23 Distributed volcanic fields of the southwestern U.S. (Posters)
This session invites submissions from various disciplines (e.g., petrology, tectonics, geophysics, geochronology) that explore distributed volcanism in the southwestern U.S. Topics of interest include spatio-temporal trends, magma compositions, and mantle and/or crustal controls on volcanism.
Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E
Did Cenozoic Volcanism in the Colorado Plateau Extend Farther in Space and Time Than Previously Accepted? Insights From the Pemada Canyon Dike.
Paper number: 23-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM
Authors
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Kevin HobbsNew Mexico Bureau of Geology and Mineral Resources, Socorro, NM
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Julia RicciNew Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico
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Laura WatersNew Mexico Tech, Socorro, NM
Abstract
The Colorado Plateau in New Mexico hosts several distributed volcanic fields. Studies of these fields’ ages, compositions, and petrogenesis have offered unique perspectives into the region’s geologic history. The Navajo volcanic field (NVF), Dulce dike (DD) swarm, and San Juan mafic dikes are examples of recorded magmatic activity that occurred primarily from the Oligocene, yet some persisted into the Miocene. These dikes are localized within the central and eastern regions of the plateau, yet amongst them resides an unstudied outlier near Aztec, New Mexico, that may shed further insight into the magmatic activity that occurred throughout the Cenozoic. Here we present new geochronology, geochemistry, and petrology results from the anomalous outlier.
The Pemada Canyon dike (PCD) is a standalone lamprophyric dike located near the center of the San Juan Basin (SJB). Preliminary geochronological studies suggest a Miocene age of emplacement. The PCD is ultramafic in composition, with low silica content (SiO₂ 27.8-37.7%) and elevated alkaline levels (Na₂O 0.313-1.72%, K₂O 0.453-2.02%). Samples show high levels of Ni (500 to >2000 ppm), Cr (550 to 950 ppm), and Cu (200 to >2000ppm), with some samples harboring Pb concentrations >1 wt% and Ag contents >800ppm. Thin section evaluations revealed a matrix of clinopyroxenes alongside olivine and phlogopites with rare primary calcite. 40Ar/39Ar analyses performed on groundmass suggest an emplacement age of about 10.0-10.8 Ma. New bulk geochemical analyses confirm the dike’s ultramafic and potassic composition, similar to other lamprophyres from within the NVF and San Juan Mountains mafic dikes. Textural evidence supports the interpretation that the PCD was emplaced during multiple injection events.
The data presented here suggest that the PCD could represent an incident of magmatism similar to the mafic dikes [spessartites (porphyritic hornblende lamprophyre) and kersantites (hypabyssal micacious lamprophyre)] from the San Juan Mountains, as the two share similar silica-undersaturated, alkali-rich compositions. They may also exhibit a structural-temporal relationship as their ages align with an interval of mantle-derived volcanism dated to 4-10 Ma which follows a N-S orientation. As this project continues, we will explore some of its unidentified components, including outlining its boundaries, limiting the timing of formation, observing its mineral modal abundance and composition, identifying its magmatic origin, and establishing its connection to other dikes in the area.
Geological Society of America Abstracts with Programs. Vol. 58, No. 4
doi: 10.1130/abs/2026RM-410694
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