HR: 1340h
AN: V33C-1518    [Abstracts]
TI: Composition of Magmas, Minerals, and Melts of Tuffs and Lavas from the San Luis Caldera Complex, Colorado: Framework for Crystal Recycling and Melt Extraction Processes
AU: * Streck, M J
EM: streckm@pdx.edu
AF: Portland State University, Department of Geology 17 Cramer Hall, 1721 SW Broadway, Portland, OR 97207-0751, United States
AU: Lipman, P W
EM: plipman@usgs.gov
AF: U.S. Geological Survey, MS 910, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,
AB: The stratigraphy of the Oligocene San Luis Caldera Complex comprises three voluminous tuffs issued from overlapping calderas and numerous, compositionally diverse pre- and post-caldera lavas. The entire complex was erupted over an amazingly short period of 50-100 k.y. From oldest to youngest, the tuffs are the Rat Creek Tuff (>150 km³) that ranges in bulk composition from dacite to rhyolite to high-silica rhyolite, the dacitic Cebolla Creek Tuff (>250 km³), and the Nelson Mountain Tuff (>500 km³) that is again compositionally zoned from dacite to high-silica rhyolite. Pre- and post-caldera lavas range in composition from andesite to high-silica rhyolite. Phenocryst contents of most units range from 15 to 25%; only high-silica rhyolites are notably crystal-poorer (?5%) and tuffs can reach crystal contents of 35-45%. The compositional and lithological framework of the San Luis Caldera complex combined with the tight temporal and spatial control provide an ideal field laboratory for investigating whether minerals in younger volcanic units may be in part derived from recycling crystals of non-erupted magma of older units and for investigating generation processes of crystal-poor high-silica rhyolites. Many eruptive units are distinct on the level of bulk rock in terms of major and trace elemental composition and/or mineral proportions. For example, the dacitic to rhyolitic Rat Creek and Nelson Mountain Tuffs are dominated by pyroxene with only minor amphibole while the intervening dacitic Cebolla Creek Tuff contains abundant amphibole with subordinate pyroxene. When glass and mineral compositions are considered, eruptive units are less distinctive and observed compositions of individual units may cluster tightly or spread over most of the observed range for all units but in either case individual units typically follow broad compositional trends defined by all units. Interstitial melts of andesites to rhyolite are rhyodacitic to high-silca rhyolite.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting