SUBDUCTION OR RIFTING: THE EOCENE MAGMAS OF THE SACRAMENTO MOUNTAINS, NM

Middle to late Eocene intrusions of the Sacramento Mountains in south-central New Mexico (ESS, Eocene Sacramento Suite) were intruded at the eastern extent of the subducting Farrallon plate, approximately 1000 km inward from the edge of North America. There has been much debate as to whether these intrusions were produced at the very end of subduction or the very beginning of Rio Grande rift extension. The ESS was intruded ca. 80 km east of the eastern edge of a coeval continental arc, represented by the Rubio Peak and Palm Park Formations. The ESS consists of NE trending dikes and subhorizontal sills with ⁴⁰Ar/³⁹Ar dates of hornblende of 44.01 + / -0.15 Ma, 41.13 +/- 0.42 Ma, 36.32 +/- 0.35 Ma, and 36.11 +/- 0.32 Ma. Ages decrease from south to north; it is not clear whether this is a real trend or an artifact of the scarcity of dates. The ESS is alkalic and mafic with MgO ranging from 1.03% to 5.67 wt% and SiO₂ between 48.39% and 64.38 wt%. The majority of the ESS consists of nepheline normative trachybasalts, basaltic trachyandesites, trachyandesites, dacites. Concentrations of high field strength elements divide the suite into two distinct groups. The first group of samples has low Nb (< 10 ppm) and moderate TiO₂ (0.5 – 1.2 wt%). The second group of samples has higher Nb (10 – 35ppm) and higher TiO₂ (1.2 – 1.6 wt%). The only dated sample in the second group is the youngest and northernmost of the suite; additional age determinations are needed to determine whether the differences in trace element geochemistry are temporal or spatial in origin.

Potential tectonic models for the ESS are 1) back arc (shoshonitic) magmatism behind the Eocene arc, 2) an eastward continuation of subduction related magmatism, or 3) earliest Rio Grande rift magmatism. The ESS contrasts with Andean shoshonites by having K₂O/Na₂O less than 1, by having low concentrations of Nb, TiO₂, and other high field strength elements, and by being silica-undersaturated. The volcanic rocks of the coeval arc are silica-oversaturated, suggesting the ESS is of different origin. However, the chemical characteristics of the ESS are similar to the earliest Rio Grande rift basalts. Thus, we conclude the Eocene magmas of the Sacramento Mountains of New Mexico represent the earliest rift magmas.