Isotopic Evidence on the Structure and Origin of Subcontinental Lithospheric Mantle in Southern Nevada

The trace element and Nd, Sr, and Pb isotopic compositions of extension-related late Cenozoic basalts in southern Nevada were determined in order to constrain models for the tectonic evolution of the subcrustal mantle in the southern Great Basin since 10 Ma. Basalts in southern Nevada between latitudes 36øN and 37øN have low ENd (--8 to -10), high A8/4 (+70 to + 107) and A7/4 (+8 to + 12), high 87Sr/86Sr, and low high field strength element (HFSE) concentrations relative to other Basin and Range basalts. Basalts with these chemical and isotopic traits have erupted semicontinuously in southern Nevada since 10 Ma. During the same period of time, basalts in central Nevada and east central California have shifted toward higher end, lower 87Sr/86Sr, lower 3,8/4 and 3,7/4, and higher HFSE contents than observed for the southern Nevada basalts. The latter isotopic characteristics are similar to other basalts in the western United States that are interpreted to have been derived from asthenospheric mantle similar isotopically to the sources of ocean island basalts. The isotopic and chemical characteristics of the southern Nevada basalts are not attributed to asthenospheric mantle but to lithospheric mantle that has been preserved beneath this region despite late Cenozoic extension. The lack of any apparent lithospheric erosion over this time period is attributed to the fact that southern Nevada has been an "amagmatic" zone throughout the Phanerozoic. As a result, the mantle lithosphere may have been more difficult to extend and displace during extension than lithosphere beneath adjacent portions of the Basin and Range. The lack of any extensive Phanerozoic magmatism in southern Nevada also allows the possibility that the lithosphere in southern Nevada originated in the Precambrian. Compared with basalt source regions in Proterozoic lithospheric mantle in northern Arizona and New Mexico, the lithospheric sources for the southern Nevada basalts have lower end, higher 87Sr/86Sr, and higher 3,8/4 values. We attribute these isotopic characteristics to lithospheric mantle associated with Proterozoic crust with Nd model ages from 2.0 to 2.3 Ga (province 1), a crustal province that is restricted spatially in the southwest United States to southern Nevada and vicinity.