THE COUGAR POINT TUFF: COMPOSITIONAL GAPS AND GRADIENTS IN HIGH-TEMPERATURE, LARGE-VOLUME SILICIC MAGMAS OF THE MIOCENE YELLOWSTONE HOTSPOT, AND IMPLICATIONS FOR MAGMA RESERVOIR LONGEVITY

The 12.7–10.5 Ma Cougar Point Tuff in southern Idaho, USA consists of ten large-volume (>10²-10³ km³ ea), high temperature (800-1000°C), rhyolitic ash flow tuffs erupted from the Bruneau-Jarbidge volcanic center of the Yellowstone hotspot. These tuffs provide evidence for compositional and thermal zonation in pre–eruptive rhyolite magma, and suggest the presence of a long–lived reservoir that was tapped by numerous large explosive eruptions. Airfall glass compositions exhibit discrete compositional modes, and many fallout tuffs contain more than one mode, indicating tapping of different magma volumes during early phases of eruption. Pyroxene compositions also cluster into discrete compositional modes with respect to Fe and Mg that define a linear spectrum punctuated by conspicuous gaps. Several eruptions evacuated multiple modes of pyroxene that are present at single stratigraphic horizons, and identical assemblages of pyroxene modes recur in successive eruptions as well as non-sequential eruptions. Identical compositional gaps are likewise preserved throughout the eruptive sequence. Three lava flows are interlayered with tuff units late in the eruption sequence, and these lavas each show pyroxene modes that are indistinguishable from those in the tuff units. In units with multiple glass and mineral modes, equilibrium assemblages of pigeonite and augite are used to reconstruct compositional and thermal gradients in pre–eruptive magma. The recurrence of identical compositional modes and of mineral pairs equilibrated at high temperatures in successive eruptive units is consistent with the persistence of their respective liquids in the magma reservoir. Recurrence intervals of identical modes range from 0.3 to 0.9 Ma and suggest possible magma residence times of similar duration. Eruption ages, magma temperatures, Nd isotopes, and pyroxene and glass compositions are consistent with a long-lived, dynamically evolving magma reservoir that was chemically and thermally zoned and composed of multiple discrete magma volumes.