The geology of the area surrounding the Venetia kimberlite pipes, Limpopo Belt, South Africa: A complex interplay of nappe tectonics and granitoid magmatism

The area surrounding the similar to530 to similar to519 Ma Venetia kimberlite pipes in the Central Zone of the Limpopo Belt, South Africa, consists of four tectonic units intruded by Proterozoic dolerite dykes and sills. The structurally lowest Krone metamorphic terrane is composed primarily of quartzofeldspathic gneisses, gently folded along approximately north-south trending axes. The ages of these rocks have not been determined but they are assumed to be Archean. Zones of mylonite separate the Krone metamorphic terrane from the Venetia and Endora klippen. The Venetia klippe, which includes a synclinal remnant of a nappe, is composed of four mappable units. The lowest consists of interlayered less than or equal to 3.2 Ga quartzofeldspathic gneisses and similar to3.1 Ga orthoamphibolite. The next highest unit consists of thin, discontinuous layers of quartzofeldspathic gneisses, ortho- and para-amphibolite, carbonate and calc-silicate rocks, magnetite-quartzite and quartzite. Over lying this is a metasedimentary unit composed primarily of quartzite and carbonate and calc-silicate rocks. U-Pb ages for detrital zircons from within the quartzite indicate that this unit is younger than similar to2.67 Ga. The upper most unit consists of granitic orthogneisses derived from an similar to2.45 Ga precursor. Most of the contacts within and between these units in the Venetia klippe have been tectonically deformed into mylonite or biotite-schist although some sedimentary contacts may be preserved. The klippe is a doubly folded synform with a strong east-west trending, similar to2.04 Ga D-1 fold axis locally refolded long similar to2-03 Ga north-south trending fold axes. Sedimentary structures preserved in the metasedimentary unit indicate that the synform is a syncline. The Endore klippe comprises a synform of quartzite and magnetite-quartzite of uncertain age. Intruding the Krone metamorphic terrane and probably the Venetia klippe is the Gotha granitic complex consisting of granite, granodiorite and tonalite of possible similar to1.9 to similar to2.0 Ga age. Igneous rocks composing this terrane are characterized by large U and Th contents. The emplacement of the Venetia kimberlite pipes into this area was largely controlled by faults related to the Dowe-Tokwe system. It is argued that rocks of both Archean and Proterozoic age are found in this portion of the Central Zone and that their juxtaposition was the result of largely horizontal tectonism and possible magmatism at similar to2.04 Ga. The Proterozoic rocks have compositional similarities to and are possibly coeval with the rocks of the Transvaal Supergroup on the Kaapvaal Craton but it is unlikely that they were derived from that source.