Paper No. 4
Presentation Time: 9:45 AM


CHRISTIANSEN, Eric H.1, MCCURRY, Michael2, BINDEMAN, Ilya N.3, CHAMPION, D.E.4, KNOTT, Tom5, BRANNEY, Michael J.5, HOLTZ, Francois6, BOLTE, Torsten6 and SHERVAIS, J.W.7, (1)Department of Geological Sciences, Brigham Young University, Provo, UT 84602, (2)Department of Geosciences, Idaho State University, Pocatello, ID 83209, (3)Geological Sciences, University of Oregon, Eugene, OR 97403, (4)U.S. Geol Survey, MS-910, 345 Middlefield Rd, Menlo Park, CA 94025, (5)Department of Geology, University of Leicester, Leicester, LE1 7RH, United Kingdom, (6)Institute for Mineralogy, University of Hannover, Hannover, D-30167, Germany, (7)Department of Geology, Utah State University, Logan, UT 84322-4505,

Disparate models have been proposed for the origin of rhyolites associated with hotspots like those on the Yellowstone hotspot track. In the central part of the track, study of these large magma systems has been hindered because the eruptive sources are buried by basalt. The 2 km Kimberly core helps fill that gap; it penetrates deep into the rhyolitic underpinnings of the southern margin of the province. The core is dominated by rhyolite lava and welded ignimbrite, with basalt and sediment between 241 m and 424 m depth.

Our initial chemical, petrographic, O-isotopic, and paleomagnetic measurements suggest that there are three major rhyolite units in the core. Rhyolite 3, the uppermost, is nearly 130 m thick. It contains phenocrysts of plag-san-px-zcn-ap-ilm-mt. Rhyolite 2 is the most highly evolved unit with ~75% SiO2; it has anorthoclase and quartz, in addition to the minerals in Rhyolite 3. Rhyolite 1, the base of which is not exposed, is nearly 1,340 m thick, has no apparent flow contacts or cooling breaks and may represent a single, thick intracaldera ignimbrite. The ignimbrite has the same minerals as the upper unit and is a low-silica rhyolite with high concentrations of Fe2O3 and TiO2; it is not obviously zoned—TiO2 ranges from 0.75 to 0.84% and Nb from 39 to 41 ppm.

All units have the characteristics of A-type rhyolites and, compared to typical arc rhyolites, they are ferroan, Sr-and Al-poor, dry, and hot (~900°C). They have low La/Nb and Pb/Ce ratios and their trace element patterns have only small negative Nb anomalies. Initial analyses show that all three units are low δ18O rhyolites; feldspar ranges from 1‰ in Rhyolite 1 to 3‰ in Rhyolites 2 and 3. In the thick lower ignimbrite, whole-rock δ 18O increases systematically from the base upward (0.5 to as much as 9‰ in the altered top).

We suggest that many of these characteristics were imposed by rhyolite derivation from low degrees of partial melting of tholeiitic basalt lodged in a mid-crustal sill. The low δ18O values reflect recycling of hydrothermally altered crustal rocks. The isotopic data also indicate progressive recycling of more hydrothermally altered material into the younger magmas. More work is needed to establish correlation with regional units, understand the volcanic settings, and ascertain the origin of these distinctive low 18O, A-type rhyolites.

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