Successive sedimentary recycling regimes in southwestern Gondwana: Evidence from detrital zircons in Neoproterozoic to Cambrian sedimentary rocks in southern Africa

Abstract

Neoproterozoic sedimentary rocks in the Gariep, Saldania and Damara Belts and the Nama Group of South Africa and Namibia, which were deposited between ca. 750 Ma and ca. 550 Ma, have detrital zircon distribution patterns dominated by four major age fractions: A: ≤ 750 Ma, epsilon-Hf = −32 to +13, B: 950 to ca. 1100 Ma, epsilon-Hf = −25 to +11, C: ca. 1100 to 1300 Ma, epsilon-Hf = −25 to +12, and D: ca. 1775 to 1950 Ma, epsilon-Hf = −21 to +5. Detrital zircons older than 2000 Ma are scarce or absent. The large ranges of initial Hf isotopic composition within each of the age groups suggest that rocks with similar ages but different crustal histories contributed to each of them. The A and B components occur in the higher stratigraphic levels of the Saldania Belt, Damara Belt and Nama Group, and in the allochtonous Marmora and Malmesbury Terranes. Lower stratigraphic units of the Nama Group, Damara and Saldania Belts, and in the entire Port Nolloth Group of the Gariep Belt have combinations of the C and D components in variable proportions without B. The A and B zircons must have been sourced from emerging Pan African mountain belts in Neoproterozoic time, but the protosource(s) of the B component must have formed significantly earlier. The C component formed at the time of assembly of the Rodinia supercontinent. Age distributions of potential bedrock protosources within the Namaqua-Natal Belt of southern Africa are such that they would yield detrital zircon with both B and C age ranges represented, and with narrower distributions of epsilon-Hf than observed in the Neoproterozoic sandstones. This makes the Namaqua-Natal belt an unlikely source for the C-component. Erosion, redeposition and mixing of older, Mesoproterozoic cover sequences of the Kalahari Craton with near-unimodal C and D-dominated distribution patterns gives a much better match to the observed age and Hf isotope patterns of detrital zircon in the lower stratigraphic levels of the Neoproterozoic successions than direct derivation from crystalline basement. Recycling of A + B components has dominated the sedimentary provenance budget in southern Africa throughout the Phanerozoic. This provenance regime was established at the time of assembly of Gondwana in the late Neoproterozoic. The earlier, C + D regime was established by ca. 1100 Ma, and was related to the assembly of Rodinia.The detrital zircon data from southern Africa highlight the importance of protosource generation during periods of supercontinent assembly, followed by storage in continental margin and intracontinental basins, and recycling, mixing and redistribution in periods of breakup and denudation. In a scenario where recycling is a dominant factor, protosources that can be identified from detrital zircon data have little or no relevance for the tracing of detritus in a deposit from “source to sink”.