Collapse and Resurgence of the Valles Caldera, Jemez Mtns, NM- 40Ar/39Ar Ages of Megabreccia Blocks and Age Constraints on Timing and Duration of Resurgence

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Thesis: Collapse and Resurgence of the Valles Caldera, Jemez Mtns, NM- 40Ar/39Ar Ages of Megabreccia Blocks and Age Constraints on Timing and Duration of Resurgence

Abstract
40Ar/39Ar ages for rocks from the Valles caldera of New Mexico imply that resurgent uplift of the caldera floor occurred within 27 &plusmin; 27 k.y. of caldera collapse. Redondo Peak, part of the resurgent dome of the Valles caldera, was uplifted at least 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. 40Ar/39Ar dating places these time constraints at 1.256 &plusmin; 0.010 Ma (2�) for the upper Bandelier Tuff and 1.229 &plusmin; 0.017 Ma for Cerro del Medio. Therefore, resurgent uplift of the Valles caldera resurgent dome occurred at an average rate of ~3.7 cm/yr and probably not less than 1.9 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 and are intricately interbedded with caldera fill sediments. Sanidine phenocrysts from the Deer Canyon Member yield 40Ar/39Ar single crystal laser fusion ages ranging from 1.229 &plusmin; 0.013 Ma to 1.283 &plusmin; 0.017 Ma (n=7). 40Ar/39Ar ages for the Redondo Creek Member range from 1.208 &plusmin; 0.017 Ma to 1.239 &plusmin; 0.017 Ma (n=2). With one exception from the Deer Canyon Member and one from the Redondo Creek Member, all ages obtained for these units are indistinguishable from the upper Bandelier Tuff at the 2� confidence interval, indicating that eruption of these post-collapse rhyolites probably commenced very soon after caldera formation. The possibility of excess argon in melt inclusions in post-collapse units was investigated. Although 40Ar/39Ar 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.Whole rock analyses of the Deer Canyon and Redondo Creek Members show that they are rhyolitic in composition and could have been part of the zoned upper Bandelier Tuff magma chamber. One possible scenario for their origin is one in which residual, zoned magma was left in the chamber after the eruption of the upper Bandelier Tuff and subsequently tapped after caldera collapse. This residual magma included the Deer Canyon rhyolites in the mid to upper parts of the felsic portion of the chamber and the Redondo Creek rhyolitic magma near the base. This model is supported by high Ba, Sr, and Ti content in the Redondo Creek Member, which is similar to enrichments observed in the least evolved upper Bandelier Tuff. In addition, magnetite/ilmenite pairs from the Redondo Creek Member yield eruption temperatures of 810 &plusmin; 6°C, which correspond to published temperatures for the latest erupted upper Bandelier Tuff.Megabreccia blocks are found surrounded by intracaldera upper Bandelier Tuff in the Valles caldera and provide insights into the events that were occurring during collapse. The distribution and sources of intracaldera megabreccia blocks are highly variable. 40Ar/39Ar ages of selected megabreccia blocks indicate that they are composed of older pre-caldera units that include the lower Bandelier Tuff (1.68 &plusmin; 0.03 Ma), and a dacitic tuff dated at 8.205 &plusmin; 0.083 Ma. The ages of these blocks and their field relationships suggest that they were emplaced due to catastrophic sliding of oversteepened caldera walls during and immediately after collapse.

Author 
Erin H. Phillips





Organization 
New Mexico Institute of Mining and Technology



Published 
Publisher Not Provided, 2004





DOI 
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Citation

Erin H. Phillips. 2004. Collapse and Resurgence of the Valles Caldera, Jemez Mtns, NM- 40Ar/39Ar Ages of Megabreccia Blocks and Age Constraints on Timing and Duration of Resurgence [Thesis]. [Socorro, N.M.]: New Mexico Institute of Mining and Technology.