HR: 1340h
AN: V13B-0542    [Abstracts]
TI: Compositional and Textural Evidence for Crystal Ancestry in the Voluminous Silicic Magmas of the Bruneau-Jarbidge Eruptive Center, Yellowstone Hotspot
AU: * Cathey, H E
EM: Cathey@earth.utah.edu
AF: University of Utah, Dept. of Geology and Geophysics 135 S. 1460 E. Rm 719, Salt Lake City, UT 84112 United States
AU: Nash, B P
EM: Nash@earth.utah.edu
AF: University of Utah, Dept. of Geology and Geophysics 135 S. 1460 E. Rm 719, Salt Lake City, UT 84112 United States
AB: The Bruneau-Jarbidge eruptive center (BJEC) of southern Idaho, USA, offers an opportunity to address the problem of crystal ancestries in large silicic volcanic systems and their origins as phenocrysts, xenocrysts, or antecrysts. This large intracontinental system was powered by the Yellowstone hotspot from 12.7 to approx. 8 Ma and lies in contradistinction to both the present day Yellowstone system and voluminous arc systems in that its estimated discharge rate of rhyolitic magma is exceptionally high (2300 km³/Ma), and the mineral assemblages reflect equilibrium conditions and near liquidus temperatures of 900-1000° C. Detailed observations of mineral compositions and textures in tuffs and lavas from this system bear on current ideas concerning the relative roles of rejuvenation and multiple source inputs in magma generation and their relevance to the problem of magma residence times. Silicic magmas of the BJEC are represented by a suite of more than twenty large-volume crystal poor rhyolitic tuffs (100-1000 km³ ea) and lavas (up to 200 km³ ea). The tuffs (known collectively as the Cougar Point Tuff) and lavas share a sparse anhydrous mineral assemblage and show striking similarities to one another with respect to mineral textures and compositions. These similarities demand an exceptional but systematic petrogenesis that is poorly understood. Many tuffs share identical doublets or triplets of glass and pyroxene compositional modes, and some units are nearly indistinguishable from one another despite significant intervals between eruptions (up to 1 Ma). Multiple discrete modes are present at the hand specimen level consistent with the syn-eruption of multiple discrete magma volumes. However, lavas succeeding the tuffs are compositionally unimodal, with a collective compositional spectrum of discrete modes that overlaps that of the tuffs. Mineral textures observed in thin section support the coexistence of phenocrysts and inherited material that may derive from source rocks and/or rejuvenated mush from earlier magma reservoirs. However, mineral compositions confound the effort to detect different crystal histories because mineral phases in contrasting textural contexts have homogeneous, identical compositions. Also, multiple modes are not found in crystal clusters, xenoliths, or glomerocrysts, which might be expected if magmas had mingled or mixed prior to eruption. Such compositional homogeneity despite textural variety, especially in the lavas, suggests a common source (represented by xenoliths and xenocrysts) rather than multiple sources, and perhaps that the source and the magmas with their crystal cargo were thermally equilibrated and spatially juxtaposed prior to eruptions.
DE: 8404 Volcanoclastic deposits
DE: 8415 Intra-plate processes (1033, 3615)
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005