The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town Petrogenesis of the False Bay Dyke Swarm, Cape Peninsula, South Africa Nils Rainer Backeberg Town Cape of University Dissertation submitted for the degree of Master of Science Department of Geological Sciences University of Cape Town 2012 I Swarm Town Cape of "That is sufficiently rad ica l to entertain alternatives ... " D L Re id 27th October, 2010; in Potsdam, G€rmany University Abstract The False Bay dyke swarm is the southern-most set of Early Cretaceous dolerite dyke in­ trusions along the western margin of southern Africa associated with rifting of Gondwana and opening of the South Atlantic Ocean. Previous work highlighted the evolution of a single magma type from olivine-tholeiite basalt to ferro-tholeiite andesite. This thesis presents de­ tailed field observations of each dyke in the False Bay - Cape Peninsula region and the focus of the study is on the finer details of the differentiation process to define combined assimilation and fractional crystallisation models. Fractional crystallisation and assimilation trends are identified in the suite using both trace element and Sr-Nd isotope data; however, evidence of crustal assimilation is only pronounced in samples with less than 5 wt. % MgO. Initial differentiation (> 5Town wt. % MgO) is characterised mainly by near closed-system fractional crystallisation, while the later stages of differenti­ ation show a progressively larger input from assimilation. Assimilation trends of the False Bay dolerites are modelled with various assimilantCape sources, including the Cape Granite Suite, the Malmesbury Group and the Bushmanland Terrane. The AFC models show that the ex­ posed country rocks of the False Bay dyke swarm,of including Cape Granites and Malmesbury Group metasediments, are compositionally unfavourable for producing the observed assimila­ tion trends in the dykes. Instead, a more suitable crustal assimilant would be Mesoproterozoic gneisses similar to those exposed in the neighbouring Namaqua Province, which may underlie the Cape Peninsula.University 11 Acknowledgements The study of the False Bay dyke swarm is part of the research group, Inkaba yeAfrica, and falls into the Chapter 2.1: Margins of Africa project. Inkaba yeAfrica is run through the University of Cape Town and the Geoforschungszentrum, Potsdam, and provided the funding for this thesis. Inkaba yeAfrica graciously exposed me to two conferences during the course of the project, first in Swaziland during 2009 and the second in Potsdam, Germany in 2010. These conferences provided great insight into the project as well as the academic world of geology and enriched my experience gained during the Masters thesis. I thoroughly enjoyed working on the geology in Cape Town for my thesis and that leads me to thank my supervisor, Professor Dave Reid. He provided much guidance and mentorship, while allowing me independence and free reign in the evolutionTown of the project and supported me along the way with any hick-ups that arose. Specifically when it came to my joy of taking the morning off for a surf and unfortunately in one occasion not coming back to the office for a whole month, due to an unexpected leg-break. A bigCape thank you goes to Professor Chris Harris, who acted as co-supervisor and a additional ofsource of support, including guidance through the oxygen isotope lab and resultant data as well providing me with strong confidence in complet­ ing this MSc. Thank you to Robert Trumbull and Rolf Romer, at the Geoforschungszentrum in Potsdam, who added much value to the manuscript that evolved out of the study and broadened the scope of topics that I got toUniversity encounter. I had much support from the entire staff and students at the Department of Geological Sciences during my 6 month recovery period and I would specifically like to thank Fayrooza Rawoot, Christel Tinguely, Shireen Govender and Petrus Ie Roux, who helped me through the lab work and took time out of their schedules to help process and prepare the samples for my project. Furthermore the list of people I would like to thank is endless and there are a few I would like to single out: Thanks to Carly Faber, Pia Viglietti and Kaylan Hamel, who brought the postgraduate "family" much needed entertainment and life up on the overheated and flooded 6th floor as well as accompanying me on short field trips to collect samples and measurements. 'pLIn(U3~""'l ~q) " ! .\P.g lnOH n'n ;jll!V'OI ';> ~ " P ~JJ ~~l 01 JSOI~ lUTOO ,'O)IA lfUJd s, UUtudU~~1 _'Ill hlOI;)(! <JUTPUU1S (<J.1Jq;J"{~Ufl S]INI J ]JS.'tu p UV PI_'ll ~ .' 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Jlu np ur.ll~od ~ S~ ;:'lUll) .\~I] '" IV Supervisors Prof. David L Reid [email protected] Department of Geological Sciences, University of Cape Town, South Africa Prof. Chris Harris [email protected] Department of Geological Sciences, UniversitJ' of CapeTown Town, South Africa Cape of University v Preface This study was initiated in July 2009 as part of Inkaba yeAfrica, a collaborative research group of the Department of Geological Sciences and AEON (Africa Earth Observation Network) at the University of Cape Town and the GeoForschungsZentrum, a German Research Centre for Geosciences at Potsdam, Germany. Inkaba yeAfrica covers all manors of geoscientific and interdisciplinary topics, with focus of the research under the following project titles: 1. Heart of Africa 2. Margins of Africa 3. Living Africa The False Bay dyke swarm records igneous activity during the openingTown of the Atlantic Ocean and is related to various dyke swarms and other igneous units along the southwest African margin that were emplaced during the Cretaceous Period. Therefore, this study falls within the "Margins of Africa; Project 2.1: Magmatic processes,Cape rifting and continental breakup". The research presented here continues from initialof work done by Prof. D. L. Reid and co-authors (1990, 1991 and 2007) and other authors since the 1950s. The results have been published in a peer-reviewed journal as: Petrogenesis ofUniversity the False Bay dyke swarm, Cape Peninsula: evidence for basement assimilation. Backeberg, N.R., Reid, D.L., Trumbull R.B. and Romer, R.L. South African Journal of Geology, 2011, Volume 114 (3-4), pages 335-352. VI Declaration I know the meaning of plagiarism and declare that all of the work in the document, save for that which is properly acknowledged, is my own. Nils Rainer Backeberg 2012 University of Cape Town Contents Abstract Acknowledgements ii Supervisors iv Preface v Declaration Town vi 1 Introduction 1 2 Regional Geological Setting 3 2.1 Namaqua-Natal Province 3 2.2 Malmesbury Group Cape 5 2.3 Cape Granite Suite 6 2.4 Cape Supergroup . of 7 2.5 Gondwana Breakup 8 2.6 West Coast Dyke Swarms and Related Magmatism 9 2.6.1 Henties Bay-Outjo ... 11 2.6.2 Mehlberg........ 12 2.6.3 Koegel Fontein Complex 12 2.6.4 Cedarberg. 13 2.6.5 Saldanha 14 2.6.6 FalseUniversity Bay . 14 3 Field Relations and Dyke Outcrops 17 3.1 Sea Point (SP) . 22 3.2 Clifton 2 (CL2) ... 22 3.3 Clifton 1 (CLI) ... 23 3.3.1 Clifton 3 (CL3) 23 3.4 Bakoven (BK) ... 23 3.5 Kasteelspoort (KS) 24 3.6 Oudekraall (001) 24 3.7 Oudekraa1 2 (002) 25 3.8 Llandudno Ravine (LR) .