Alkali-rich basaltic rocks have been erupted in small volumes throughout the Western Great Basin (WGB) since 17 Ma. SiO2 ranges from 46 to 53% in samples with >4% MgO, whereas Fe2O3 ranges from 8 to 10% and TiO2 from 1 to 2% in the most magnesian rocks. They have high incompatible element contents, except for relatively low high field strength elements abundances, and thus their minor and trace element patterns closely resemble subduction-related rather than intraplate magmas. Their radiogenic isotope ratios are also enriched with 87Sr/86Sr=0.7040--0.7078, 143Nd/144Nd=0.5129--0.5120 and variable Pb isotopes (206Pb/204Pb=18.0--19.2), generally displaced above the Northern Hemisphere Reference Line in both the 207Pb/204Pb and 208Pb/204Pb diagrams. The WGB straddles the isotopic and tectonic boundary between continental North America, underlain by Proterozoic basement, and the younger accreted terrain that forms the basement to most of California.

There are no discernible differences in the major and trace element contents of the basalts either side of this boundary except that those erupted through Proterozoic basement tend to lower silica contents and have higher 87Sr/86Sr and lower 143Nd/144Nd ratios. Nd model ages and a secondary Pb isochron from basalts east of the boundary suggest an age of 1.6--1.8 Ga, significantly younger than the age of crust generation inferred from Nd model ages of granites. Correlated trace element and isotope ratios in basaltic rocks to the west of the boundary indicate a contribution from recent subduction, recognized by a decrease in 87Sr/86Sr as Nb/Sr and Zr/Sr decrease. A second, less well defined trend to higher Nb/Sr and Zr/Sr and lower 87/Sr86Sr is interpreted to reflect a minor asthenospheric contribution to contemporary magmatism. Relative to basalts from the Basin and Range, the basaltic rocks from the WGB have higher SiO2 and lower TiO2 and Fe2O3 at similar degrees of fractionation. These differences are interpreted to reflect different pressures of segregation from their mantle source regions, the WGB basalts being derived from shallower depths than the Basin and Range basalts, within the lithospheric mantle.

This conclusion is consistent with their lithospheric trace element and isotope characteristics and trace element models that allow minimal garnet in the source region. By contrast the lower silica contents of the Basin and Range alkali basalts are more consistent with derivation from depths of 90 km or more, which may place their source regions in the asthenosphere, also consistent with their trace element and isotope variations. ¿ American Geophysical Union 1995