HR: 12:05h
AN: V52B-08    [Abstracts]
TI: Establishment and Evolution of a new Silicic Magma System North of Yellowstone Caldera: Geochronology, Geochemistry and Petrographic Relationships of Extracaldera Basalts and Rhyolites in the Norris-Mammoth Corridor
AU: * Spell, T L
EM: tspell@ccmail.nevada.edu
AF: Department of Geoscience, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4010 United States
AU: Smith, E I
EM: gsmith@ccmail.nevada.edu
AF: Department of Geoscience, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4010 United States
AU: Nastanski, N M
EM: nastansk@unlv.nevada.edu
AF: Department of Geoscience, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4010 United States
AU: Bennett, K
EM: kristeenb@hotmail.com
AF: Department of Geoscience, University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4010 United States
AB: Basalts and rhyolites erupted north of the Yellowstone Caldera following collapse at 640 ka. The geochronology, petrographic relationships and geochemistry of these rocks record the development of a new rhyolitic magma system in the Yellowstone Plateau Volcanic Field. Basaltic lavas were erupted from at least seven shield volcanoes and a cinder cone between 358 ka to 209 ka (40Ar/39Ar eruption ages). Lavas for each center are associated with separate magma batches derived by melting of asthenospheric mantle and incorporating differing amounts of lithospheric mantle. For example, the Swan Lake Flat basalt erupted from four centers and displays decreasing 87Sr/86Sr and increasing $\epsilon$Nd with decreasing age (87Sr/86Sr from 0.7062 to 0.7053, $\epsilon$Nd from -2.5 to +0.18). Trace-element concentrations decrease (e.g., Ba decreases from OIB levels to 0.1 OIB). This change is modeled by a larger MORB and a smaller lithospheric component in the younger lavas. These patterns are similar to those seen prior to previous caldera-forming eruptions at Yellowstone. For example, just before the eruption of the Huckleberry Ridge tuff at 2.2 Ma, the Hepburn Mesa basalt evolved from $\epsilon$Nd -6 to -1 signifying an increased input of asthenospheric mantle derived basalt just prior to caldera formation. Aside from two older rhyolite lavas erupted at 358 ka and 526 ka which are spatially and chemically distinct, all extracaldera rhyolites have characteristics which suggest they are derived from an evolving silicic magma system of substantial longevity ($>$326 ka to 80 ka). Eruptions occurred in an area ~5-7 km wide by 22 km long (north-south). Mingled lavas consisting of andesite enclaves in rhyolite characterize the early magma system from 316 ka to 263 ka, contemporaneous with Swan Lake Flat basaltic volcanism. Andesite enclave chemistry can be modeled as mixing of Swan Lake Flat basalt with rhyolite from mingled lavas. 206Pb/238U zircon ages from these early rhyolites indicate mean magma residence ages of 30-40 ka, with some zircons exhibiting simple magmatic oscillatory zoning (CL images) having ages 80-140 ka older than 40Ar/39Ar eruptive ages. Subsequently, high-silica rhyolites exhibit Sr, Nd and Pb isotope systematics consistent with derivation from a common source having subequal mantle and crustal contributions. 226 ka to 134 ka porphyritic rhyolites record progressive changes in chemistry consistent with simple FXL of modal phenocryst phases. From 118 ka to 80 ka rhyolites are sparsely porphyritic to aphyric and exhibit a shift to significantly less evolved trace element compositions followed by evolution consistent with renewed FXL. 206Pb/238U zircon ages from younger rhyolites indicate magma residence times of up to ~100 ka prior to eruption. Identification of one mingled lava in this younger group suggests renewed/enhanced basaltic input into the crustal magma system. Eruptions of associated basalts and rhyolites between 358 ka and 80 ka north of Yellowstone caldera record establishment of a new rhyolitic magma system at the leading edge of the melting anomaly, and may represent the initial stages of a 4th caldera cycle. Early input of basaltic magmas into the crust established this silicic magma system prior to 326 ka, and undoubtedly played a fundamental role in driving it for the subsequent ~250 ka.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting