Neoproterozoic to Permian evolution of the Cape Fold Belt, South Africa: Constraints on sediment provenance and orogenesis from high-precision 40Ar/39Ar dating of detrital and metamorphic micas
AffiliationSchool of Earth Sciences
Document TypePhD thesis
Access StatusOpen Access
© 2019 Scarlett Caroline Joyce Blewett
The Permian Cape Fold Belt extends 1300 km along the western and southern coastal margins of South Africa. It comprises complexly deformed rocks of the lower-Palaeozoic Cape Supergroup, and Mesozoic parts of the Karoo Supergroup. Large-scale thrusts expose portions of the underlying Saldania Belt; a low-grade metamorphic belt intruded by granites of the ca. 550-500 Ma Cape Granite Suite. Both the Cape Fold Belt and Saldania Belt are segments of ancient continent-scale orogenic systems. The former is thought to be a portion of the Permian Gondwanides Orogen that extended from the Sierra de la Ventana Fold Belt of Argentina, across southern Africa, and into the Falkland (Malvinas) Islands, and Ellsworth-Whitmore Mountains of Antarctica, whereas the latter is considered one of the many Neoproterozoic-Cambrian Pan-African/Brasilliano terranes developed during the amalgamation of west Gondwana. However, fragmentation of Gondwana and separation of the Cape Fold Belt from its neighbouring terranes during the Cretaceous has provided major challenges in understanding both the geodynamic evolution of the poorly exposed Saldania Belt and the mechanics by which the Cape Orogen formed within the Gondwana interior. Gondwanan tectonic models often rely on geochronological provenance studies to not only link sedimentary sources and sinks, but to also correlate sedimentary successions in separated terranes. Previous geochronological provenance studies on the Cape Fold Belt have utilised U-Pb dating of detrital zircons to suggest that sediments of the Saldania Belt and Cape Supergroup were largely sourced from Mesoproterozoic rocks of the Namaqua-Natal Metamorphic Belt to the immediate north and underlying the Cape Fold Belt, as well as undifferentiated Pan-African and/or Brasilliano terranes. However, as zircon is able to survive orogenic recycling and long-distance transport, U-Pb detrital zircon studies have been unable to identify the most recent and proximal sources of sediments. In addition to having only broadly defined sediment provenance, the timing and extent of orogenesis during the Pan-African and Permian periods are poorly constrained. The timing of deformation in the Saldania Belt is only defined relative to the Cape Granite Suite, whereas the Cape Orogeny has been dated in a handful of limited 40Ar/39Ar studies. Early studies attempting to constrain Cape deformation utilised bulk mineral aliquots that yielded largely discordant 40Ar/39Ar age spectra, which the authors interpreted to represent multiple phases of deformation. A more recent study performed 40Ar/39Ar dating of single mica grains and proposed a bi-model evolution for the Cape Orogeny; however, this is based on only eight analyses. In this study, high precision 40Ar/39Ar geochronology is used to constrain the ages of individual detrital and metamorphic micas from low-grade rocks, as well as localised zones of variable deformation intensity in the southern Cape Fold Belt branch. Fundamental to this study, was the collection of detailed structural observations, petrographic and mineral chemistry data used to delineate detrital and neocrystallised mica age populations, from partially reset, altered, and/or complexly intergrown micas. A total of 648 individual mica grains were dated from 57 samples representing a variety of relatively undeformed and deformed Cape Supergroup and eastern Saldania Belt sediments, including crenulated metasediments, axial planar cleavages, thrust planes in duplex structures, and major shear zones. Only seven samples, collected from zones of intense deformation and focused fluid flow, yielded reproducible mica ages indicating that Cape Orogenesis was most pronounced at 257-248 Ma. These samples were located along, or in close proximity to the Worcester and Kango Fault systems, which are considered major decollement structures responsible for thin-skinned deformation of the Saldania Belt and Cape Supergroup sediments inland from the Gondwana margin. Biotite fusion ages of 272-270 Ma from a metamorphosed mafic dyke in the Kaaimans Inlier provide possible evidence for an earlier onset of Cape Orogenesis, preserved only in the Saldania basement. Samples from less deformed zones contained partially recrystallised detrital mica grains, mixed detrital and neocrystallised mica grains, and/or complex micas hosting clay and chlorite intergrowths, resulting in a spread of apparent ages older than the Cape Orogeny. 40Ar/39Ar detrital mica age populations were defined for a number of Saldania Belt and Cape Supergroup samples; these data were integrated with published U-Pb detrital zircon ages for provenance analysis. Detrital muscovite and zircon ages for the Lime Bank sequence and possibly part of the Kleinrivier sequence (Gamtoos Inlier) suggest exclusive provenance from the Mesoproterozoic Namaqua-Natal Metamorphic Belt or the similar-aged Maud Belt of Antarctica. In contrast, detrital ages for the Groenefontein and Huis River formations (Upper Cango Caves Group, Kango Inlier) indicate a source that experienced early Pan-African Orogenesis (580-550 Ma), such as the Sor Rondane Mountains or the Dronning Maud Land sector of the East African-Antarctic Orogen. The Cango Caves Group was folded prior to deposition of the overlying Kansa Group, which hosts abundant 530-510 Ma zircons. This suggests that deformation of the Cango Caves Group is Pan-African in age (i.e. 550-530 Ma) - possibly related to tectonic loading of the Kaaimans Inlier to the south, or the western Saldania Belt and Gariep Belt to the west. Later deformation of the Kango Inlier folded both the Cango Caves and Kansa Group, after which the conglomeratic Schoemanspoort Formation was deposited. A tightly constrained detrital mica age population of 510-500 Ma in the Schoemanspoort Formation represents either cooling/exhumation of the source terrane during the late stages of Pan-African tectonism or a younger tectonic and thermal pulse in the source area. These events could have occurred in the proximal Kaaimans Inlier, and may have been responsible for the combined folding of the Cango Caves and Kansa Groups. In the Kaaimans Inlier, 40Ar/39Ar incremental step-heating of muscovite from a pegmatitic vein suggest that parts of the Saldania Belt were affected by a post-Pan-African Ordovician thermal overprint. This overprint, identified in other Cape Granite Suite intrusives by previous studies, may represent a raised geothermal gradient in the basement as a result of Cape Supergroup sediment load and mantle flow coupled to far-field subduction along the Proto-Andean margin, and/or asthenospheric upwelling in the southern East African-Antarctic Orogen to the east of the Cape Fold Belt. 40Ar/39Ar analysis of individual mica grains from the Cape Supergroup reveals a dominant Ordovician (490-465 Ma) detrital muscovite population, suggesting provenance from orogenic belts possibly associated with the aforementioned Ordovician thermal overprint in the Cape Fold Belt basement; i.e. the Famatinian Orogen of western Argentina and/or the East African-Antarctic Orogen. Lesser detrital muscovite populations of 650-500 Ma and >730 Ma corroborate previous zircon provenance studies suggesting Pan-African and Namaqua-Natal Metamorphic Belts sources, respectively. The sediment provenance investigations presented in this study provide insights into the evolution of late Neoproterozoic to Devonian sedimentation in the Cape Fold Belt, and enable correlation of orogenic terranes spanning South American, African and Antarctic. In addition, well-constrained deformation ages from intensely deformed metasediments and major shear zones permitted formulation of a tectonic model for the Cape Orogeny, considering the onset, duration of deformation, and structural development of the Cape Fold Belt along the southwest Gondwana margin.
KeywordsCape Fold Belt; Saldania Belt; Gondwana; 40Ar/39Ar geochronology; Orogenesis; Provenance; Tectonics
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