24 Tectonics, Volcanism, and Sediment Dispersal Within and Proximal to the Rio Grande Rift and Southern Rocky Mountains (Posters)

This session integrates research on coupled tectonics, volcanism, and sediment dispersal in the Rio Grande Rift and southern Rockies. We examine how extensional faulting controls volcanic processes and reshapes sediment routing from Colorado through Texas.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

New Petrographic, Mineralogic, and Rock Magnetic Data Bearing on the Tectonic-Magmatic Setting of the Tooth of Time, New Mexico.

Paper number: 24-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Tooth of Time is a notable geological feature in the Cimmaron Range, a subrange of the Sangre de Cristo Mountains of New Mexico. It is a prominent landmark of the Santa Fe Trail and an iconic summit of the Philmont Scout Ranch. The feature is part of a series of stacked laccolithic sheets that comprise the Paleogene Cimmaron pluton (absolute age unknown). This project reports new petrographic, mineralogical, and rock magnetic data that shed insight into the Tooth of Time’s origin and emplacement. Petrographic analysis shows major plagioclase (30%) and quartz (7%) phenocrysts (1.5-3.0 mm diameter) and hornblende (4%) and biotite (4%) subphenocyrsts (0.5-1.5 mm diameter) in a quartz-rich aphanitic groundmass. Minor minerals include disseminated 0.10 mm-diameter equant Fe-Ti oxides along with accessory euhedral titanite crystals. Plagioclase and quartz occur as single phenocrysts or glomerocrysts, indicating fractionation of and accumulation within the parent magma. Quartz phenocrysts range from subhedral polygonal-shaped to round, scalloped, and highly embayed crystals indicative of late-stage magmatic absorption. Alteration is absent in the inner sheet but incipient to pervasive in the outer sheet characterized by sericitized plagioclase and hematite and chlorite overprinting at the edges and in the cleavage planes of hornblende and biotite. Single crystal x-ray diffraction analysis of 4 feldspar crystal separates each from the inner and outer portions of the Tooth of Time sheet reveals oligoclase variety (30% Ca-composition). The modeled crystal structures show a high degree of consistency among the samples and a high degree of disorder within the samples consistent with rapid cooling and shallow emplacement. The rock magnetic experiments show that ferromagnetic minerals dominate the inner sheet samples, mostly magnetite and minor contributions of another Fe-Ti or Fe-S minerals with a Curie temperature around 257 °C. The rock magnetic results from the outer sheet samples are dominated by paramagnetic minerals supporting the petrographic observations of significant alteration. 40Ar/39Ar age dating, in collaboration with the New Mexico Bureau of Geology and Mineral Resources, is pending. We submit, based on rock composition and igneous structure, that the Tooth of Time sheet and its parent Cimmaron Pluton were emplaced during compression related to the waning stages of the Laramide orogeny prior to Rio Grande rift extension and mafic volcanism that dominate the eastern Great Plains.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410711

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