The crust-mantle boundary beneath cratons and craton margins: A transect across the south-west margin of the Kaapvaal craton

''Lower-crustal suite'' xenoliths occur in ''on-craton'' and ''off-craton'' kimberlites located across the south-western margin of the Kaapvaal craton, southern Africa. Rock types include mafic granulite (plagioclase-bearing assemblages), eclogite (plagioclase-absent assemblages with omphacitic clinopyroxene) and garnet pyroxenite (''orthopyroxene-bearing eclogite''). The mafic granulites are subdivided into three groups: garnet granulites (cpx + grt + plag + qtz); two pyroxene garnet granulites (cpx + opx + grt + plag); kyanite granulites (cpx + grt + ky + plag + qtz). Reaction microstructures preserved in many of the granulite xenoliths involve the breakdown of plagioclase by a combination of reactions: (1) cpx + plag --> grt + qtz; (2) plag --> grt + ky + qtz; (3) plag --> cpx (jd-rich) + qtz. Compositional zoning in minerals associated with these reactions records the continuous transition from granulite facies mineral assemblages and pressure (P) - temperature (T) conditions to those of eclogite facies. Two distinct P-T arrays are produced: (i) ''off-craton'' granulites away from the craton margin define a trend from 680 degrees C, 7.5 kbar to 850 degrees C, 12 kbar; (2) granulite xenoliths from kimberlites near the craton margin and ''on-craton'' granulites produce a trend with similar geothermal gradient but displaced to lower T by similar to 100 degrees C. Both P-T fields define higher geothermal gradients than the model steady state conductive continental geotherm (40 mWm(-2)) and are not consistent with the paleogeotherm constructed from mantle-derived garnet peridotite xenoliths. A model involving intrusion of basic magmas around the crust/mantle boundary followed by isobaric cooling is proposed to explain the thermal history of the lower crust beneath the craton margin, The model is consistent with the thermal evolution of the exposed Namaqua-Natal mobile belt low pressure granulites and the addition of material from the mantle during the Namaqua thermal event (c. 1150 Ma). The xenolith P-T arrays are not interpreted as representing paleogeotherms at the time of entrainment in the host kimberlite. They most likely record P-T conditions ''frozen-in'' during various stages of the tectonic juxtaposition of the Namaqua Mobile Belt with the Kaapvaal craton.