HR: 16:45h
AN: V52G-04    [PDF]
TI: $^{40}$Ar/$^{39}$Ar Age Constraints on the Timing and Duration of Resurgence of the Valles Caldera, New Mexico
AU: * Phillips, E H
EM: ehp@nmt.edu
AF: Department of Earth and Environmental Science and New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: McIntosh, W C
EM: mcintosh@nmt.edu
AF: Department of Earth and Environmental Science and New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: Kyle, P R
EM: kyle@nmt.edu
AF: Department of Earth and Environmental Science and New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: Goff, F
EM: fraser@lanl.gov
AF: Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Gardner, J N
EM: jgardner@lanl.gov
AF: Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AB: New $^{40}$Ar/$^{39}$Ar ages for rocks from the Valles caldera of New Mexico imply that resurgent uplift of the caldera floor occurred within at most, 51,000 years of caldera collapse. Redondo Peak, part of the resurgent dome of the Valles caldera, was uplifted approximately 1000 m above the surrounding caldera floor during resurgence. The lower time constraint on resurgence is the eruption of the upper Bandelier Tuff, which occurred concurrently with collapse of the Valles caldera. The upper time constraint is the emplacement of the oldest post-collapse ring fracture dome complex, Cerro Del Medio, which displays no apparent faulting or deformation due to resurgence. $^{40}$Ar/$^{39}$Ar dating places these time constraints at 1.256 $\pm$ 0.009 Ma (2$\sigma$; ages relative to Fish Canyon Tuff sanidine at 28.02 Ma) for the upper Bandelier Tuff and 1.231 $\pm$ 0.017 Ma for Cerro Del Medio. Therefore, resurgent uplift of the Valles caldera resurgent dome occurred at a minimum rate of approximately 2 cm/yr. The Deer Canyon and Redondo Creek members of the Valles Rhyolite formation erupted prior to and during resurgence; both units stratigraphically overlie the upper Bandelier Tuff. The Deer Canyon member erupted immediately prior to resurgence, and is mainly overlain by caldera fill sediments. The Redondo Creek member erupted before and during resurgence, and is intensely faulted and tilted by resurgent uplift. It is mainly interbedded with caldera fill sediments. The Deer Canyon member is characterized by a phenocryst assemblage that includes abundant sanidine and quartz, minor oxides, and rare plagioclase and zircon. The Redondo Creek member differs from other intracaldera units, with phenocrysts of abundant biotite and plagioclase (often rimmed by sanidine), minor oxides and homogeneous sanidine, and rare pyroxene and zircon. Magnetite/ilmenite pairs from the Redondo Creek member yield eruption temperatures of 810 $\pm$ $6\deg$C. Preliminary whole rock analyses of the Deer Canyon and Redondo Creek members show that they are rhyolitic in composition and may be related to the upper Bandelier Tuff magma. Sanidine phenocrysts from the Deer Canyon member yield $^{40}$Ar/$^{39}$Ar single crystal laser fusion ages ranging from 1.257 $\pm$ 0.022 Ma to 1.281 $\pm$ 0.017 Ma (n=5). With one exception at 1.208 $\pm$ 0.017 Ma, the Redondo Creek member gives ages between 1.237 $\pm$ 0.018 Ma and 1.292 $\pm$ 0.043 Ma (n=3). Field work demonstrates that the Deer Canyon and Redondo Creek members are younger than the upper Bandelier Tuff. However, most ages from the Deer Canyon and Redondo Creek members are within 2$\sigma$ error of the upper Bandelier Tuff age (1.256 $\pm$ 0.009 Ma), suggesting that resurgence commenced soon after collapse of the Valles caldera and eruption of the upper Bandelier Tuff. The possibility of excess argon in melt inclusions in post-collapse units was investigated. Although $^{40}$Ar/$^{39}$Ar analysis of melt inclusion bearing quartz phenocrysts from the Deer Canyon member demonstrates that some amount of excess argon resides in melt inclusions, there is no measurable effect on sanidine phenocrysts in either the Deer Canyon or Redondo Creek members.
DE: 1035 Geochronology
DE: 3640 Igneous petrology
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting