A separate basement terrane, Mojavia or the Mojave province with characteristic 2.0–2.3 Ga model ages, has been proposed to underlie much of the western U.S. Its existence, posited on patterns of Pb and Nd isotope data, has been propagated in the literature for more than two decades.
New and compiled U/Pb geochronology shows there is no direct evidence of >2.0 Ga juvenile basement rocks exposed within Mojavia of the eastern Great Basin. Pb and Nd model ages, by contrast, vary from Archean to Neoproterozoic with large variations exhibited, commonly within small regions. Archean ages are concentrated northward, suggesting the influence of sediment shed southward from the Wyoming province onto Paleoproterozoic basement terranes. In places, including the Farmington Canyon complex, sediment has been tectonically reprocessed and now is preserved as high-grade metamorphic rock in accretionary mélanges. There is no strong evidence that Mojavian basement exists outside of the Mojave Desert region proper.
A statistical evaluation of common Pb and Sr isotopes in Phanerozoic igneous rocks shows distinct differences north and south of one proposed boundary between Mojavia and the Yavapai province. However, close examination suggests that variation in these parameters could be produced by the influence of Archean material shed from the Wyoming province rather than representing a distinct difference in the age and isotopic character of the basement. Statistical discrimination of candidate terranes and terrane boundaries may be valuable for their recognition, but such differences alone do not prove their existence.
The northern boundary of Mojavia with the Wyoming province in the eastern Great Basin has been given the same name, the Cheyenne Belt, as the exposed suture be tween Paleoproterozoic and Archean basement in southern Wyoming. We reassess the location of this boundary in the Great Basin. New age controls on key basement outcrops in the Uinta Mountains and Farmington Canyon complex that were previously considered Archean indicate that these rocks are Paleoproterozoic in age (∼1.7 Ga). Thus, the Cheyenne Belt has traditionally been placed too far south; it must lie in a poorly defined location north of these localities.
- Received 3 March 2010.
- Revision received 7 September 2010.
- Accepted 17 October 2010.
- © 2011 Geological Society of America