Diamond exploration conducted by Rio Tinto Namibia in the Tsumkwe region of northeast Namibia from 1994 to 1996 produced significant enough populations of kimberlitic indicator minerals, particularly G10 garnets, to warrant further study of the poorly exposed basement. Although representing a geochemically diverse population, most of the garnets studied appear to have been derived from a mantle with a cratonic geotherm, a diamond window of 900° to 1250°C, and a lithosphere thickness of at least 190 km. G10 garnets are moderately abundant and fall within the diamond window. Zircons from basement megacrystic granite produced a 206Pb/238U magmatic crystallization age of 852 ± 11 Ma, and zircons from fine-grained granitic gneiss produced a crystallization age of 2022 ± 15 Ma with lead loss, possibly relating to a metamorphic event, at about 970 Ma. Combined, these results indicate the presence of a stabilized continental segment, established by at least 2.0 Ga, which included mantle with some Archean-like characteristics at the time of kimberlite eruption. This portion of the lithosphere may have been a thinned southern edge of the Congo craton.