Detrital zircon in sandstones from the Palaeoproterozoic Waterberg and Nylstroom basins, South Africa: Provenance and recycling

Laser-ablation ICPMS U-Pb and Lu-Hf isotope data on detrital zircon in sandstone from the Palaeoproterozoic Waterberg Group in the Waterberg and Nylstroom basins of northeastern South Africa contain a dominant Palaeoproterozoic (2 000 to 2 150 Ma) age fraction and minor, late Archaean zircon (2 650 to 2 780 Ma). Zircon in the 2 200 to 2 350 Ma age range is very scarce. There are no statistically significant differences in detrital zircon age distributions of sandstones in the two basins. In contrast, conglomerate from the northern margin of the Waterberg basin (Mogalakwena Formation) is dominated by a late Archaean age fraction (2 600 to 2 700 Ma, with minor 3 200 to 3 300 Ma), which resides in quartzite clasts. Zircon ages alone cannot distinguish between potential sources of detritus in the Kaapvaal Craton and Limpopo Belt. Hf isotope data, however, suggest that the main input to the basins was from recycled Archaean to Palaeoproterozoic sedimentary cover successions on the Kaapvaal Craton, supplemented by material from late Archaean granitic intrusions and the felsic igneous rocks of the Rooiberg Group and / or felsic members of the Bushveld Complex. The only known source of a prominent group of ca. 2.0 Ga detrital zircon with epsilon-Hf = 0 to -3 would be the youngest generation of granites in the Limpopo Belt (e.g. Mahalapye Granite). The Waterberg Group or its equivalents have provided only minor amounts of detritus to the Neoproterozoic to Cambrian basins at the western margin of the Kalahari Craton, directly or through recycling of intermediate deposits. However, erosion of the rocks in the Waterberg and Nylstroom basins provided material to the lower stratigraphic levels of the adjacent, Karoo-age Ellisras and Springbok Flats basins. Downloaded from https://pubs.geoscienceworld. org/gssa/sajg/article-pdf/122/1/79/4681030/79_0008_andersen_et_al. pdf by Dalhousie Univ Libraries Serials/Killam Library user on 05 August 2019 DETRITAL ZIRCON IN SANDSTONES FROM THE PALAEOPROTEROZOIC WATERBERG AND NYLSTROOM BASINS, SOUTH AFRICA: PROVENANCE AND RECYCLING 80 SOUTH AFRICAN JOURNAL OF GEOLOGY that are indistinguishable in terms of age may have sufficiently different ranges of Hf isotope composition to allow them to be distinguished (e.g. Veevers et al., 2005; Howard et al., 2009). Although detrital zircon geochronology is not a problemfree method, it has certain advantages over provenance indicators such as heavy mineral assemblages. Poly-mineralic heavy mineral assemblages may be affected by hydraulic sorting during transport, controlled by differences in density, grainshape and surface properties (Garzanti et al., 2009). Regardless of age, zircon crystals show little variation in density or other parameters promoting differentiation by sedimentary processes. On the other hand, radiation-damaged zircon is more susceptible to abrasion during transport than crystalline zircon. However, the degree of radiation damage depends on the combination of age and uranium (and thorium) concentration. Whereas old and U-rich zircon is likely to suffer, especially when older sediments are recycled, grains with lower U concentration are less affected. It is therefore unlikely that an age fraction is completely removed from a sediment by abrasion during transport, and low-U members of any age fraction, even Archaean ones, will be able to survive to the present day. This is demonstrated by examples of Phanerozoic sandstones containing abundant Archaean zircon (e.g. in the Karoo-age Ellisras Basin, Veevers and Saeed, 2007). It should, however, always be kept in mind that the U-Pb age and Hf isotopic composition of a detrital zircon grain in a clastic sediment reflect the age and Lu-Hf characteristics of the rock in which it formed, the protosource. Because a sediment may be the result of erosion and redeposition of older sedimentary strata, the protosource is not necessarily the immediate source of detritus. This is well illustrated by Neoproterozoic to Cenozoic strata in southern Africa (Figure 1). Detrital zircon data indicate the existence of two, successive provenance regimes in the region: In the early Neoproterozoic, Mesoproterozoic continental cover • successions containing a mixture of late Palaeoproterozoic (1775 to 1950 Ma) and late Mesoproterozoic (1 100 to 1 300 Ma) detrital zircons were recycled into continental margin basins forming the Port Nolloth Group of the Gariep Belt, the lower stratigraphic units in the Saldania Belt, and in the Nama Group (Andersen et al., 2018b). Since the later Neoproterozoic, detritus derived from 950 to • 1100 Ma and 550 to 750 Ma protosources have been recycled through Neoproterozoic, Phanerozoic and Cenozoic deposits, with only minor younger contributions (Andersen et al., 2016a, b and 2018b). Archaean zircon is scarce or absent from most of these deposits, even from Cenozoic sands deposited by rivers eroding Archaean rocks of the Kaapvaal Craton (Andersen et al., 2016a). This excludes not only potential source rocks in the crystalline bedrock of the Kaapvaal Craton and Palaeoproterozoic to early Mesoproterozoic terranes along its western boundary, but also recycling of sedimentary cover sequences deposited before the early Mesoproterozoic, including the lower part of the Bushmanland Group, the Keis Supergroup and Palaeoproterozoic cover sequences on the Kaapvaal Craton (Dorland, 2004; van Niekerk, 2006; McClung, 2006; Schröder et al., 2016; see also review by Andersen et al., 2018b). The Palaeoproterozoic red bed-type sandstones and conglomerates of the Waterberg Group that are exposed in northern part of the Kaapvaal Craton are potential intermediate reservoirs for material to be recycled into younger basins. Moreover, the detrital zircon signature of the Waterberg Group is important to correlate the stratigraphy of the Transvaal, Kanye and Griqualand West basins (e.g. Beukes et al., 2019). Only few detrital zircon U-Pb data from the Waterberg Group have been published (a total of 84 analyses from two samples in Dorland, 2004 and 63 analyses from one sample by Corcoran et al., 2013); neither of these studies reports Hf isotope data. The relatively scarce data from these studies indicate dominant, Palaeoproterozoic detrital zircon age fractions that do not match with those of younger deposits. In order to better understand the provenance characteristics of the Waterberg Group, as well as its role in subsequent sedimentary recycling processes in southern Africa, U-Pb and Lu-Hf analyses have been performed on eleven samples from the Waterberg and Nylstroom basins, and the results are presented here.