The Quaternary volcanic centres of the Bockfjord area, northwestern Spitsbergen, are probably the world's largest and richest source of xenoliths from the uppermost mantle and lower crust. Pressure-temperature estimates on garnet websterites, and temperature estimates on two-pyroxene granulites and spinel lherzolites, allow definition of a stratigraphic sequence and the ambient geotherm. The crust is ca. 27 km thick and its lower part consists of mafic (gabbroic to anorthositic) meta-igneous granulites. The uppermost mantle consists of Cr-diopside spinel lherzolite, mixed with pyroxenites and metapyroxenites. The xenolith data are compatible with available geophysical data. The xenolith-defined geothermal gradient is even higher than one previously derived for eastern Australia, and comparable to that for the Rio Grande Rift. The high ambient temperatures probably reflect the addition of heat by basaltic magmas, to the base of a continental crust thinned by rifting only 10-15 Ma ago.