NEW U-PB ZIRCON AGES FOR THE MIOCENE PEACH SPRINGS TUFF (SOUTHWESTERN US)

The Miocene Peach Springs Tuff (PST) is the eruptive product of a massive volcanic event originating from a recently located caldera exposed within the Northern Colorado River Extensional Corridor along the CA-NV-AZ border. Both intrusive and extrusive units have been studied extensively using textural, petrological, geochemical and geochronological methods. Combined results from these methods suggest that the magma body was compositionally and thermally zoned and that the cataclysmic eruption was triggered by an episode of mafic injection (1).

We present new U-Pb zircon SHRIMP and CA-TIMS ages from intra-caldera and outflow deposits which confirm the hypothesis that the magma body was crystallized at the millennial time scale, and that the eruption occurred close to 18.8 Ma as recorded by the youngest zircon CA-TIMS ages, which is in close agreement with a ⁴⁰Ar/³⁹Ar age (2). In order to resolve intra-crystal age variation, crystal tips, as well as the interior portions of zircon crystals, where subjected to U-Pb SHRIMP analyses. In addition, SHRIMP analyses were performed on crystal faces using indium mounts in order to capture the age of the final stage of zircon crystallization. SHRIMP analyses were applied to untreated and thermally annealed/chemically abraded crystals, the latter showing improved coherence. The U-Pb SHRIMP ages largely also confirm an eruption age close to 18.8 Ma. Combined with U-Pb CA-TIMS ages from extrusive and intrusive units (3) the magmatic system leading to the PST was thermally active (i.e., near or above solidus) from ca 19.0 Ma to 18.2 Ma, with a thermal flare-up that lasted ca 200 ka prior to, and triggering, the cataclysmic eruption. To better integrate SHRIMP, TIMS, and ⁴⁰Ar/³⁹Ar age constraints, we consider Bayesian approaches using the Chron.jl framework to address pre-eruptive zircon crystallization.

1. A. S. Pamukcu, T. L. Carley, G. A. R. Gualda, C. F. Miller, C. A. Ferguson, Journal of Petrology 54, 1109 (2013).
2. C. A. Ferguson, W. C. McIntosh, C. F. Miller, Geology 41, 3 (2013).
3. S. McDowell, C. Miller, R. Mundil, C. Ferguson, J. Wooden, Contributions to Mineralogy and Petrology 168, 1 (2014).