Potassic mafic intrusions at Two Buttes, Colorado, enable examination of melt generation and mantle evolution at the eastern extent of Oligocene–Eocene magmatism in the western United States, and comparison of these processes to those in similar tectonic settings but within mantle lithosphere of contrasting age. Average K-Ar age determinations from four intrusions yield a 36.8 ± 0.4 Ma (1-σ) age for the Two Buttes complex. This late Eocene age precludes a genetic connection with inception of the Rio Grande rift, documented to have begun ca. 26 Ma, as advocated by others. The magmatism may have been related to Eocene subduction, although no isotopic component from Cenozoic subduction was recognized. Initial 87Sr/86Sr of whole-rock samples and clinopyroxene separates ranges from 0.7061 to 0.7073 and ϵNd (37 Ma) from −3.5 to −5.1. 207Pb/204Pb ranges from 15.56 to 15.60, 206Pb/204Pb ranges from 18.69 to 19.00, and 208Pb/204Pb ranges from 38.13 to 38.73. These isotopic ratios primarily reflect processes within heterogeneous, enriched mantle sources. The Two Buttes minettes have depletions in high field strength elements similar to those characteristic of subduction-related magmas, but the subduction event giving rise to these compositions cannot be Cenozoic because Nd depleted-mantle model ages are near 1.0 Ga. The Two Buttes magmas appear to have been produced from mantle enriched by Proterozoic subduction-related processes, as were some of the similar Cenozoic alkalic rocks intrusive into Proterozoic and younger crust in the Rocky Mountains region. The regional pattern of model Nd ages for similar alkalic rocks near the Rocky Mountain front may document successive periods of Precambrian crust formation from north to south.
Two Buttes magmatism may have resulted from slab collision with the western edge of a keel of thick continental lithosphere beneath the Great Plains. A similar slab-lithosphere interaction may have immediately preceded small-volume alkalic magmatism along the Rocky Mountain front in Montana and Wyoming ca. 48–54 Ma and in Colorado and New Mexico ca. 37 Ma. These small-volume alkalic igneous rocks may mark the locations of the western edge of the keel in mid-Cenozoic time, the times of slab-keel collision, and the times of regional uplift consequent to the thermal event responsible for mantle melting.