Miocene crustal extension in Arizona was accompanied by uplift and uproofing of metamorphic core complexes and crustal thinning, creating the presently observed variation in crustal thickness from about 25 km in the Basin and Range to about 45 km beneath the Colorado Plateau. The effect of thinning on the lower crust is poorly understood. Researchers at two universities will study four basalt- rhyolite volcanic fields that define a transect spanning the Basin and Range - Colorado Plateau province boundary. We will use trace element and isotopic compositions of these lavas to constrain their source compositions. Inferred source compositions will be compared with data collected from crustal xenoliths from nearby localities. Preliminary work demonstrates that the rhyolites contain a large crustal component and that the lavas change composition systematically along the transect. Our major goals are to test hypotheses of the origins of the rhyolite and to relate our crustal source constraints to recent and ancient tectonic and crust formation events. Additionally, we will investigate the relationship between the high-silica and low-silica rhyolites that occur in each field and study the time- space relations of alkali-olivine and tholeiitic basalt volcanism.
