Detrital zircon constraints on the tectonostratigraphy of the Paleoproterozoic Pretoria Group, South Africa

Absolute age constraints are a key requirement to understand links between global tectonics, sedimentation and changing surface environments in the Neoarchean and Paleoproterozoic, in particular oxygenation of the ocean-atmosphere system (the Great Oxidation Event, GOE) or increased carbon burial during the Lomagundi δ¹³C excursion. This paper presents new detrital zircon U-Pb dates from the 2.4 to 2.1 Ga Pretoria Group (Transvaal Supergroup, South Africa), a clastic-dominated succession spanning surface oxidation and the δ¹³C excursion. Zircon ages cover all major igneous source terrains on the Kaapvaal craton and thus constitute a representative cross section of craton structure. Maximum depositional ages have been determined for the Duitschland Formation (2424 ± 12 Ma - youngest single grain age), Timeball Hill Formation (2324 ± 17 Ma - youngest single grain age), Hekpoort Formation (2247 ± 10 Ma - weighed mean peak age), Daspoort Formation (2265 ± 20 Ma - weighed mean peak age), and Magaliesberg Formation (2214 ± 11 Ma - youngest 2-grain cluster). These ages are consistent with previously published age constraints, but represent the first direct age constraints for the Daspoort and Magaliesberg formations. Maximum depositional ages and zircon populations show clear younging trends up through the succession that reflect a progressively thicker sedimentary cover and increasing internal recycling. The lower Pretoria Group (Duitschland to Hekpoort formations) contains a prominent ~2575-2410 Ma zircon population that likely derived from Pilbara craton volcanic rocks, but its equivalent was probably eroded from the Kaapvaal craton. Major disconformities exist at the base of Duitschland, Hekpoort and Daspoort formations. The Duitschland Formation represents significant tectonic uplift and renewed clastic input across the craton. The Duitschland/Timeball Hill contact is reconsidered as a conformable contact to paraconformity with a minor time gap on the basis of sedimentary observations and similar zircon signatures, thus placing the top of the Duitschland Formation at about 2320 Ma. Although no age constraints were obtained for the basal Duitschland glacial period, it was likely associated with tectonic activity between ~2420 Ma and sometime before 2320 Ma. The Pretoria Group tectonostratigraphic record is largely consistent with global supercontinent assembly and rifting in the interval ~2.4-2.1 Ga. It includes possible crustal uplift and tectonic activity between deposition of iron formation and the upper Duitschland Formation, and two potential rifting events in the Bushy Bend lavas and Hekpoort basalts. Only two glacial intervals are confirmed for South Africa (Makganyene-Rietfontein diamictite ~2250-2220 Ma, basal Duitschland diamictite ~2420-2320 Ma), with a third possibly represented by a cryptic glacial surface in the Duitschland Formation.