Petrogenesis of the Ambohiby Complex, Madagascar and the Role of Marion Hotspot Plume
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Petrogenesis of the Ambohiby Complex, Madagascar and the Role of Marion Hotspot plume by Ndivhuwo Cecilia Mukosi Dissertation presented for the degree of Masters of Science in the Faculty of Science at Stellenbosch University Promoter: Dr Jodie Miller and Dr Paul Macey December 2012 Stellenbosch University http://scholar.sun.ac.za Stellenbosch University http://scholar.sun.ac.za i PLAGIARISM DECLARATION I, Ndivhuwo Cecilia Mukosi declare that this work submitted for the degree of MSc at the University of Stellenbosch is the result of my own work and has not been previously submitted by me at another University for any degree. An effort to the best of my abilities is made to cite the contribution of others with due reference to the literature. Signature: ………………………………………. Date: ……………………………………………. Stellenbosch University http://scholar.sun.ac.za ii Stellenbosch University http://scholar.sun.ac.za iii Abstract The Cretaceous Ambohiby Complex is an alkaline ring complex located in the central part of Madagascar and covers a mountainous area of approximately 225km2. The complex intrudes into Precambrian basement gneisses and consists of the following rock types in a chronological order; gabbros, monzonite, alkali-syenite, micro-granite and granites. Both mafic and felsic rocks are dominated by sodic mineralogies. Pyroxenes are generally aegirine, aegirine-augite, and hedenbergite and commonly occur in granites, micro-granites, syenites and monzonite. In gabbros and mafic dykes, augite is the more common composition. Amphiboles are represented by bluish to brownish-green varieties with arfvedsonite to eckermannite compositions in granites, and magnesia-arfvedsonite compositions in micro-granites. Ferro-edenite is present in some alkali-syenites and monzonite. Feldspars are usually single phase and are therefore hypersolvus. In granites, micro-granites and alkali-syenites, path and string perthite is very common. Graphic intergrowth of quartz and alkali feldspars is also common in granites and some alkali- syenites. Major elements variation diagrams plotted against SiO2 indicate that the mafic and felsic rocks of the Ambohiby Complex were formed by processes similar to those of Fractional crystallization. Chondrite normalised mafic rocks have slightly positive Eu anomalies while the felsic rocks have negative Eu anomalies, indicating fractionation of plagioclase feldspars. The Chondrite normalised gabbroic rocks shared similar trends of heavy rare earth with Chondrite normalised Marion Hotspot data. This suggests that the basaltic parent magma for the Ambohiby Complex, possibly related to the Marion hotspot plume. The Fractional crystallization model with an inclusion of olivine in the mineral assemblage seems to fit very well with the actual Ambohiby felsic end member rocks (i.e. granites). It is therefore clear that differentiation mainly occurred by fractional crystallization but variable initial Sr and Nd values indicate the magmas assimilated crustal material during emplacement. The Rb-Sr geochronology gave an age of 90±2.4 Ma for the intrusion of the Ambohiby Complex, which confirms that the Ambohiby Complex is associated with the Gondwana break-up. In addition the Marion Hotspot plume is believed to have been located in the southern tip of the island at around 90 Ma ago. Stellenbosch University http://scholar.sun.ac.za iv Stellenbosch University http://scholar.sun.ac.za v Acknowledgement I’d like to thank my supervisors, Jodie miller and Paul Macey who offered me the opportunity to undertake this work under their supervision. I thank them for their patience and their advices which have been of great help. I thank my field assistance Santa for his support and guidance during my fieldwork in Madagascar. I thank the Council For Geoscience for their support during the writing up of the thesis. I thank all the following people John Clemens, Martin Klaussen, Madelaine Frazenburg, and Dave Raid for their assistance with mineral chemistry and geochemical data. I also thank Natalie Fish, Cynthia Sanchez-Garrido, Loxie Conradie, Arnaud Villaros, Rob Ward, Mafusi Rapopo and Risa Matsumura for their kind support. I thank my family and friends for their unconditional love and support throughout my studies. Finally I thank God for his grace and faithfulness. Stellenbosch University http://scholar.sun.ac.za vi Stellenbosch University http://scholar.sun.ac.za vii Table of Contents Declaration………………………………………………………………………………………… i Abstract…………………………………………………………………………………………… ii Acknowledgements ……………………………………………………………………………… iii Contents…………………………………………………………………………………………… iv Appendix………………………………………………………………………………………….. vii List of Figures……………………………………………………………………………………… viii List of Tables……………………………………………………………………………………… xv List of Abbreviations and Acronyms…………………………………………………………… xvi List of Mineral Abbreviations…………………………………………………………………… xvii CHAPTER 1: INTRODUCTIONS………………………………………………………………………... 