40AR/39AR AGE CONSTRAINTS on THE DURATION OF RESURGENCE AND POST-COLLAPSE RHYOLITES AT VALLES CALDERA, JEMEZ MOUNTAINS, NEW MEXICO

The Valles caldera with its resurgent dome, Redondo Peak, is the type locality for resurgent calderas. The young age (ca. 1.25 Ma) and excellent exposures make Valles caldera an exquisite site to study caldera formation and resurgence. Resurgence occurred after catastrophic eruption of the Tshirege Member of the Bandelier Tuff (BTT) and collapse of Valles caldera. The BTT therefore serves as a lower age constraint on the duration of resurgence, which resulted in at least 1000 m of uplift above the surrounding caldera floor. Resurgence was complete before emplacement of Cerro del Medio (CdM), the oldest post-collapse ring fracture rhyolite (ca 1.23 Ma). The oldest lobes of CdM show no deformation from resurgence and provide an upper time constraint on resurgence. The duration of resurgence was determined by precise ⁴⁰Ar/³⁹Ar dating of these two time constraints. Sanidine yield a mean single crystal laser fusion age of 1.256 ± 0.010 Ma for the BTT and 1.229 ± 0.017 Ma for an older flow of CdM (2σ; ages relative to Fish Canyon tuff sanidine at 28.02 Ma). These ages imply that resurgence occurred within 27 ± 27 ka of caldera collapse. Resurgence therefore was complete within 54 ka and the uplift rate was at least 1.9 cm/yr.

Consideration of mineral xenocrysts is important when comparing such similarly aged rocks. Heterogeneous feldspars that are likely candidates for xenocrystic contamination were identified in CdM rhyolites. However, homogeneous sanidine crystals with no visible evidence of alteration have chemical compositions of Or₄₅Ab₅₃ to Or₄₈Ab₅₀, significantly different from BTT feldspar, which typically range from Or₃₄Ab₆₃ to Or₄₄Ab₅₅. Additionally, ⁴⁰Ar/³⁹Ar age data show only one population of sanidine in this CdM sample.

Further insight into the post-collapse history of Valles caldera is gleaned from ages of the intracaldera Deer Canyon and Redondo Creek rhyolites. These units yield ⁴⁰Ar/³⁹Ar ages ranging from 1.229 ± 0.013 Ma to 1.283 ± 0.017 Ma (n=8) for the Deer Canyon Member and 1.208 ± 0.017 Ma to 1.239 ± 0.017 Ma (n=4) for the Redondo Creek Member. Excepting one sample from each unit, these ages are statistically indistinguishable from the age of the BTT suggesting that they erupted shortly after collapse and continued to erupt during resurgence.