Bimodal basalt-rhyolite fields that erupted since 15 Ma form a NE trending line that crosses the boundary between the Basin and Range and the Colorado Plateau in western Arizona. The bimodal volcanism traverses physiographic province boundaries marked by changes in crustal thickness. Silicic volcanism in these fields postdates detachment faulting in the Basin and Range and generally decreases in age from south to north. We distinguish two rhyolite types that occur in these fields and summarize their geology, petrography, and geochemistry. Both high-silica (HSR) and low-silica rhyolites (LSR) occur in these fields. HSR lavas contain >75 wt % SiO2 (normalized to 100% volatile free), have high concentrations of incompatible elements (Rb, U, Th, Cs, Ta), low concentrations of compatible elements (Ba, Sr, Eu, Mg, Cr), and relatively flat rare earth element (REE) patterns La/LuN 8). Further distinction of HSR and LSR lavas is made based on phenocryst assemblages. Trace element and isotopic studies suggest that both the HSR and LSR lavas contain a major crustal component and that the HSR lavas may have formed by extensive crystal fractionation of LSR parent magmas. HSR and LSR lavas display systematic compositional variations along the transect; however, their compositions diverge from south to north. HSR lavas erupted on the Colorado Plateau are more differentiated than those erupted in the Basin and Range, perhaps reflecting more extensive crystal fractionation while ascending the thicker crust of the Colorado Plateau province. In contrast, LSR lavas on the Colorado Plateau are more primitive that those in the Basin and Range.

This seeming paradox may be related to lateral changes in source composition, possibly caused by a northerly increase in either metamorphic grade or bulk composition. Alternative models for the LSR compositional variations, such as magma mixing and mantle input into lower crustal sources, cannot be tested with our present data. ¿ American Geophysical Union 1989