Archaean-Palaeoproterozoic geochronometry of the central Kaapvaal Craton, South Africa
Date
2021
Authors
Beaton, Kimberley Jane
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Abstract
The Kaapvaal Craton hosts one of the most complete records of Archaean to Palaeoproterozoic crust formation on Earth. Thermal and magmatic events, that initially stabilised and subsequently affected this crust and its underlying mantle root, were integral in the shaping of the economic Bushveld Complex and Witwatersrand Supergroup deposits. However, considerable uncertainty remains about the exact timing and origin of these events. This is particularly well-illustrated in the Vredefort Dome, a 2.02 Ga impact-induced feature located centrally within the Witwatersrand Basin. This study focuses on 4 field areas: the Bushveld Complex contact metamorphic aureole, a lower crustal xenolith from the Lace kimberlite, the Archaean basement core complex, and the supracrustal collar of the Vredefort Dome. Multiple metamorphic events have been identified using Ar-Ar white mica, U-Pb zircon, and Lu-Hf and Sm-Nd garnet chronology. M1 metamorphism is preserved in the core of the Vredefort Dome with ages between3102.59 ± 4.35 and 2940.27 ± 1.27 Ma, related to TTG magmatism and the stabilisation of the central Kaapvaal Craton. This metamorphism is evident in U-Pb zircon ages from metapelitic rafts. The metapelites from the core of the Dome preserve P-T conditions of 870-880 °C at 6.5-6.6 kbar. Additional Sm-Nd garnet chronology was conducted on similar metapelite samples, yielding inconclusive, intermediate ages. M2 metamorphism is related to the Ventersdorp large igneous province (LIP) between 2.79 and 2.69 Ga. This metamorphism is preserved in the Sm-Nd garnet ages from a lower crustal xenolith from the Lace kimberlite, yielding UHT (ultrahigh-temperature) metamorphic conditionsof 1115 °C at 9.54 kbar with an age of 2755 ± 13 Ma. Additional evidence of Ventersdorp-related metamorphism on the central Kaapvaal Craton is preserved in U-Pb zircon ages from a metapelite sample from the core of the Vredefort Dome, as well as Sm-Nd garnet ages from the inner collar of the Vredefort Dome. U-Pb zircon chronology from a metapelite from the core of the Dome yields a concordant age of 2742.30 ± 0.96 Ma. Two garnet-bearing samples from the inner collar of the Vredefort Dome preserve ages of 2789.1 ± 1.6 Ma and 2689.6 ± 3.1 Ma, with proposed P-T conditions of 500 °C at 3.1 kbar and 570 °C at 2.9 kbar, respectively. In the past, the metamorphism in the collar of the Dome has been ascribed to the emplacement of the Bushveld Complex based on circumstantial evidence, with no decisive chronological support. This work, however, shows that the mid-amphibolite facies metamorphism in the collar of the Dome is substantially older, and a result of the emplacement and thermal perturbations related to the Ventersdorp LIP.M3 metamorphism is related to orogeny and the collision between the Kaapvaal and Pilbara Cratons at 2.15-2.12 Ga. In this study, evidence of this event is preserved in Ar-Ar white mica ages from the collar of the Vredefort Dome, yielding repeat ages of 2112.9 ± 12.2 and 2115.2 ± 12.1 Ma. M4 metamorphism is a result of the emplacement of the Bushveld Complex at 2.06-2.05 Ga. This metamorphism is primarily confined to the thermal aureole of the Complex. Sm-Nd garnet chronology on rocks in the aureole yield P-T conditions of 3.5-3.9 kbar and 780-785 °C close to the contact with the Rustenburg Layered Suite (RLS), and 570-580 °C at 2.9-3.1 kbar further from the contact. Sm-Nd and Lu-Hf garnet ages are inconclusive with and age of 2034 ± 14 Ma for the sample further from the contact with the RLS, and 2046 ± 48 Ma (Sm-Nd) and 2013 ± 140 Ma (Lu-Hf) for the sample close to the contact. Additional evidence of the thermal effects of the emplacement of the Bushveld Complex has been identified in the Vredefort Dome. A concordant cluster of U-Pb zircon points from the core of the Vredefort Dome yields an age of 2055.03 ± 1.68 Ma. Irregular garnet overgrowths on the samples from the collar of the Vredefort Dome have P-T conditions of 528 °C at 4.95 kbar, suggesting a Bushveld timing is necessary to achieve the modelled pressures. This research has implications for the existence of the Vaalbara Craton between 2.79 and 2.69 Ga. The effects of the emplacement of the Bushveld Complex are more limited than previously proposed, and the Ventersdorp Supergroup emplacement appears to be the dominant thermal event on a craton scale
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree Doctor of Philosophy, 2021