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Paper No. 6
Presentation Time: 8:00 AM-6:00 PM


KELLY, Evan A.1, MILLER, Calvin F.2, GUALDA, Guilherme A.R.3, COLOMBINI, Lindy L.S.2, PADILLA, Abraham De Jesus2 and CRIBB, J. Warner4, (1)Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40508, (2)Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, (3)Dept. of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, (4)Geosciences, Middle Tennessee State Univ, PO Box 9, Murfreesboro, TN 37132,

The middle of the Miocene Highland Range volcanic sequence (HRV) comprises low- and high-Si dacite lavas (62 & 67-70 wt% SiO2; 16.6 Ma); low- to high-silica rhyolite tuffs and lavas (70-77% SiO2); mingled andesite and rhyolite lava (16.0 Ma); and andesite lavas (58% SiO2, 16.0 Ma at their base). The dacite and rhyolite underlying the andesite are thought to have erupted as part of the magmatic system that formed nearby Searchlight pluton (SLP), which they match in age and composition.

A 250 m thick coulee, exposed for 1.8 km near the base of the rhyolite portion of HRV, represents a single, large, composite effusive eruption and comprises 3 lithologies: (1) dominant rhyolite (69-77% SiO2; phenocrysts sanidine + qtz + plag), brecciated at the coulee base and top, in part flow-banded; (2) subordinate but widespread dacite (61-62% SiO2; cpx + plag), commonly in distinct pods 25-300 cm wide in the middle of the flow, locally more massive or displaying flow bands mingled with rhyolite; (3) andesite (58% SiO2; plag + cpx + oliv), present as common 1-20 cm enclaves with crenulate margins.

Two magma mingling events, prior to and during eruption, can account for observed compositions, textures, and field relations: (1) injection of andesite into rhyolite, with little or no mixing; (2) mingling of rhyolite and dacite, resulting in limited contamination of rhyolite. The rhyolite and dacite were in contact while in the conduit but remained largely separate until the eruption, at which point further mingling proceeded, including net-veining of rhyolite in dacite, plucking of brittlely-behaving dacite from massive pods into rhyolite, and cm-scale flow banding of rhyolite with dacite. Andesite enclaves were preserved undeformed during dacite-rhyolite mingling.

The components of the coulee match other erupted and intruded portions of the SLP-HRV system: the more silicic rhyolite is almost identical to overlying lavas and tuffs, and leucogranites in SLP; dacite to underlying, low-Si dacite and older portions of the pluton; and andesite enclaves to the andesites that overlie the silicic sequence. Thus, the coulee demonstrates that multiple magmas, with compositions that span much of the compositional and age range of the SLP-HRV system, erupted simultaneously and were present together in the conduit and underlying SLP magma chamber.

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