New Mexico Geological Society Annual Spring Meeting — Abstracts

The Uvas Basaltic Andesite (UBA) is a series of lava flows, cinder cones, and dikes emplaced between the post-Laramide volcanism of the Mogollon-Datil volcanic field and the early stages of magmatism in the Rio Grande rift. The timing of volcanism has been poorly constrained with only some bracketing ages and unpublished K-Ar dates. In addition, the source of volcanism is still unknown; both asthenospheric vs. lithospheric mantle sources have been postulated. This study employs high-resolution ⁴⁰Ar/³⁹Ar geochronology (n = 9) to constrain the eruptive age of lava flows from five localities, showing that volcanism lasted from as early as 29.13±0.19 Ma to 27.39±0.03 Ma. This timeframe includes an eruptive sequence that comprises the interlayered Bell Top Formation Tuff 7, previously dated at 28.57±0.03 Ma, and the Thurman Formation, previously constrained by a single crystal 40Ar/39Ar sanidine maximum deposition age of 27.4 Ma.

Most of the UBA lava flows have higher LILE/HFSE ratios, higher LREE/HREE ratios, and negative Nb-Ta anomalies. Furthermore, the low ε_Nd values (-4.3 to -3.8) for these samples mirror those of other basaltic rocks associated with post-Laramide volcanism (e.g., Southern Cordillera Basaltic Andesites suite) suggesting that the source region was an enriched sub-continental lithospheric mantle.

In contrast, the geochemical signatures of the two youngest UBA samples, collected from Rincon Hills and located directly above potential early rift-related sediments of the Thurman Formation, suggest the incorporation of material from a more depleted and likely asthenospheric mantle source, in line with rift-related magmatism. These samples exhibit a lower LILE/HFSE ratio, reduced LREE/HREE ratios, the absence of a negative Nb-Ta anomaly, and a higher εNd value (+2.5). This transition to a geochemical signature characteristic of a rift-related depleted mantle melt source, occurring alongside the initial stages of rifting in the southern Rio Grande rift, is consistent with the mixing of asthenospheric mantle melts into the magmatic system, marking a temporal link between the geochemical evolution and the onset of rifting.

In summary, this study employs high-resolution ⁴⁰Ar/³⁹Ar geochronology to constrain the eruptive period of the Uvas Basaltic Andesite, revealing a geochemical shift indicative of a source transition from an enriched lithospheric mantle source to a more depleted and likely asthenospheric mantle source in conjunction with the initial stages of rifting within the southern Rio Grande rift.