Most of the volcanic rocks in the Colorado River Trough (CRT) and the Mogollon-Datil Volcanic Field (MDVF) in the Basin and Range exhibit calc-alkaline major element trends and relatively low high field strength element abundances, similar to those erupted from the volcanoes of Aso and Towada in Japan. Such features are widely regarded as characteristic of subduction-related magmatism, and yet the rocks in the Basin and Range were generated in response to lithospheric extension. The preextensional to synextensional rocks of the CRT and the MDVF have higher Na2O, K2O, and TiO2, in the range 47--55% SiO2, and relatively low Al2O3, and overall, they tend to have higher Sr contents and Zr/Y and La/Nb ratios than those from Aso and Towada. In addition, the basalts in the Basin and Range tend to be more aphyric than those in Japan, consistent with more rapid movement of magma through the crust during extension in the Basin and Range, and the rate of melt generation appears to have been significantly less in the Basin and Range than along recent destructive plate margins.

The geochemical differences are attributed to smaller degrees of partial melting in the Basin and Range and to source regions that had been enriched in incompatible elements since the Proterozoic, resulting in parental magmas with higher alkali contents than those commonly observed in subduction-related calc-alkaline suites. Within the CRT the subsequent calc-alkaline trend was due at least in part to mixing with crustal derived melts, whereas in the MDVF such trends reflect both crustal contamination and fractional crystallization involving magnetite and amphibole. The small volumes of magma with minor and trace element features similar to oceanic basalts indicate that relatively little melt was generated in underlying asthenosphere. Thus it is inferred that magmatism in the Basin and Range was not associated with a significant increase in temperature, such as might be attributed to a mantle plume, but rather it was in response to lithospheric extension. Calculations are presented which demonstrate that the magma volumes and inferred source regions, extension, present-day heat flow, and topography are consistent with a model of convective lithospheric thinning after thickening in the Laramide and Sevier orogenies. ¿ American Geophysical Union 1995