U–Pb and Lu–Hf zircon data in young sediments reflect sedimentary recycling in eastern South Africa

Abstract

Detrital zircon from unconsolidated, Cenozoic sediments from eastern South Africa has been analysed for U–Pb and Lu–Hf isotopes by laser ablation inductively coupled plasma mass spectrometry. Identifiable bedrock sources have made local contributions to the detrital zircon populations, but the dominant zircon components are of regional distribution: late Mesoproterozoic (e Hf = –5 to +10), Neoproterozoic to early Palaeozoic (e Hf = –10 to +10), and minor late Palaeozoic (e Hf ≈ 0). Archaean zircons are scarce even in sediments deposited on exposed Archaean basement or by rivers eroding it. The dominant components cannot be tied to specific first-generation sources in southern Africa or its former Gondwana neighbours. Instead, we see the effect of mixing and remobilization of debris from large parts of the supercontinent in the early Phanerozoic, which was stored in the Karoo basin and other continental cover sequences and shed from there to the present site of deposition. Therefore, data from detrital zircon in these deposits tell us less about the path of detritus from source to sink in a recent sedimentary system than about processes in much earlier erosion–transport–deposition cycles. To facilitate comparison of detrital zircon age distribution patterns, a simple and intuitive method that takes sampling uncertainty explicitly into account is proposed. Supplementary materials: U–Pb and Lu–Hf data, and concordia diagrams and discussion of effects of discordance are available at http://www.geolsoc.org.uk/SUP18884.