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

Highlights

• ⁴⁰Ar/³⁹Ar dating of 253 individual muscovites from 39 Cape Supergroup samples

• Complimented with detailed petrographic observations and mineral chemistry

• Ordovician detrital muscovite population likely sourced from South America

• Peak, or final phase, of CFB deformation constrained to 253.4–249.6 Ma.

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.

Introduction

The Cape Fold Belt (CFB) is a Permian, low-metamorphic grade, fold and thrust belt that extends ~1300 km along the western and southern coastal margins of southern Africa. It incorporates Neoproterozoic to Cambrian metasediments and Cape Granite Suite igneous rocks of the Saldania Belt, lower to mid-Palaeozoic siliciclastic sediments of the Cape Supergroup, and upper Palaeozoic to lower Mesozoic strata of the overlying Karoo Supergroup (Hälbich, 1983). The CFB is considered a central component of a larger Permian Gondwanide orogenic belt flanking the margin of southwestern Gondwana, extending from the Sierra de la Ventana Fold Belt in Argentina, across southern Africa, and continuing east through the Falkland (Malvinas) Islands and Ellsworth-Whitmore Mountains of Antarctica (Du Toit, 1937; Curtis and Hyam, 1998; Rapela et al., 2003; Lindeque et al., 2011; Pángaro and Ramos, 2012; Fig. 1). However, the relationship of the CFB to other terranes in this Permian orogenic belt, the provenance of the CFB sediments and the timing of regional deformation are poorly constrained, thus hampering efforts to construct a coherent tectonic and structural model for the region.

The Cape Supergroup is a passive margin sedimentary succession of basal quartzites overlain by interbedded sandstone, siltstone and shale units (Thamm and Johnson, 2006). Deposition of the Cape Supergroup within the Cape Basin is estimated to have occurred between ca. 510 and 350 Ma, based on palaeontological evidence and U-Pb detrital zircon ages (Broquet, 1992; Miller et al., 2016a, Miller et al., 2016b; Gess, 2016). U-Pb detrital zircon provenance studies suggest the Cape Supergroup is largely derived from the adjacent Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) and Pan-African and Brasiliano orogenic terranes (Fourie et al., 2011; Naidoo et al., 2013; Miller et al., 2016a, Miller et al., 2016b), with lesser contributions from Ordovician South American sources (Fourie et al., 2011; Ramos et al., 2014). However, further provenance investigations are required to confirm a South American source.

Current constraints on the timing of CFB orogenesis rely on a handful of limited ⁴⁰Ar/³⁹Ar studies. The first ⁴⁰Ar/³⁹Ar age constraint for CFB orogenesis was provided by Gentle et al. (1978), who produced an age of 248 ± 2 Ma. Subsequent ⁴⁰Ar/³⁹Ar studies by Hälbich et al. (1983) and Gresse et al. (1992) suggested that deformation progressed via a series of tectonic pulses within an 80 m.y. period between ~300 and 220 Ma. More recently, Hansma et al. (2015) carried out ⁴⁰Ar/³⁹Ar experiments on single muscovite grains and bulk muscovite aliquots, and proposed bimodal CFB orogenesis at ca. 275–260 Ma and ca. 255–245 Ma.

A preliminary high precision ⁴⁰Ar/³⁹Ar dating study by Blewett and Phillips (2016) revealed a clustering of muscovite ages at 253 ± 2 Ma (2σ), which was interpreted as the main/final phase of CFB deformation. However, these ages were confined to three samples from only two locations. Ages >440 Ma reported in this study were interpreted as detrital ages consistent with sediment contributions from Pan-African terranes. At the same time, a large number of grains yielded intermediate apparent ages ranging from 255 and 440 Ma; these results were ascribed to varying degrees of argon loss from older detrital grains and/or the presence of intergrown detrital/neocrystallised mica grains.

The aims of this study are to provide new insights into the provenance of sediments in the Cape Basin and improve constraints on the timing of CFB Orogenesis. This study presents ⁴⁰Ar/³⁹Ar data for muscovite grains from 27 samples collected in a series of structurally controlled traverses across the CFB. The age results of this study are combined with those of Blewett and Phillips (2016), providing a combined ⁴⁰Ar/³⁹Ar dataset of over 400 muscovite ages from 41 Cape Supergroup samples across the southern CFB branch. These data were complemented by detailed field observations, documentation of micro-structural features, and muscovite mineral chemistry. The latter information permitted the selection of samples with the greatest potential for defining either sedimentary provenance or constraining the age of CFB deformation.