Recent models for the Cenozoic tectonic evolution of the western margin of North America propose that delamination of ancient lithosphere accompanied asthenospheric upwelling, magmatism, and uplift subsequent to Laramide deformation. On the basis of the age of an alkaline dike in south-central Montana, thermometry of mantle xenoliths from the dike, and Sr, Nd, and Pb isotopic compositions of the dike and a xenocryst, we show that refractory lithosphere, derived from ancient, depleted mantle, remained in place under the Wyoming Craton as late as 42 Ma. The Haymond School Dike, a camptonite, yields a 40Ar/39Ar plateau date of 41.97 ?? 0.19 Ma (2??). Paleomagnetic data are consistent with this date and indicate intrusion during chron C19r. The dike has Sr, Nd, and Pb isotopic compositions similar to those of other Eocene alkaline rocks from central Montana. A clinopyroxene megacryst from the dike has ??42 = 17, and 87Sr/86Sr = 0.70288, indicating that it derives from ancient, depleted mantle isotopically distinct from the source of the host camptonite. Thermometry of xenoliths from the dike shows pyroxene populations that formed at 880?? and 1200??C. Combining thermometry with previous estimates of the regional Eocene geotherm inferred from xenoliths in kimberlites, and with the Al-in-orthopyroxene barometer, we infer that lithospheric mantle remained intact to depths of 110-150 km as late as 42 Ma. Eocene magmatism was not accompanied by complete removal of ancient lithosphere.