Modeling of continental crustal evolution requires improved understanding of lower crustal rock types, compositions, and structure. Recent geophysical transects in the Basin and Range and the Basin and Range-to-Colorado Plateau (BR-CP) transition zone have raised questions about the nature and development of the lower crust and the crust-mantle boundary in this region. Flat Moho topography beneath the Basin and Range, and multiple, sub-horizontal Moho reflectors beneath the Basin and Range and the BR-CP transition zone have lead to suggestions that basaltic underplating played an important role in the development of the Basin and Range lower crust and crust-mantle boundary during Cenozoic extension. Xenoliths from Wikieup, Arizona provide actual rock samples from the crust-mantle boundary beneath the BR-CP transition zone. This xenolith suite includes abundant composite nodules of upper mantle peridotite in contact with lower crustal gabbro and granulite. Complete petrographic and compositional analysis (minerals, whole rock major, trace, and rare elements, isotopes) will be used to determine the petrogenesis of lower crustal rock types. Xenolith thermobarometry will provide estimates of the thermal state of the lower crust in this region. Integration of complete petrologic data with existing geophysical data will allow evaluation of the role of processes such as basaltic underplating in the evolution of the Basin and Range lower crust and the development of the crust-mantle boundary.
