Zircon U-Pb-Hf isotope systematics of Limpopo Belt quartzites and igneous rocks, implications for Kaapvaal – Zimbabwe Craton accretion

Abstract

There are ongoing debates about the nature and timing of Kaapvaal-Zimbabwe Craton accretion. A critical role in this discussion plays the Central Zone (CZ) of the Limpopo Belt, which represents a polymetamorphic gneiss terrane limited by the Kaapvaal Craton to the South and the Zimbabwe Craton to the North. Uncertainties mainly result from complex field relationships, the lack of age data, and the complexity of investigated zircon grains. In this study we present new results of combined U-Th-Pb and Lu-Hf isotope analyses of zircon grains from quartzite, granitoid, and metabasite samples from different areas of the CZ. These data indicate that the oldest lithologies are mafic rocks which were emplaced at 3.35–3.29 Ga from near-chondritic mantle source. They also reveal that quartzite precursors throughout the CZ were deposited during two major sedimentary cycles, with quartzite-I members between 3.36 and 2.64 Ga, and quartzite-II between 2.55 and 2.04 Ga, and that the detritus was supplied from distinct sources. Detrital zircon grains of quartzite-I were delivered from Eo- to Mesoarchean granite-gneiss terranes, affected by magmatism between 3.92 and 2.90 Ga, and characterized by hafnium model ages (TDM) between 4.40 and 3.30 Ga. In contrast, zircon grains of quartzite-II stem predominately from Neoarchean granitoids, which intruded between 2.73 and 2.62 Ga, and mostly show TDM of 2.88 ± 0.06 Ga (n = 8) or 3.09 ± 0.10 Ga (n = 4). Comparison with published age-εHft data and spatial relationships suggest quartzite-I being sourced mainly from the Zimbabwe Craton, which was connected to parts of the CZ since the Paleoarchean, and quartzite-II from granitoids of an independent Neoarchean terrane (with TDM 3.1–2.9 Ga), which was derived from the same mantle source as the crust of the adjacent Pietersburg Block (northern Kaapvaal Craton). All data in combination provide evidence that the CZ of the Limpopo Belt hosts a composite terrane that comprises relics of (1) Zimbabwe Craton-related crust, and (2) a south-to-north developing accretionary system of granite-greenstone terranes younger than 3.1 Ga, which finally collided with the Zimbabwe Craton at ca. 2.62 Ga.