HR: 1340h
AN: V33C-1509    [Abstracts]
TI: Postcollapse Volcanism in the Valles Caldera, New Mexico: Magma System Dynamics During a Transition Between Large Volume Explosive and Small Volume Effusive Eruptions
AU: * Spell, T L
EM: terry.spell@unlv.edu
AF: University of Nevada, Las Vegas, Department of Geoscience 4505 Maryland Parkway, Las Vegas, NV 89154, United States
AU: Gibler, K I
EM: kgibler@converseconsultants.com
AF: Converse Consultants, 731 Pilot Road, Suite H, Las Vegas, NV 89119, United States
AB: The Valles Caldera formed during eruption of the ~250 km³ upper Bandelier Tuff at 1.26 Ma. Following caldera collapse a series of 3 intracaldera rhyolites (Deer Canyon, Redondo Creek and Del Medio) erupted within 54 ka. They are petrographically diverse, ranging from coarsely porphyritic (Deer Canyon type B, Redondo Creek) to nearly aphyric (Deer Canyon type A, Del Medio) and contain distinctive phenocryst assemblages. Disequilibrium textures are common and include resorbed quartz, strongly resorbed plagioclase, antirapikivi feldspars, and zoned sanidine and plagioclase. Plagioclase in Deer Canyon rhyolite ranges widely from labradorite (An₆₀) to oligoclase (An₂₄). Most are andesine in composition and often exhibit oscillatory zonation with overgrowths up to ~200 um wide. Alkali feldspars include both anorthoclase and sanidine with a total variability in Or content of ~50%. Plagioclase and alkali feldspars in Redondo Creek rhyolite show less variability, but similarly large ranges. One phenocryst exhibits an andesine core, oligoclase overgrowth, and sanidine rim. Del Medio rhyolite lacks plagioclase, but contains alkali feldspars (both anorthoclase and sanidine) with variability in Or content of ~30%. Patchy zonation is common, with Or content within phenocrysts varying by ~7%. Redondo Creek rhyolite ranges from ~72-74 wt.% SiO₂ whereas Deer Canyon and Del Medio are high silica rhyolites with ~76-78 wt.% SiO₂. Trace elements show more significant variability. Redondo Creek samples have low Nb, Rb, Sc, and Lu and high La, Sr, Ba and Hf compared to Del Medio samples. In contrast, Deer Canyon samples exhibit extreme variability in trace element concentrations (e.g., Nb ranging from 32-83 ppm). Incompatible trace element ratios (Th/Nb, Th/Yb, Ta/Yb) versus Nb plots show that Redondo Creek and Del Medio samples have distinctive, limited compositions, whereas Deer Canyon exhibits widely varying values (e.g., Th/Yb ranging from ~3.5-10.5). ²³⁸U/²⁰⁶Pb ion microprobe dating of zircons from Redondo Creek rhyolite show crystallization ages from 0.97 Ma to 1.60 Ma, with a mean age of 1.29 ± 0.15 Ma. These ages range from the timing of eruption of Redondo Creek rhyolite to the eruption of the lower Bandelier Tuff (1.61 Ma). In contrast, zircons from Deer Canyon rhyolite (type A) range from 4.8 to 5.6 Ma, with a mean age of 5.3 ± 0.3 Ma. All zircons are >3.6 Ma older than the eruption age of the Deer Canyon rhyolite. No zircons were recovered from Del Medio samples. These data indicate that postcollapse rhyolites erupted within 54 ka of caldera collapse represent distinct magma batches which are not comagmatic with each other, nor with the preceding upper Bandelier Tuff. Deer Canyon rhyolite represents two distinct melt compositions which were mingled and erupted quickly, such that sufficient time for homogenization did not occur. Type A Deer Canyon rhyolite was produced by remelting of a pre-caldera pluton at depth, whereas type B rhyolite originated within the residual Bandelier crystal mush by crystal-liquid separation. Redondo Creek rhyolite was produced by interaction of a hotter, more mafic magma with residual Bandelier crystal mush, production of new rhyolitic melt and crystal transfer, followed by interaction of this magma with a third, hotter, more K-rich rhyolitic melt prior to eruption. Del Medio rhyolite appears to be a single batch of crystal poor rhyolite separated from residual Bandelier crystal mush.
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1115 Radioisotope geochronology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting