Mantle and crustal xenoliths and megacrysts hosted in an alnöite diatreme emplaced in the Mesozoic Okenyenya igneous complex in northwestern Namibia include ultramafic varieties - lherzolite, wehrlite and clinopyroxenite (all ± spinel ± amphibole), and mafic varieties - granulite, amphibolite, and eclogite. The megacryst suite includes amphibole, clinopyroxene, ilmenite, plagioclase and olivine. The peridotite xenoliths have textures ranging from coarse granular through porphyroclastic to granuloblastic and many show evidence of modal metasomatism by the presence of abundant amphibole. Olivine (Fo90 – Fo91), orthopyroxene (Wo1 En89Fs10 – Wo1 En90Fs9 ) and clinopyroxene (Wo46En48Fs6 – Wo48En52Fs8 ) in the lherzolites are significantly more Mg-rich than those in the wehrlites (Fo89-Fo69; Wo1 En77Fs22). Calculated temperatures of equilibration of the lherzolites range from 950 to 1050°C, corresponding to a pressure range for the spinel lherzolites of 18 to 19 kbar. Pressure estimates of the wehrlites are constrained to between 7 and 20 kbar based on the amphibole stability field. Mafic xenoliths include two-pyroxene granulite (opx, cpx, plag, sp), amphibole-clinopyroxene granulite (cpx, amph, plag, sp, scapolite), amphibolite and amphibole eclogite with a mosaic granuloblastic texture. Clinopyroxenes show systematic differences in composition between the different mafic xenolith types, as does amphibole (titanian-magnesio-hastingsite in the eclogite; pargasite in the amphibolite and granulite). Pyroxene equilibration temperatures for the granulites range from 663 to 741°C, at pressures of less than 10 kbar on the basis of the equilibrium mineral assemblage. Equilibration temperatures of the eclogites are estimated at 1120°C (20 kbar) and 1160°C (30 kbar). On the basis of major and trace element compositions, the megacryst suite of minerals are interpreted as xenocrysts that crystallised from parental magmas more primitive (amphibole) and more evolved (clinopyroxene, ilmenite) than the host alnöite. A conceptual model for the mantle and lower crust beneath the Damara mobile belt is proposed in which a lower crust of two pyroxene and amphibole-pyroxene granulite overlies mantle comprising a heterogeneous mixture of spinel lherzolite, wehrlite, eclogite and clinopyroxenite down to a depth equivalent to at least 20 kbar, and which has experienced extensive invasive modal metasomatism and veining by incipient alkaline melts or fluids.