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

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 (ε_Hf = –5 to +10), Neoproterozoic to early Palaeozoic (ε_Hf = –10 to +10), and minor late Palaeozoic (ε_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.

针对南非东部未固结新生代沉积物中的碎屑锆石（detrital zircon），研究人员通过激光剥蚀电感耦合等离子体质谱法（laser ablation inductively coupled plasma mass spectrometry）完成了铀-铅与镥-铪同位素分析。可识别的基岩物源对碎屑锆石群体存在局部贡献，但占主导的锆石组分具有区域分布特征：包括中元古代晚期（ε_Hf = –5 至 +10）、新元古代至早古生代（ε_Hf = –10 至 +10）以及少量晚古生代（ε_Hf ≈ 0）组分。即便在沉积于出露太古宙基底之上或由侵蚀该基底的河流形成的沉积物中，太古宙锆石也极为稀缺。上述占主导的锆石组分无法与非洲南部及其原冈瓦纳（Gondwana）邻区的特定原生物源相匹配。取而代之的是，我们观测到早古生代时期超大陆大范围区域的碎屑物质发生混合与再活化的效应，这些物质曾被储存在卡鲁盆地（Karoo basin）与其他大陆盖层序列中，并自此被搬运至现今的沉积位点。因此，相较于揭示现代沉积系统中碎屑物从物源区到沉积汇的搬运路径，这些沉积物中的碎屑锆石数据所能反映的，是更早时期的侵蚀-搬运-沉积循环过程。为便于对比碎屑锆石年龄分布模式，本文提出了一种可明确纳入采样不确定性的简洁直观分析方法。