18O/16O ratios of granitic rocks in western North America reflect source-rock lithologies, providing insight into the geology of the deep continental crust. In a 400-km-wide, 800-km-long, east-west transect through the northern Great Basin, the Mesozoic and Cenozoic plutons define three zones: western low-18O zone (WZ), δ18O = +6 to +8.5; central high-18O zone (CZ), δ18O = +9 to +13; eastern zone (EZ) (Utah), δ18O = +7 to +9. The WZ-CZ boundary is analogous to the δ18O "step" in the Peninsular Ranges batholith (PRB). A similarly abrupt (87Sr/86Sr)i "step" (<0.708 to the west and >0.710 to the east) defined by Farmer and DePaolo (1983) lies 150-200 km east of the WZ-CZ 18O/16O step, dividing the CZ into two subzones: V-type in the west, corresponding to the eastern half of the PRB (but much wider due to regional Cenozoic extension); and S-type to the east. A plausible source for the high-87Sr, low-ϵNd, S-type subzone is a late Precambrian (miogeoclinal) metasedimentary section, whereas the V-type source appears to be a Phanerozoic volcanogenic (eugeoclinal) accreted terrane. Both source regions have very high δ18O (+9 to +16) but radically different 87Sr/86Sr (and ϵNd); thus, we would not place the buried edge of the > 1.5 Ga craton beneath the 87Sr/86Sr step, but farther east near the EZ boundary. We suggest instead that this step is a sharp lithologic and "age" boundary (a suture-zone?) within the giant prism of sediments and volcanics in Nevada.