Gabbro xenoliths in Tertiary alkali olivine basalts near Wikieup, Arizona provide samples from the crust-mantle boundary beneath the Basin and Range to Colorado Plateau transition zone. The xenoliths consist of two pyroxenes+plagioclase+spinel (gabbronorite) and olivine+clinopyroxene+plagioclase+spinel (olivine gabbro). Pyroxenes and spinel are aluminous; plagioclase is Ca-rich (An59--71), and olivine is Mg-rich (Fo69--79). Major element compositions suggest that the gabbros are cumulates formed by fractional crystallization from an alkalic parent magma. Thermobarometry indicates high equilibration temperatures of 900--1050 ¿C. Abundant composite xenoliths of gabbro intruded into spinel and plagioclase peridotite constrain the depth of origin to the uppermost mantle or lowermost crust (about 30 km depth, 0.8 GPa pressure) and suggest that these rocks represent fragments from a crust-mantle transition zone formed by basaltic underplating during extension-related magmatism. This transition zone consists of peridotite intruded by veins of gabbro. The amount of peridotite decreases and gabbro increases upward until, in the lower crust, there is only gabbro. Elevated lower crust and upper mantle temperatures estimated from the xenoliths are in accord with the elevated geothermal gradient determined by surface heat flow (>80 mW m-2). Estimated compressional seismic velocities (Vp), corrected for temperature and pressure, are about 6.4 to 7.2 km s-1 for these gabbros. Geophysical data may be compatible with presence of a thin, <2-km thick, crust-mantle transition zone beneath Wikieup. The Wikieup gabbro xenoliths are samples from this thin transition between crust and mantle. There is no evidence that these gabbros are representative of the lower crust as a whole. ¿American Geophysical Union 1994