MANTLE PLUME ORIGIN FOR THE BUSHVELD AND VENTERSDORP MAGMATIC PROVINCES

Geochemical and isotopic data demonstrate that the parental magma of t he lower zone of the Bushveld complex has been contaminated by the cru st Magmas similar in composition are identified in the Ventersdorp Sup ergroup, but these magmas are part of a series which includes members that are free of crustal contamination. Primary mantle melts can there fore be identified and their conditions of origin have been determined . The depth of origin is between 18 and 40 km. The presence of mantle, at such shallow depths in an intracratonic setting, is only explicabl e if a mantle diapir temporarily intruded the crust up to the base of the strong middle and upper crustal layer. Crustal melting and contami nation occurred at the top of the diapir. In the Ventersdorp, crustal melts failed to impede the passage of mantle melts to the surface. In the Bushveld, mantle melts were impeded and only intruded outside the central ellipse of the crustal melts.