HR: 1330h
AN: V12A-0562    [PDF]
TI: The Mafic Holocene Sand Mountain-Nash Crater Chain, Oregon Cascade Range: Preliminary Insights Into Enigmatic Crustal Contamination Processes
AU: * Ramos, F C
EM: framos@wsu.edu
AF: Earth and Marine Sciences, UC Santa Cruz, Santa Cruz, CA 95064 United States
AU: Conrey, R M
EM: conrey@mail.wsu.edu
AF: Dept. of Geology, Washington State University, Pullman, WA 99164 United States
AB: The ca. 3 ka Sand Mtn-Nash Crater chain is a 10 km long bifurcated cinder cone alignment formed during an intra-arc extensional fissure-style eruption in the central Oregon Cascades. Eruptions began with at least 6 separate basalt flows (MgO 6.5 to 8.8 wt %), followed by several flows of two distinct basaltic andesites. Measurable paleomagnetic secular variation is absent (12 of 13 flows sampled; Duane Champion, pers. comm., 2003), suggesting the eruptions spanned at most a few decades. Nash Crater basaltic andesite is similar to the most common Mt. Washington type mafic lava in the Cascades, while Sand Mtn. basaltic andesite is similar to less common Sr-rich (900-1200 ppm Sr) mafic lava. Both basalt and basaltic andesite typically contain olivine and plagioclase phenocrysts, with rare clinopyroxene. Rare glass-bearing gabbroic xenoliths appear to preserve a record of both assimilation and crystallization of plag-oliv-cpx. Each basalt is unique, but basalts collectively record a wide compositional diversity (e.g., K2O 0.65 - 0.90; Ba 230 - 350; Ba/Sr 0.33 - 0.53), which may result from either source variation and/or crustal assimilation. Sr isotopes range from 0.7031 - 0.7034 in basalt whole rocks (WR), and are positively correlated with Ba/Sr ratio. A similar correlation is found in Sr isotopes from Sand Mtn basaltic andesites (WR), which range from 0.7031 - 0.7033. Preliminary laser ablation sampling of Sr isotopes in Sand Mtn. basaltic andesite plagioclase phenocrysts and groundmass demonstrates equilibrium between WR, groundmass, and crystals, strongly suggesting that compositional variation in at least these lavas was acquired below the plagioclase stability field (i.e., in the lower crust or upper mantle). Variations in the more typical Nash Crater basaltic andesites appear to reflect in part mixing with a Sand Mtn type component. Derivation of basaltic andesites from their co-erupted basalts, suggested by the field relations, is not readily apparent. The enigma presented by the compositional variation in these lavas may shed light on common assimilation processes, especially in the deep crust, which likely affect many mafic arc magmas.
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting