Neoarchaean Clastic Rocks of the Kaapvaal Craton – Provenance Analyses and Geotectonic Implications Eva Anita Schneiderhan Neoarchaean Clastic Rocks on the Kaapvaal Craton – Provenance Analyses and Geotectonic Implications by Eva Anita Schneiderhan THESIS presented in fulfilment of the requirements for the degree of PHILOSOPHIAE DOCTOR in GEOLOGY in the FACULTY OF SCIENCE of the UNIVERSITY OF JOHANNESBURG Supervisor Dr. U. Zimmermann Co-Supervisor Prof. J. Gutzmer May 2007 Declaration I declare that this thesis is my own original work, conducted under the supervision of Dr. Udo Zimmermann and Prof. Jens Gutzmer. It is submitted for the degree Doctor of Philosophy at the Faculty of Science at the University of Johannesburg. No part of this research has been submitted in the past, or is being submitted, for a degree or examination at any other university. E. A. Schneiderhan I Acknowledgements There are a number of people whom I would like to thank, because without them I would not have been able to succeed with this thesis. I appreciate your time and your support that you expressed in various ways, especially during the difficult last months. First of all, I am thankful to my supervisor Dr. Udo Zimmermann and co-supervisor Prof. Jens Gutzmer for providing me with the topic of this thesis. I am grateful for their support, constructive criticism and discussions, as well as for the bursary and research funding granted. I am indebted to the German Academic Exchange Service (DAAD) for providing a generous scholarship under grant number D/02/28359 for the first two years. Avgold Ltd., Mr. Rod Tucker and Mr. Dries Ferreira are thanked for giving access to their exploration drill core and their hospitality in Allanridge. Furthermore, Dr. Herman Dorland is thanked for helping me with the sample collection in the field. In this respect, I would also like to thank Mr. Bill Wheater who gave us access to his Farm Kransberg, from where some of the studied samples originate. Regarding the logistics and sample analyses, the support by the staff of the Department of Geology, University of Johannesburg, and SPECTRAU is thankfully appreciated. I greatly appreciate the support by Dr. Richard Armstrong, Australian National University, Canberra, who performed SHRIMP II analyses on a large number of zircons. Furthermore, I gratefully acknowledge that Prof. Klaus Mezger granted me access to the facilities of the Zentrallaboratorium für Geochronologie of the Westfälische Wilhelms Universität Münster, Germany, to carry out analyses on Sm-Nd and Rb-Sr isotopic systems. Also Miss Heidi Baier and Prof. Erik Scherer are thanked for their support in acquiring a lot of numbers and processing them into useful data. I appreciate their great hospitality during my research visit, as well as fruitful discussion of the data later on. II Also Prof. Harald Strauss and Mr. Artur Fugman from the Stabile Isotopen Labor of the Geologisch-Paläontologisches Institut of the Westfälische Wilhelms Universität Münster are thanked for taking care of the analyses of stable isotopes of carbonates and organic carbon of selected samples. I appreciate the support of all those who helped me get going in my first weeks in South Africa. Thanks to André. You made me a much stronger person, taught me how to catch life’s curveballs, and proved a lot of patience. Jy is my beste maatjie! Lastly, I am eternally indebted to my parents for their unconditional support during the last years. I appreciate that they let me go - which brought me closer to them again. This thesis is dedicated to my mother, and especially to my father who won’t see the result of my studies anymore. I know that he would have seriously tried to read this thesis entirely, and I appreciate it. III Abstract The provenance of the Neoarchaean Ventersdorp Supergroup and several age-related supracrustal successions was analysed to gain insight into the geotectonic evolution of the Kaapvaal Craton during the transition from the Archaean to Proterozoic Eras. The studied successions include, besides the siliciclastic formations of the Ventersdorp Supergroup, the upper Wolkberg and Buffelsfontein Groups, the Godwan Formation and the Schmidtsdrift Subgroup of the basal Transvaal Supergroup in Griqualand West. Petrographic, whole rock geochemical and Sm-Nd isotopic analyses were combined with SHRIMP U-Pb age dating of detrital zircons. Furthermore, Rb-Sr isotopic studies were carried out on carefully selected suites of samples from surface exposure or, wherever possible, on deep diamond drill core. The Ventersdorp Supergroup is an up to 5 km thick undeformed, only slightly metamorphosed volcano-sedimentary succession deposited on the Kaapvaal Craton between 2714 Ma and 2665 Ma. A lack of major time hiati to the underlying