U-Pb and Hf isotopes of detrital zircons from the Pongola Supergroup: Constraints on deposition ages, provenance and Archean evolution of the Kaapvaal craton

Abstract

The c.3.0–2.9 Ga Pongola Supergroup located on the south-eastern margin of the Kaapvaal Craton preserves some of the most pristine Archean successions but the timing of volcanic and sedimentary processes, and sediment sources remain relatively poorly known. Combined zircon U-Pb ages and Hf isotope data of sandstones from the Nsuze and Mozaan Groups are presented from key locations of the Northern, Central and Southern Domains of the 270 km long belt. These data indicate important lateral and temporal variations and therefore changes in source areas. The basal Pongola sedimentary rocks (Mantonga Formation) in the Northern Domain were derived from the c.3.4 Ga basement, and also most likely from reworked Moodies Group sedimentary rocks of the Barberton greenstone belt. Zircon detritus in Nsuze and lower Mozaan Groups sandstones shows similar age-Hf isotope spectra, and a remarkable overlap with data from magmatic and (meta)sedimentary rocks of the southeastern Kaapvaal craton. The data point to a proximal source for Nsuze sandstones in the Northern Domain and a different provenance for the Southern Domain sandstones. A possible signature of the extensive 3.10–3.07 Ga Mpuluzi granite is seen only in the Nsuze sandstones and basal Mozaan of the Central Domain. The data indicate for the first time that Cr-rich sandstones of the Ndikwe Thrust Segment of the Southern Domain were deposited at <2.92 Ma, and therefore belong to the uppermost Mozaan Group. Furthermore, they reveal a significant switch in provenance from the lower to upper Mozaan Group from a reworked Archean basement towards a more juvenile hinterland with εHft = +0.9 to +6.3 between 3.07 and 2.92 Ga; similar to the source previously reported for the Central Rand Group of the Witwatersrand Basin. The juvenile zircon detritus in the upper Mozaan Group requires sources as far as the Pietersburg and/or Kimberley Blocks, which became amalgamated to the central block of the Kaapvaal Craton at 2.97–2.87 Ga, but perhaps also a source outside of the present day Kaapvaal Craton, and possibly as far as the Pilbara Craton.