Carl M. Bunker Charles A Bush Peter W. Lipman 1973 <p>Late Cenozoic basaltic volcanism in southern Colorado&nbsp;and northern New Mexico was most intense near the Rio Grande rift&nbsp;depression but extended onto stable platforms to the west (Colorado&nbsp;Plateau) and to the east (High Plains). Tholeiitic rocks are largely&nbsp;confined to the Rio Grande depression, and the basalts become&nbsp;increasingly alkalic with distance from the depression. The K, Th, and&nbsp;U contents and the Th/K and U/K ratios consistently increase away&nbsp;from the depression, and Th/U ratios also tend to increase slightly.&nbsp;Geographically distinct suites of petrographicaUy related basalts that&nbsp;are very similar in major-oxide compositions are readily distinguishable&nbsp;by K, Th, and U contents. Sialic crustal contamination did not&nbsp;contribute significantly to development of these compositional variations,&nbsp;and the lateral change from tholeiitic to alkalic basaltic volcanism&nbsp;may be related to different depths of or degrees of partial melting in&nbsp;the mantle. The compositions and compositional ranges of basalts in&nbsp;the southern Rocky Mountain region are similar to those of many&nbsp;Pacific islands, despite the contrasting geologic settings.</p> application/pdf en U.S. Geological Survey Potassium, thorium, and uranium contents of upper Cenozoic basalts of the southern Rocky Mountain region, and their relation to the Rio Grande depression article