Mesoarchaean Witwatersrand Supergroup and covering Neoarchaean to Palaeoproterozoic Transvaal Supergroup render the Ventersdorp Supergroup very well suited for the investigation of the geotectonic evolution of the Kaapvaal Craton near the Archaean-Proterozoic boundary. This is supported by its excellent preservation, which also allowed detailed studies of sedimentological structures, such as seismites indicating Neoarchaean earthquakes. The provenance analyses carried out on the clastic formations of the Ventersdorp Supergroup point to a gradual change in tectonic evolution from typically Archaean to post-Archaean processes rather than a drastic, unique transition in the case of the Kaapvaal Craton. Texturally immature wackes of the Kameeldoorns Formation, representing the oldest clastic units of the Ventersdorp Supergroup, are derived mainly from Mesoarchaean source rocks, whereas the stratigraphically younger Bothaville Formation displays geochemical signatures comparable with Archaean trondhjemite-tonalite granodiorite-suites (TTGs), thus suggesting crustal addition in the so-called ‘Archaean-style’. The extension of provenance analyses to supracrustal successions that are tentatively correlated with the Bothaville Formation, revealed contributions from granitoid IV sources that formed under post-Archaean and Archaean conditions. Furthermore, the geochemical data for all analysed formations support a passive margin setting. Arc settings, as indicated in some samples, are due to the input of less fractionated volcanic material that provides evidence of distal volcanism. Analyses of Nd-isotopic systematics and U-Pb ages of detrital zircons revealed a Mesoarchaean age for the source rocks of the formations. U-Pb age dating of detrital zircons of the Godwan Formation suggests that this formation is of Mesoarchaean age, and therefore not a correlative of the other Neoarchaean successions. Hence, the results suggest that the continental crust of the Kaapvaal Craton was thick enough since the Mesoarchaean (2.8 - 3.1 Ga) to allow long-term crustal recycling, and therefore modern plate tectonic processes could have operated earlier than on other well-studied cratonic blocks. During the Neoarchaean, however, crustal thickening of the Kaapvaal Craton took place by accretion of Archaean-style TTGs along the margins of the craton. Thus, Archaean and post-Archaean tectono-magmatic processes co-existed. Furthermore, the Neoarchaean supracrustal successions represent the first sedimentation events on an entirely stabilised and tectonically quiescent Kaapvaal Craton. Input from distal volcanic sources marks the last sign of volcanic activity prior to the craton-wide deposition of carbonate rocks of the Transvaal Supergroup. Geochronological data also imply a connection of the Neoarchaean Kaapvaal Craton to further cratonic blocks that may hold source rocks for the studied formations, as for some small age populations of older detrital zircons (ca. 3.1 - 3.4 Ga), no suitable source area could be identified on the Kaapvaal Craton itself. However, it seems unlikely that the Zimbabwe Craton was one of these cratonic blocks, because the Rb-Sr whole rock ages of all studied formations yield a model age of 2092 ± 55 Ma, which is thought to correspond to a craton-wide influence of the 2.05 Ga old Bushveld Igneous Complex on the Rb-Sr isotope systematics of all analysed clastic successions. This influence is apparently missing in the Southern and Central Marginal Zones of the Limpopo Belt, suggesting that the collision between the Kaapvaal and Zimbabwe Cratons only took place after the emplacement of the Bushveld Igneous Complex, i.e. after 2.05 Ga. V Table of Contents Table of Contents DECLARATION I ACKNOWLEDGEMENTS II ABSTRACT IV 1 INTRODUCTION 1 1.1 Objective 5 1.2 Analytical Methodology 6 1.3 Plate-tectonic Models and crustal Growth 7 1.3.1 Geotectonic models 8 1.3.1.1 Vertical geotectonic (plume) model 9 1.3.1.2 Horizontal geotectonic (arc) model 10 1.3.1.3 Supercontinents and Vaalbara 10 1.3.2 Archaean and post-Archaean crustal growth models 12 1.3.2.1 Archaean crustal growth 12 1.3.2.1.1 Greenstone Belts 13 1.3.2.1.2 Trondhjemite-Tonalite Granodiorites (TTG) 14 1.3.2.2 Post-Archaean crustal growth 15 1.3.2.3 Geochemical characteristics 15 1.3.3 Summary 16 1.4 References 17 2 GEOLOGICAL SETTING 29 2.1 Previous Work 38 2.2 Ventersdorp Supergroup 41 2.2.1 Lithostratigraphy 41 2.2.2 Palaeoenvironmental Significance 46 2.3 Transvaal proto-basinal Successions 47 2.3.1 Godwan Formation 47 2.3.1.1 Lithostratigraphy 47 2.3.1.2 Palaeoenvironmental significance 48 2.3.2 Wolkberg Group 49 2.3.2.1 Lithostratigraphy 49 2.3.2.2 Palaeoenvironmental Significance 50 2.3.3 Buffelsfontein Group 51 2.3.3.1 Lithostratigraphy 51 2.3.3.2 Palaeoenvironmental