New Mexico Geological Society Annual Spring Meeting — Abstracts

A series of Eocene intrusions in the Sacramento mountains of south-central New Mexico were emplaced as the tectonic regime of western North America shifted from Laramide subduction to Rio Grande rift/Basin and Basin extension. The suite contains dioritic sills and a cogenetic NE-trending dike swarm. 40Ar/39Ar age determinations for three sets of sills suggest that intrusion occurred over at least an 8 million year period (sills at Oliver Lee State Park: 44.01±0.15 Ma; McLemore (1998); sill at Ortega Peak: 41.13±0.42 Ma; and sill at Three Rivers Petroglyph site: 36.32±0.35 and 36.11±0.32 Ma). Fresh material from dikes has yet to be retrieved from radiometric dating.

The ESI (Eocene Sacramento Intrusions) have the following unusual geochemical signature: 1) high alkalis, 2) nepheline-normative compositions, 3) low concentrations of high field strength elements (HFSE, Nb, Ti, Zr), and 4) K₂O < Na₂O. These characteristics preclude an arc origin for the ESI, because arc magmas are rarely, if ever, silica-undersaturated. Although the paleogeographic position of the ESI is consistent with a back-arc, or shoshonitic model, the ESI lack key characteristics of shoshonites (Druelle, 1981): K₂O > Na₂O, silica-oversaturation, and high HFSE consentrations. A model that is consistent with all the data is that the ESI are the easternmost early rift magmas, produced from the subcontinental lithosphere as asthenosphere migrated into the space previously occupied nu the subducted Farallon Plate. ESI compositions are similar to early-rift magmas erupted in south-central New Mexico that have been interpreted as partial melts of subduction-modified lithospheric mantle.