1 1.1. Introduction…………………………………………………………………………………… 1 1.2 Aim of this study……………………………………………………………………………… 8 1.3 Geological mapping and Sampling Methodology………………………………………… 9 1.4 Analytical Methodology……………………………………………………………………… 9 1.4.1. Major and Trace Element Analysis……………………………………………….... 9 1.4.2. Mineral Compositions……………………………………………………………… 12 1.4.3 Radiogenic Isotopes………………………………………………………………… 13 CHAPTER TWO: TECTONOSTRATIGRAPHY……………………………………………… 14 2.1. Introduction…………………………………………………………………………………… 14 2.2. Tectonostratigraphic Units………………………………………………………………… 16 2.2.1 Precambrian Basement……………………………………………………...................... 19 2.2.1.1. Antongil Block………………………………………………………….…..………...… 19 2.2.1.2 Antananarivo Block……………………………………………………………………… 19 2.2.1.3 Tsaratanana Thrust Sheet……………………………………………………………… 20 2.2.1.4 Bemarivo Orogenic Belt……………………………………………………………… 20 2.2.1.5 Neoproterozoic metasedimentary belts………………………………………………… 20 Stellenbosch University http://scholar.sun.ac.za viii 2.2.2.Late Neoproterozoic-Cambrian magmatism in Madagascar…………………………... 23 2.2.3 Karoo Supergroup …………………………………………………………………............ 23 2.2.4 Cretaceous Magmatism…………………………………………………………………… 23 2.3.3 Younger Deposits…………………………………………………………………............. 23 CHAPTER THREE: LITHOLOGY AND PETROGRAPHY…………………………………... 27 3.1. Introduction and Overview………………………………………………………………….. 27 3.2 Precambrian gneiss………………………………………………………………………….. 31 3.2.1 Migmatitic orthogneiss……………………………………………………………………... 31 3.2.2 Granitic augen orthogneiss ………………………………………………………............ 32 3.3 Ambohiby Complex…………………………………………………………………………... 35 3.3.1 Gabbro………………………………………………………………………………… 35 3.3.2 Monzonite…………………………………………………………………………….. 38 3.3.3. Alkali syenite………………………………………………………………………… 41 3.3.4 Granites……………………………………………………………………………….. 44 3.3.5 Dykes………………………………………………………………………………….. 54 CHAPTER FOUR: WHOLE-ROCK GEOCHEMISTRY………………………………………. 61 4.1 Introduction……………………………………………………………………………………. 61 4.2. Classification of Rock types………………………………………………………………… 61 4.3 Mafic rocks…………………………………………………………………………………….. 63 4.3.1 Major elements………………………………………………………………………. 56 4.3.2 Trace and rare earth elements……………………………………………………... 67 4.4 Intermediate to felsic rocks………………………………………………………………….. 71 4.4.1 Major elements………………………………………………………………………. 71 4.4.2 Trace and rare earth elements……………………………………………………... 75 Stellenbosch University http://scholar.sun.ac.za ix 4.5. Radiogenic isotopes…………………………………………………………………………. 81 4.5.1 Processing of Sr and Nd Isotope Data……………………………………………. 81 4.5.2. Results……………………………………………………………………………….. 82 CHAPTER FIVE: DISCUSSIONS …………………..………………………………………….. 86 5.1 Introduction……………………………………………………………………………………. 86 5.2 Comparisons of the Ambohiby complex with other Madagascan complexes………….. 86 5.2.1 Major and trace element…………………………………………………………….. 88 5.2.2 Radiogenic isotope…………………………………………………………………... 92 5.3. Comparisons of the Ambohiby Complex with the Marion hotspot Plume …………….. 93 5.3.2 Major and trace elements………………………………………………………....... 94 5.3.2 Radiogenic isotope…………………………………………………………………... 95 5.4. Summary……………………………………………………………………………………… 98 5.5 Modelling ……………………………………………………………………………………… 99 5.5. 1 Fractional Crystallization (FC) Model…………………………………………….. 99 5.5.1.1 Fractional crystallisation not involving olivine………………………………… 99 5.5.1.2 Fractional crystallisation with olivine…………………………………….…….. 102 5.5.2. Assimilation-fractional crystallization (AFC) model ……………………………... 104 5.6. Summary……………………………………………………………………………………… 108 CHAPTER SIX: CONCLUSIONS……………………………………………………………….. 109 REFERENCES……………………………………………………………………………………. 112 Stellenbosch University http://scholar.sun.ac.za x APPENDIX Appendix1.1. List of the geographic positions of all the collected Ambohiby Complex samples Appendix 1.2 Thin sections petrographic descriptions of the Ambohiby Complex Appendix 1.3 Mineral Chemistry Analysis Appendix 1.4 Major and trace element data of the selected samples of the Ambohiby Complex Appendix 1.5 Radiogenic isotope data for Sm-Nd and Rb-Sr geochronology Stellenbosch University http://scholar.sun.ac.za xi LIST OF FIGURES Figure 1.1: (a) Main outcrops of middle Cretaceous Volcanic rocks on Madagascar after Besairie (1964), Mahoney et al., 2008. (b) Plate reconstruction at 83.5 Ma, shortly after the Androy eruptions. The locations of the Marion Hotspot Plume (M) and the Crozet (C) hotspots assume these hotspots have remained stationary relative to the earth’s spin axis. (Kent et al., 2002, Mahoney et al., 2008) 3 Figure 1.2: Bonin (1998) demonstrates structural levels of alkaline magmatism: