CHEMICAL CONSTRAINTS ON LITHOSPHERE COMPOSITION AND EVOLUTION BENEATH THE COLORADO PLATEAU

We have measured Sr and Nd isotopic composition and rare earth element (REE) abundances in a variety of inclusions and host minettes from the Navajo Volcanic Field of the Colorado Plateau in order to develop geochemical constraints on the composition and evolution of the mantle beneath the relatively undisturbed Proterozoic crust of the Colorado Plateau. The data for the Colorado Plateau suggest that highly depleted, shallow lithosphere is underlain by material enriched in incompatible elements. This latter material was located near the inferred Oligocene base of the lithosphere and its composition may reflect mixing between asthenosphere and lithosphere. Melting of this material as a consequence of warming of the Colorado Plateau lithosphere during the late Tertiary may explain the origin of the Hopi Buttes alkaline magmas. -from Authors