THE ARBON VALLEY TUFF: A NEW LOOK AT A HIGHLY ANOMALOUS IGNIMBRITE FROM THE YELLOWSTONE-SNAKE RIVER PLAIN VOLCANIC TRACK

The 10.2 Ma Arbon Valley Tuff is a regionally distributed ignimbrite erupted from vents associated with Picabo volcanic field along the east-central part of the Yellowstone-Snake River Plain hot spot track.  Coeval fallout is widely distributed and used as a distinctive marker bed in tephrochronologic studies in the region.   Previous work has documented some anomalous features of the ignimbrite and fallout, relative to typical SR-type ignimbrites (Branney et al. 2008), such as presence of biotite, lower degree of welding, and vertical zonations in phenocryst content.  We have initiated a new study of the volcanology, petrology and geochemistry of the unit with the purpose of better understanding the nature and magmatic origin of its distinctive characteristics.  We present preliminary documentation of strong, systematically vertical gradients in phenocryst abundance, assemblage and compositions, as well as in bulk isotopic composition of the ignimbrite.  Phenocryst content varies from ~1% in comagmatic fallout , and from ~5 (base) to ~40% at the top of the ignimbrite.  Fallout contains sparse Sa (Or₅₉) + Pl (An₁₀).  Lower parts of the ignimbrite contain Sa (Or₇₀)> Q > Pl (An₃₀) > Bt (Fe/Fe+Mg=0.94-0.56) + Hbl + Mt > Ilm > Chevkinite, Zrn, Ap.  Upper parts of ignimbrite contain Sa (Or_65-70) > Q > Pl(An₃₅) > Bt (Fe/Fe+Mg=0.44) + Opx (Fs₄₅) > Mt > Ilm > Chevkinte + Allanite + Zrn + Ap.  Amphibole geobarometry indicates pre-eruption crystallization occurred at a pressure of ~3 kb (~10 depth).  Bulk isotopic composition of the ignimbrite is highly evolved relative to typical SR-type rhyolite having ε_Nd(-18.3 to -19.3), ⁸⁷Sr/⁸⁶Sr_i(0.7151-0.7156) and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb of 17.5, 16.6 and 38.1, respectively. These evolved ratios indicate that magmas that erupted to generate the Arbon Valley Tuff had a much higher mass contribution from crustal sources, relative to the mantle, when compared to the majority of SR-type ignimbrites.