Potassium, thorium, and uranium contents of upper Cenozoic basalts of the southern Rocky Mountain region, and their relation to the Rio Grande depression

Late Cenozoic basaltic volcanism in southern Colorado and northern New Mexico was most intense near the Rio Grande rift depression but extended onto stable platforms to the west (Colorado Plateau) and to the east (High Plains). Tholeiitic rocks are largely confined to the Rio Grande depression, and the basalts become increasingly alkalic with distance from the depression. The K, Th, and U contents and the Th/K and U/K ratios consistently increase away from the depression, and Th/U ratios also tend to increase slightly. Geographically distinct suites of petrographicaUy related basalts that are very similar in major-oxide compositions are readily distinguishable by K, Th, and U contents. Sialic crustal contamination did not contribute significantly to development of these compositional variations, and the lateral change from tholeiitic to alkalic basaltic volcanism may be related to different depths of or degrees of partial melting in the mantle. The compositions and compositional ranges of basalts in the southern Rocky Mountain region are similar to those of many Pacific islands, despite the contrasting geologic settings.