • Ren Thompson
    Denver, CO

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318051

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