HR: 0800h
AN: V21B-0606    [Abstracts]
TI: Generation of Magmas Within the Southwest Nevada Volcanic Field: Constraints Based on Trace Element Concentrations in Melt Inclusions and Sanidine
AU: * Tefend, K S
EM: tefendka@msu.edu
AF: Michigan State University, Department Geological Sciences, East Lansing, MI 48824-1115
AU: Vogel, T A
EM: vogel@msu.edu
AF: Michigan State University, Department Geological Sciences, East Lansing, MI 48824-1115
AU: Patino, L C
EM: patinol@msu.edu
AF: Michigan State University, Department Geological Sciences, East Lansing, MI 48824-1115
AB: The southwest Nevada volcanic field contains four large compositionally zoned ash-flow tuffs, which are among the best studied in the world. This study presents trace element data from melt inclusions and sanidines in order to evaluate interpretations that the compositional zoning is due to the emplacement of discrete magma batches. The earlier ash-flows are the Topopah Spring (13.4 Ma, 1200 km$^{3}$) and the Tiva Canyon (12.9 Ma, ~900 km$^{3}$) tuffs. The later ash-flow tuffs are the Rainier Mesa (11.6 Ma, 1200 km$^{3}$) and the Ammonia Tanks tuff (11.4 Ma, 900 km$^{3}$), which erupted following a period of major extension that occurred within this region of the southern Great Basin. Each unit consists of a lower portion, dominated by rhyolitic pumice fragments and an upper portion dominated by more mafic pumice fragments - each of these contain distinct Sr, Nd and $\delta$$^{18}$O isotopic compositions. These data are inconsistent with fractional crystallization relating the rhyolitic and mafic portions within each ash-flow tuff. In addition, the compositional variation among ash-flow units cannot be related by fractional crystallization. All melt inclusions within phenocrysts from the more mafic pumice fragments of each tuff have identical trace element concentrations. Furthermore, melt inclusions within the high-silica pumice fragments of each tuff also have identical trace element concentrations, except for a group of high-Rb, high-Nb inclusions from Rainier Mesa. Magma mixing occurred during evolution of each tuff, and is recorded in both the melt inclusions and in the host sanidines. These mixing events are subtle in the Topopah Spring tuff, but extensive mixing produced magmas of intermediate composition within the Tiva Canyon, Rainier Mesa, and Ammonia Tanks tuffs. The chemical analyses of pumice fragments from the Topopah Spring, Tiva Canyon, and Ammonia Tanks tuffs are consistent with their generation from a common source. However, Rainier Mesa magmas are different, having been formed from a more Th-enriched, La-depleted source region with a higher $\delta$$^{18}$O signature, and the existence of melt inclusions with unusual compositions. Comparison of isotopic compositions among ash flows indicates an increase in mantle melt contribution over time. However, the Rainier Mesa source region was sufficiently high in $\delta$$^{18}$O so that this signature was dominant.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting