New Mexico Geological Society Annual Spring Meeting — Abstracts

Ongoing geochronology research of volcanic fields from northern NM and southern CO provide insight into the timescales of postcaldera processes and associated hazards. A small, but growing dataset of ultra-high-precision Ar/Ar ages of ring-fracture domes at Valles caldera, combined with published ages, suggests a change in the eruption style and duration of individual domes throughout the current postcaldera stage. Published ages for two of the earliest postcaldera domes, Cerro del Medio and Cerro Santa Rosa, indicate total dome growth episodes of 130 ± 29 ka and 128 ± 19 ka, respectively. New geochronology of the younger domes indicates short-lived eruptive cycles where the ages of individual flows for each dome are indistinguishable, or define dome growth and repose periods of less than 10 ka. A comprehensive dating campaign, in conjunction with new isotope geochemical methods that involve analyzing fused single crystals from the Ar/Ar analyses, is planned for the next several years to test our hypotheses of dome eruption evolution, assess related hazards at Valles caldera, and determine best practices for interpreting ultra-high-precision sanidine ages. In contrast to dating volcanic rocks in Quaternary systems, which provides snapshots of rapid or short-lived events, geochronology of intrusive rocks at eroded mid-Tertiary systems offers insight into prolonged postcaldera magmatic processes. Numerous studies have shown that the duration of postcaldera pluton emplacement commonly exceeds that of postcaldera volcanism and can continue for as much as 5 Ma after caldera formation. Nearly completed research on the Dulce-Platoro dike swarm, which originates from the ca. 28.6-30.1 Ma Platoro caldera of the Southern Rocky Mountain volcanic field and extends nearly 125 km south into northern NM, shows that dike swarm emplacement may be a previously under appreciated postcaldera process. Approximately 40 new Ar/Ar ages indicate that the Dulce-Platoro dike swarm was emplaced in a 15 Ma period both before and after caldera collapse, although most dikes were emplaced between 24 and 27 Ma during prolonged consolidation and crystallization of the subcaldera magmatic system. Similarly, a new age of 23.71 ± 0.60 Ma for the Tinaja Dike, exposed along I-25 south of Raton, suggests that it may be related to postcaldera magmatism associated with the 25.4 Ma Questa caldera, located approximately 80 km to the west. Dating of nearby Tertiary intrusions that crop out on the High Plains will test this preliminary interpretation. At both mid-Tertiary caldera systems erosion has removed any direct evidence that would indicate whether the injected dikes reached the surface and erupted. Regardless, widespread and shallow dike emplacement could have certainly caused damaging seismicity as well as scattered, small-volume eruptions. These examples, as well as those from other caldera systems around the globe, suggest that proximal and distal magmatic-tectonic hazards should be considered for volcanic fields currently experiencing postcaldera unrest.