EOCENE POTASSIC MAGMATISM AT 2 BUTTES, COLORADO, WITH IMPLICATIONS FOR CENOZOIC TECTONICS AND MAGMA GENERATION IN THE WESTERN UNITED-STATES

Potassic mafic intrusions at Two Buttes, Colorado, enable examination of melt generation and mantle evolution at the eastern extent of Oligo cene-Eocene magmatism in the western United States, and comparison of these processes to those in similar tectonic settings but within mantl e lithosphere of contrasting age. Average K-Ar age determinations from four intrusions yield a 36.8 +/- 0.4 Ma (1<(sigma)over bar>) age for the Two Buttes complex. This late Eocene age precludes a genetic conne ction 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 su bduction was recognized. Initial Sr-87/Sr-86 of whole-rock samples and clinopyroxene separates ranges from 0.7061 to 0.7073 and epsilon(Nd) (37 Ma) from 3.5 to 5.1. Pb-207/Pb-204 ranges from 15.56 to 15.60, Pb- 206/Pb-204 ranges from 18.69 to 19.00, and Pb-208/Pb-204 ranges from 3 8.13 to 38.73. These isotopic ratios primarily reflect processes withi n heterogeneous, enriched mantle sources. The Two Buttes minettes have depletions in high field strength elements similar to those character istic of subduction-related magmas, but the subduction event giving ri se 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 proce sses, as were some of the similar Cenozoic alkalic rocks intrusive int o Proterozoic and younger crust in the Rocky Mountains region. The reg ional pattern of model Nd ages for similar alkalic rocks near the Rock y 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 continent al lithosphere beneath the Great Plains. A similar slab-lithosphere in teraction may have immediately preceded small-volume alkalic magmatism along the Rocky Mountain front in Montana and Wyoming ca. 48-54 Ma an d in Colorado and New Mexico ca. 37 Ma. These small-volume alkalic ign eous rocks may mark the locations of the western edge of the keel in m id-Cenozoic time, the times of slab-keel collision, and the times of r egional uplift consequent to the thermal event responsible for mantle melting.