Magmatic Recharge during the Formation and Resurgence of the Valles Caldera, New Mexico, USA: Evidence from Quartz Compositional Zoning and Geothermometry

Abstract

The Valles caldera complex in north-central New Mexico, USA, represents the type example of resurgent caldera system, characterised by eruption of two voluminous high-silica rhyolite ignimbrites (the Otowi and Tshirege Members of the Bandelier Tuff) at 1.608 ± 0.010 Ma and 1.256 ± 0.010 Ma, respectively. Refined dating has shown that resurgence occurred shortly after eruption of the Tshirege, or Upper Bandelier Tuff (UBT). Central resurgence of ~1000 m was accompanied by small-volume eruptions of the Deer Canyon Rhyolite, followed closely by the Redondo Creek Rhyodacite. The Cerro del Medio Rhyolite lava dome complex is a product of ring fracture volcanism following resurgence, erupting at 1.229 ± 0.017 Ma. A central aim of this study was to find evidence for magmatic recharge during this geologically short (~ 27 ka) time period. We have combined cathodoluminescence (CL) imaging and titanium-in-quartz geothermometry techniques to individual quartz crystals from 1) different stratigraphic horizons of the UBT ignimbrite, 2) samples of the Deer Canyon Rhyolite and 3) the Cerro del Medio Rhyolite lavas. CL imaging reveals that ~80% of the erupted volume UBT ignimbrite contains unzoned quartz crystals (average concentration = 28 ± 2 ppm Ti), recording relatively isothermal temperatures of 647-696 °C. An abrupt occurrence of compositionally zoned quartz crystals ) within the mid-to-late erupted UBT ignimbrite units 3-5 reveals evidence for interaction with hotter magma. Corresponding titanium-in-quartz measurements of outer, bright CL rims (71 ± 9 ppm Ti) reveal temperature increases of ~100 °C relative to the start of the UBT eruption. We have discovered an interesting heterogeneity within the Deer Canyon Rhyolite lavas, with strong spatial control on eruption of porphyritic lavas containing complexly zoned quartz crystals onto the western regions of the resurgent dome. Conversely, crystal-poor to aphyric lavas containing small, unzoned quartz crystals are conf%%%%Le complexe de caldeiras de Valles au centre-nord du Nouveau-Mexique, Etats-Unis, constitue l'exemple type d'un systeme de caldeira resurgente, caracterise par l'eruption de deux volumineuses ignimbrites rhyolitiques riche en silice (les Membres Otowi et Tshirege de la Bandelier Tuff) a 1.608 ± 0.010 Ma et 1.256 ± 0.010 Ma, respectivement. Des techniques de datation de haute precision ont montre que la resurgence s'est produite rapidement apres l'eruption de la Tshirege ou Upper Bandelier Tuff (UBT). La resurgence du centre de la caldeira d'environ 1000 m s'est accompagnee des eruptions mineures des rhyolites de Deer Canyon, puis de la rhyodacite de Redondo Creek. Le complexe de domes de lave rhyolitiques de Cerro del Medio est le produit de volcanisme de fracture annulaire post-resurgence a 1.229 ± 0.017 Ma. Un point cle de cette etude etait de trouver des evidences de recharge magmatique durant ce court episode ~ 27 ka. Nous avons analyse des cristaux de quartz par imagerie de cathodoluminescence (CL) et geothermometrie du titane dans 1) plusieurs niveaux…