CALC-ALKALINE MAGMATISM, LITHOSPHERIC THINNING AND EXTENSION IN THE BASIN AND RANGE

Most of the volcanic rocks in the Colorado River Trough (CRT) and the Mogollon-Datil Volcanic Field (MDVF) in the Basin and Range exhibit ca lc-alkaline major element trends and relatively low high field strengt h element abundances, similar to those erupted from the volcanoes of A so and Towada in Japan. Such features are widely regarded as character istic of subduction-related magmatism, and yet the rocks in the Basin and Range were generated in response to lithospheric extension. The pr eextensional to synextensional rocks of the CRT and the MDVF have high er Na2O, K2O, and TiO2, in the range 47-55% SiO2, and relatively low A l2O3, and overall, they tend to have higher Sr contents and Zr/Y and L a/Nb ratios than those from Aso and Towada. In addition, the basalts i n the Basin and Range tend to be more aphyric than those in Japan, con sistent with more rapid movement of magma through the crust during ext ension in the Basin and Range, and the rate of melt generation appears to have been significantly less in the Basin and Range than along rec ent destructive plate margins. The geochemical differences are attribu ted to smaller degrees of partial melting in the Basin and Range and t o source regions that had been enriched in incompatible elements since the Proterozoic, resulting in parental magmas with higher alkali cont ents than those commonly observed in subduction-related calc-alkaline suites. Within the CRT the subsequent calc-alkaline trend was due at l east in part to mixing with crustal derived melts, whereas in the MDVF such trends reflect both crustal contamination and fractional crystal lization involving magnetite and amphibole. The small volumes of magma with minor and trace element features similar to oceanic basalts indi cate that relatively little melt was generated in underlying asthenosp here. Thus it is inferred that magmatism in the Basin and Range was no t associated with a significant increase in temperature, such as might be attributed to a mantle plume, but rather it was in response to lit hospheric 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 l ithospheric thinning after thickening in the Laramide and Sevier oroge nies.