The significance of 3000 Ma granulite-facies mafic dikes in the central zone of the Limpopo belt, southern Africa

Structural, petrographic and chemical relationships indicate that a suite of mafic dikes, possibly 3000 Ma old, were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the Limpopo belt. These dikes were subsequently metamorphosed at granulite grade conditions, probably during the Limpopo orogeny about 2700 Ma ago. The earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 Ma and 3100 Ma ago. These relationships indicate that the rocks comprising the central zone did not follow a single P-T-time loop but instead were involved in two distinct orogenies. The orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface. This tectonic history also implies that the supracrustal rocks of the central zone and the Sand River gneisses are of great antiquity. If the Sand River gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks, then the Sand River gneisses have been involved in three orogenies. Pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average μ ratio at some stage in its evolution prior to 3200 Ma. All Archean lithologic units within the central zone of the Limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 Ma ago until it was destroyed by or after the Limpopo orogeny. The tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the Limpopo orogeny.