New Mexico Geological Society Annual Spring Meeting — Abstracts

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 topographic landmark of the Santa Fe Trail and an iconic attribute of the Philmont Scout Ranch. The Tooth of Time is a trachydacitic sill that is part of a series of stacked laccolithic bodies that constitute the Paleogene Cimmaron pluton (numerical age unknown). We conducted petrographic, single crystal x-ray diffractometry, and rock magnetic analyses to gain insight into the Tooth of Time’s origin and emplacement. Petrographic analysis shows major (30%) plagioclase 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 0.10 mm-diameter equant Fe-Ti oxides disseminated throughout the samples, along with accessory euhedral titanite crystals. Plagioclase and quartz occur as single phenocrysts or glomerocrysts, suggesting fractionation of and accumulation within the parent magma. Quartz phenocrysts range from subhedral polygonal-shaped crystals indicative of formation in the presence of melt to round, scalloped, and highly embayed forms indicative of late-stage magmatic absorption. Alteration is absent in the upper sheet but incipient to pervasive in the lower sheet characterized by sericitized plagioclase and recrystallized hornblende and biotite defined hematite and chlorite overprinting at crystal edges and in cleavage planes. Single crystal x-ray diffractometry analyses of 8 feldspar crystal separates from the lower (4) and upper (4) portions of the Tooth of Time sheet confirm plagioclase of the oligoclase variety (30% Ca-composition). The modeled crystal structure shows a high degree of uniformity among the samples and a high degree of disorder within the samples consistent with rapid cooling and shallow emplacement. Thermomagnetic experiments show that ferromagnetic minerals dominate the inner sheet samples, mostly magnetite with a minor contribution of a Fe-Ti mineral with a Curie temperature around 257 °C, most likely a titanomaghemite. The magnetic hysteresis curves and First-Order-Reversal-Curve (FORC) measurements indicate the presence of coarse-grained multidomain magnetite and a minor amount of smaller-grained minerals. The rock magnetic results from the outer sheet samples are dominated by paramagnetic minerals with a very small amount of fine-grained, high coercivity minerals such as hematite. Rock magnetic results indicate that alteration in the outer sheet significantly affected the magnetic mineralogy, which supports the incipient alteration observed in its thin section. 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.