New Mexico Geological Society Annual Spring Meeting — Abstracts

Presented here are 27 ⁴⁰Ar/³⁹Ar ages for the Taos Plateau volcanic field (TPVF) in northern New Mexico ani southern Colorado. These ages are from a 111 sample suite collected for a thesis study designed to produce a refined geochronologic framework for the TPVF. The ages are from plateaus in spectra produced from step-heating whole rock separates, and ranges in age from 2.14 to 17.45 Ma with average errors of ±0.15 or less. The samples were mainly collected from eruptive centers of the TPVF, with a smaller amount consisting of sequences through exposed sections of the Servilleta Basalt were multiple flows are present.

The TPVF is one of several dominantly basaltic fields erupted within New Mexico during the last 10 m.y. Located in the Rio Grande Rift in northern New Mexico and southern Colorado, the volcanism of the TPVF is volumetrically dominated by the eruption of the Servilleta Basalt, a series of highly fluid olivine tholeittes. Intermixed spatially and contemporaneously with the Servilleta basalt is volumetrically lesser suite of andesites, dacites, and akalic basalts. Also present is a scattering of older pre-TPVF units exposed by tectonic and erosional processes within the area.

Ages for the Servilleta Basalt range from 3.21±0.08 Ma for a flow north of San Antonio Mountain to 4.33±0.01 Ma for a flow at the base of the Rio Grande Gorge at Dunn Bridge. Other Servilleta Basalt ages are for La Segita Peaks (3.28±0.06, 3.29±0.11, and 3.32±0.12 Ma), a roadcut through two flows north of No Agua on US 285 (3.61±0.13 and 3.82±0.03 Ma), and a sequence from bottom to top in the Rio Grande Gorge at Dunn Bridge (3.95±0.05, 4.00±0.3, and 4.33±0.01). Ages for San Antonio Mountain include 2.99±0.03 and 3.30±0.06 Ma for dacites of the mountain, 2.43±0.04 and 2.72±0.07 Ma for an akalic basalt flow and cone on the north-eastern slope, and 2.49±0.06 Ma for and andesite on the northern slope of the mountain. Other ages for the TPVF are 2.14±0.03 Ma for an akalic basalt at La segita Peaks, 2.32±0.13 Ma for Pinatabosa Peaks, 2.83±0.02 Ma for a flow west of No Aguan, 3.52±0.05 Ma for Cerro del Aire, 3.31±0.08 and 3.82±0.03 Ma for the Cerritos de la Cruz. 3.96±0.06 Ma for Los Mogotes, 4.56 ±0.11 Ma for Cerro de Taos, and 4.60±0.5, 4.77±0.13, 4.85±0.06, and 4.85±0.03 Ma for Tres Orejas. The final age presented here is 17.45±0.04 Ma for a pre-TPVF akalic basalt cone exposed north of Cerro Cheiflo.

The ages presented here for the TPVF are within the commonly cited range of 1.5 to 5.0 Ma. While no ages below 2.14 Ma have been collected as of yet, K-Ar dates for the area suggest that the younger limit for the TPVF is still valid. The ages given for the San Antonio Mountain area are in close agreement with previous K-Ar dates, as is the age for Cerro del Aire. The age range for the Servilleta Basalt however differs at its youngest end member from previously published dates. This is due to the younger ages found for Servilleta Basalt elsewhere in the TPVF, other then those in the Dunn Bridge area, with which this study shows good agreement, which are the commonly cited age ranges. Over all the TPVF shows a fairly well distributed range of ages starting at 4.85 Ma and lasting until 2.14 Ma, with no major breaks or hiatus in activity. Non-basaltic activity within the field appears to be well distributed through out the eruptive sequence. The alkalic basalts within the field seem to have the narrowest range of ages, from 2.43 to 2.83 Ma, but this mayan artifact of the small population measured to date.