Provenance of Cape Supergroup sediments and timing of Cape Fold Belt orogenesis: Constraints from high-precision 40Ar/39Ar dating of muscovite

Abstract

The Permian Cape Fold Belt (CFB) of South Africa forms part of a major orogenic belt that originally extended from Argentina, across southern Africa and into Antarctica. The CFB is dominated by complexly folded and faulted rocks of the siliciclastic Cape Supergroup that were deposited in the Cape Basin. The provenance of the Cape Supergroup, timing of deformation and tectonic setting are poorly constrained. U-Pb detrital zircon provenance studies suggest that the Cape Basin received sedimentary detritus from the African Mesoproterozoic Namaqua-Natal Metamorphic Belt, Neoproterozoic-Cambrian Pan-African Belts and the Brasiliano orogenic belts of South America, Africa and Antarctica. However, as zircon is able to survive multiple orogenic and sedimentary transport cycles, complementary provenance tools are required to confirm Cape Supergroup provenance. Previous studies have suggested both uni-modal and multi-modal models for the timing of CFB orogenesis. In the current study, structurally controlled, muscovite-bearing samples were collected along several north-south traverses across the CFB. Detailed textural and mineral chemistry analyses identified multiple muscovite populations, commonly with complex intergrowth features. High precision 40Ar/39Ar analyses reveal a dominant 490–465 Ma detrital muscovite population, lending support to a largely South American provenance for the Cape Supergroup. Lesser detrital muscovite populations in the range 650–500 Ma and >730 Ma, corroborate previous zircon provenance studies suggesting Pan-African/Brasiliano terranes and the Namaqua-Natal Metamorphic Belt as significant sediment sources, respectively. Detailed 40Ar/39Ar analyses of multiple neo-crystallised muscovite samples are consistent with a single major phase of CFB deformation occurring between 253.4 and 249.6 Ma. This age is interpreted to represent either the peak or final dominant phase of CFB deformation.