The Cougar Point Tuff; implications for thermochemical zonation andlongevity of high-temperature, large-volume silicic magmas of theMiocene Yellowstone Hotspot

Abstract

The 12.7-10.5 Ma Cougar Point Tuff in southern Idaho, USA, consists of10 large-volume <10 super 2 -10 super 3 km super 3 each,high-temperature 800-1000 degrees C, rhyolitic ash-flow tuffserupted from the Bruneau-Farbidge volcanic center of the Yellow-stonehotspot. These tuffs provide evidence for compositional and thermalzonation in pre-eruptive rhyolite magma, and suggest the presence of along-lived reservoir that was tapped by numerous large explosiveeruptions. Pyroxene compositions exhibit discrete compositional modeswith respect to Fe and Mg that define a linear spectrum punctuated byconspicuous gaps. Airfall glass compositions also cluster into modes,and the presence of multiple modes indicates tapping of differentmagma volumes during early phases of eruption. Equilibrium assemblagesof pigeonite and augite are used to reconstruct compositional andthermal gradients in the pre-eruptive reservoir. The recurrence ofidentical compositional modes and of mineral pairs equilibrated athigh temperatures in successive eruptive units is consistent with thepersistence of their respective liquids in the magma reservoir.Recurrence intervals of identical modes range from 0.3 to 0.9 Myr andsuggest possible magma residence times of similar duration. Eruptionages, magma temperatures, Nd isotopes, and pyroxene and glasscompositions are consistent with a long-lived, dynamically evolvingmagma reservoir that was chemically and thermally zoned and composedof multiple discrete magma volumes.

31 samples from this reference