Xenoliths hosted by Cenozoic volcanic flows and plugs from the Central Sierra Nevada and Eastern Sierra Nevada, Owens Valley, and Inyo Mountains were studied for petrography and thermobarometry. The Central Sierra Nevada suite consists of abundant lower crustal feldspathic granulites, garnet clinopyroxenites, and mantle-derived peridotites and garnet websterites. Mafic crustal assemblages occur down to ~65--70 km, although below 35--40 km, they are mainly in the eclogite facies. In contrast, the Eastern Sierra Region suites show peridotitic, pyroxenitic, and harzburgitic assemblages at depths of ≥35--40 km. They define an adiabat in PT space (T~1180--1250 ¿C), suggesting the presence of the asthenospheric upper mantle close to the base of the crust. The ultramafic mantle rocks from the Central Sierra Nevada also define an adiabatic slope in PT space, possibly an artifact of side heating from the east. There is xenolith evidence that the Sierra Nevada lost about half of its original crust on the eastern side of the range. Regardless of the actual mechanism of crustal thinning, the loss of the eclogitic lowermost crust and replacement by peridotite in the eastern Sierra Nevada is a process accompanied by a substantial density decrease (>100 kg/m3). Overall, if the mechanism of eclogitic lowermost crust removal is viable, there are enough buoyancy sources to explain topographic differences between the Sierra Nevada and the adjacent Basin and Range, assuming isostatic equilibrium. ¿ American Geophysical Union 1996