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Petrological, geochemical and U-Pb isotopic studies of Archaean granitoid rocks of the Makoppa Dome, northwest Limpopo Province, South Africa

Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. WITS 2050, Johannesburg, South Africa, e-mail: anhaeusserc{at}geosciences.wits.ac.za

M. Poujol

Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. WITS 2050, Johannesburg, South Africa, Present address: Department of Earth Sciences, Memorial University of Newfoundland, St John’s, Newfoundland, NF A1B 3X5, Canada, e-mail: mpoujol{at}esd.mun.ca

The Makoppa Dome, located in the far northwestern part of the Kaapvaal Craton, constitutes one of the least well-exposed Archaean granite-greenstone terranes in southern Africa. For this reason it has received very little attention in the past and its characteristics have largely gone undocumented. This study presents new field, petrological, geochemical and single zircon dating results for a suite of granitoid rocks that comprises foliated trondhjemitic gneisses and homogeneous monzogranitic and granodioritic phases, all of which post-date remnants of Archaean greenstone belts metamorphosed to amphibolite facies. Three separate granitoid types are recognized on the basis of their distinctive field as well as major, trace and rare-earth element characteristics. Confirmation of the three granite types follows from new ID-TIMS and LAM-ICP-MS dating of single zircons extracted from samples collected on the dome. The oldest granitoid rocks so far recognized are represented by the Vaalpenskraal trondhjemitic gneisses, two samples of which yielded ages that are identical within error and are dated at 3013±11 Ma and 3034±64 Ma, respectively. A single sample of homogeneous granitoid, referred to as Makoppa monzogranite, yielded an age of 2886+3/–2 Ma, while a further two samples of homogeneous granodiorite, referred to as Rooibokvlei granodiorite, are representative of the youngest granitic event recorded on the dome and yielded ages of 2777±35 Ma and 2797±2 Ma, respectively. The granitoid rocks of the Makoppa Dome have many features in common with those of the Kraaipan-Amalia terrane on the western side of the Kaapvaal Craton, as well as those of the Murchison-Giyani-Pietersburg terrane on the northern side, supporting suggestions that these rocks developed in, or adjacent to, a crescent-shaped magmatic arc that began forming at ~3.1 to ~3.0 Ga around the northern, northwestern and western rim of the Kaapvaal Craton. Continent-arc collision followed in the period up to ~2.8 Ga, the time of emplacement of the Rooibokvlei granitoids on the Makoppa Dome and the 2791±8 Ma Mosita granitoids in the Kraaipan terrane. These ages closely approximate the age of the ~2.79 Ga Gaborone Granite Complex in neighbouring Botswana and may represent the proto-magmatic stage leading to the development, on thickened crust, of the A-type Gaborone granite suite and Kanye volcanic formation, followed, in turn, by the rift-related ~2.71 Ga volcanism of the Ventersdorp Supergroup.