Volcanology, geochemistry and structural geology of the Upper Cretaceous Hidalgo Formation, southwestern New Mexico
— John R. Young, Nancy J. McMillan, Timothy F. Lawton, and Richard P. Esser

The Hidalgo Formation represents the proximal cone-forming lithofacies association of a dominantly andesitic, Laramide stratovolcano. New ⁴⁰Ar/³⁹Ar age determinations of 70.53 ± 0.48, 70.69 ± 0.44 Ma, and 71.44 ± 0.38 Ma demonstrate an Upper Cretaceous age of volcanism. Early Hidalgo volcanism is recorded by plagioclase-bearing lava flows, lapilli tuff, ash-fall tuff, and debris-flow breccia intercalated with sandstone, shale and conglomerate of the underlying Upper Cretaceous Ringbone Formation. The base of the Hidalgo Formation is marked by an abrupt transition to predominantly autoclastic, monolithologic and heterolithologic breccia, and cogenetic massive lava flows with subordinate intrusions and sedimentary deposits. Laramide shortening accompanied volcanism and resulted in the folding of the Hidalgo Formation into an overturned footwall syncline adjacent to the Hidalgo thrust fault. Whole-rock analyses of 19 lava, intrusion, and cumulate xenolith samples indicate that Hidalgo magmas range from basalt to andesite (48-60% SiO₂, 2,7-7.3% MgO). Hidalgo lavas were variably altered during cooling, diagenesis, and intrusion of the mid-Tertiary Eureka complex; the rocks now contain high and variable volatile contents (1.3-8.5% loss on ignition). Concentrations of the soluble elements K, Na, Rb, Ba, and Sr have been modified and do not reflect the original igneous concentrations; however, insoluble elements such as the rare earth elements, Ta, Ti, Zr, and Hf, appear to have been unaffected by alteration. Fractionation of the observed mineral assemblage plagioclase + hornblende + augite in lavas, dikes, and cumulate xenoliths is consistent with major element trends and a slight depletion of the middle rare-earth elements in evolved lavas, Low Ta/La ratios (0.035-0.076) are similar to modern continental arcs and suggest that Hidalgo volcanism was triggered by subduction of the Farallon Plate during Laramide shortening.