2007 GSA Denver Annual Meeting (2831 October 2007)
Paper No. 169-9
Presentation Time: 4:00 PM-4:15 PM


WATTS, Kathryn E., Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, kwatts@uoregon.edu, BINDEMAN, Ilya N., Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, SCHMITT, Axel K., Dept. of Earth and Space Sciences, Univ. of California, Los Angeles, CA 90095-1567, and MORGAN, Lisa A., U.S. Geological Survey, Federal Center, Box 25046, MS 973, Denver, CO 80225

Low-d18O magmas are conspicuous features of Snake River Plain caldera complexes. Heise volcanic field, Yellowstone's immediate predecessor, contains the largest single known low-d18O unit (Kilgore tuff, 1800 km3) in the Snake River Plain and perhaps worldwide. Here we present new oxygen isotope analyses of phenocrysts and U-Pb ages of zircons (quoted) from major caldera-forming ignimbrites and post-caldera lavas of Heise volcanic field. The 6.9-2 Ma volcanism at Heise is represented by three normal-d18O caldera-forming ignimbrites (Blacktail Creek, 6.920.28 Ma; Walcott; Conant Creek 5.700.19 Ma) with d18OQz = 6.4 and d18OZircon = 4.8, followed by a dramatic 18O depletion in the final Kilgore cycle (4.45-2.07 Ma) with d18OQz = 4.3 and d18OZircon = 1.5. Post-Kilgore lavas exhibit equally low d18O compositions and mark the final stage of low-d18O volcanism at Heise volcanic field prior to the onset of normal-d18O volcanism at Yellowstone Plateau volcanic field. One low-d18O post-Kilgore unit (rhyolite of Sheridan Reservoir, 2.070.19 Ma), is coeval with the first, normal-d18O caldera-forming ignimbrite in the Yellowstone Plateau field (Huckleberry Ridge Tuff, 2.050.006 Ma). The U-Pb ages of zircons in Kilgore's post-caldera lavas did not reveal any inheritance, nor d18O zoning, from previously erupted volcanic units, in contrast to what has been found at Yellowstone. However, zircons from one post-Kilgore rhyolite (Indian Creek, 4.46-3.96 Ma) contain significant (0.5 Ma) zoning between older cores and younger rims. At both Heise and Yellowstone, low-d18O magmas occur exclusively in the later stages of individual volcanic cycles. The abrupt d18O depletions of late stage volcanic units in the caldera complexes at Heise and Yellowstone fields indicate that an 18O depleting mechanism must be present, excluding the possibility of derivation from a low-d18O mantle source or crustal protolith, in which case all volcanic units would be low-d18O. We argue that low-d18O magmas at Heise (and Yellowstone) are generated by heat of the mantle plume remelting shallow hydrothermally altered intracaldera rocks and terminate once volcanism begins at a new place tapping fertile, normal-d18O crust. This trend may be applicable to older caldera complexes in the Snake River Plain that are poorly exposed.

2007 GSA Denver Annual Meeting (2831 October 2007)
General Information for this Meeting
Session No. 169
The Track of the Yellowstone Hot Spot II: What do Neotectonics, Climate Indicators, Volcanism, and Petrogenesis Reveal about Subsurface Processes?
Colorado Convention Center: 401/402
1:30 PM-5:30 PM, Tuesday, 30 October 2007

Geological Society of America Abstracts with Programs, Vol. 39, No. 6, p. 456

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