title: Cenozoic potassic magmatism and uplift of the Western United States
Creators: name: Nowell, Geoffrey Mark
Publisher: name: The Open University
Year: 1994

description: The geochemistry and temporal evolution of small volume potassium-rich mafic volcanics within the non-extensional, uplifted central Colorado Plateau and Sierra Nevada have been studied in detail and a general model is proposed for non-extensional, uplift-related, magmatism within the Western Cordillera. <br><br>The Oligo-Miocene potassic-ultrapotassic lamprophyres of the Navajo Volcanic Field, Colorado Plateau, and the late Miocene-Pliocene shoshonites of the San Joaquin- Kings Volcanic Field, Sierra Nevada, represent primary or near primary mantle-derived melts that have experienced limited crystal fractionation and/or crustal contamination. Yet despite this each suite of samples is characterised by a significant range of trace element and isotope ratios. The variation in LIL/HFS ratios (Ba/Nb=11-77) and isotopic ratios (<sup>87</sup>Sr/<sup>86</sup>Sr=0.7044-0.7071) of the Navajo lamprophyres are attributed to small degrees of melting of a compositionally and mineralogically heterogeneous lithospheric mantle source. By contrast a similar, though slightly more extreme, variation in trace element and isotope ratios of shoshonites from the Sierra Nevada (Ba/Nb=70-325,<sup>87</sup>Sr/<sup>86</sup>Sr=0.7050-0.7069) are attributed to mixing of magmas derived from incompatible element enriched lithospheric mantle and depleted asthenospheric mantle. <br><br>Despite the differences in petrogenetic model for the magmatism within the each study area, the magmatic evolution of both the Colorado Plateau as a whole and the Sierra Nevada are similar. Initial magmatism within each province is dominated by a comparatively long history of incompatible element-enriched lithosphere-derived potassic magmatism. After a period of volcanic quiescence, lasting approximately 11 and 5Ma within the Colorado Plateau and Sierra Nevada respectively, the final, relatively short period of volcanism is characterised by compositions more typical of asthenosphere-derived magmas. New Ar-Ar agesf or the Navajo lamprophyresa nd existing K-Ar agesf or the Sierran shoshonites suggest that the onset of lithosphere-dominated magmatism within both the central Colorado Plateau and Sierra Nevada was coeval with the onset of major uplift during the late Cenozoic. <br><br>The temporal evolution of magmatism and the degree of uplift, combined with geophysical and xenolith evidence, are consistent with a model in which uplift and preextensional magmatism of the Colorado plateau and Sierra Nevada resulted from the convective removal of the lower lithosphere. In the case of the Colorado Plateau convective thinning of the lithosphere is in response to its previous thickening during the Laramide orogeny whereas in the Sierra Nevada it is related to the cessation of subduction and the development of the San Andreas transform margin during the late Cenozoic. Uplift resulting from convective thinning of the lithosphere increases the gravitational potential of the orogen and the likelihood for later collapse and provides an explanation for the close temporal relationship of mafic mantle-derived magmatism, uplift and major crustal extension typical of the Western Cordillera of the United States.