Structure, metamorphism, and geochronology of the Rhenosterkoppies Greenstone Belt, South Africa

The Rhenosterkoppies Greenstone Belt, situated in the northern Kaapvaal Craton close to the Southern Marginal Zone of the Limpopo Belt, was studied by structural mapping, petrography, and geochronology. The belt consists mainly of a 300 - 500-m-thick alternating sequence of amphibolite and banded iron formation horizons, with intercalated talc-tremolite schist, rare metapelite, and strongly foliated sheets of mainly granodioritic gneiss. This mainly supracrustal sequence broadly defines an antiform with axis plunging shallowly to the east. Granodioritic gneisses are exposed in the centre of the antiform. Their contact with the supracrustal sequence is sheared. The gneisses have T(DM) model ages between ~2950 and ~3100 Ma, in agreement with T(DM) ages of gneisses from the northernmost Kaapvaal Craton and the Limpopo Belt Southern Marginal Zone in general. Four deformational phases were recognized, the most prominent of which produced a pervasive layer-parallel foliation and a consistent northeast-trending stretching lineation, and probably represents the regional north-vergent thrusting episode recognized elsewhere in the northern Kaapvaal Craton. The shearing occurred under upper greenschist to lower amphibolite-facies conditions. Upper amphibolite-facies relics are found throughout the greenstone belt. The shearing was dated directly at 2729 ± 19 Ma by stepwise leaching Pb/Pb analysis of synkinematic titanite. Dating of a retrograde post-kinematic titanite-epidote paragenesis within the granitoid gneisses in the core of the anticline yielded 2743 ± 21 Ma, placing a lower age limit of ~2720 Ma on this tectonism. It was followed by the updoming of the anticline, recognizeable as a separate event as the main lineation is rotated by it. In the northernmost part of the greenstone belt, close to the Hout River Shear Zone, a local medium- to high-pressure, lower amphibolite-facies metamorphic episode associated with localized metasomatism was dated at 2634 ± 49 Ma by Pb stepwise leaching of kyanite. This is consistent with other age constraints on the tectonics of the Southern Marginal Zone of the Limpopo Belt. North-vergent tectonism in the northernmost Kaapvaal Craton thus predates tectono-metamorphism in the Southern Marginal Zone by more than 50 Ma.