Latest Eocene 40Ar/39Ar age from the Yeso Hills Dikes, Eddy County, New Mexico

The Yeso Hills dikes in southeastern New Mexico are the only surface expression of a minor set of latest Eocene–earliest Oligocene, mafic, alkaline intrusions stretching for ~150 km in the subsurface of the northern Delaware Basin. Preliminary mapping reveals a complex intrusive architecture, with each intrusion being composed of multiple dikes interconnected by sills. New high-precision 40Ar/39Ar geochronology analysis provides a latest Eocene age of 34.45±0.28 Ma for the Yeso Hills dikes. We propose that the Yeso Hills dikes are part of a coherent “Cordilleran magmatic peripheral belt” that may stretch as far north as the Canadian border in central Montana, composed of minor intrusions derived from small-volume melting beneath the western edge of the Great Plains. This distal magmatism represents farther east outliers distinct from the North American alkaline magmatic belt of the Cordilleran interior.