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Petrogenesis of the Peralkaline Granite (And Associated Syenite) Dykes of the Straumsvola Complex, Western Dronning Maud Land, Antarctica

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Petrogenesis of the Peralkaline Granite (And Associated Syenite) Dykes of the Straumsvola Complex, Western Dronning Maud Land, Antarctica Petrogenesis of the peralkaline granite (and associated syenite) dykes of the Straumsvola Complex, Western Dronning Maud Land, Antarctica Tanya Dreyer Dissertation presented for the degree of Master of Science in the Department of Geological Sciences University of Cape Town July 2015 Supervisors: Professor Chris Harris and Dr Petrus Le Roux University of Cape Town 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 Plagiarism declaration I know the meaning of plagiarism and declare that all of the work in this dissertation, save for that which is properly acknowledged, is my own. Signed: Date: Abstract The Straumsvola nepheline syenite complex in Western Dronning Maud Land, Antarctica consists of an outer massive and inner layered nepheline syenite that is itself intruded by ijolite. The complex and its immediate country rock was intruded by numerous dykes. A small proportion of these dykes are peralkaline syenite and microgranite, and these are found only to intrude the nepheline syenite and not the country rock. The presence of peralkaline granite dykes intruding a silica-undersaturated complex is unexpected, given the thermal divide that exists at low pressures between silica-under and -oversaturated phases. Major and trace element variations in the dykes are found to be consistent with fractional crystallisation of a parental peralkaline magma of trachyte composition. The dykes give an age of 171±4.4 Ma (Rb-Sr isochron) and have an initial 87Sr/86Sr ratio =0.707500 and average initial 143Nd/144Nd = 0.51190. The δ18O values of quartz and aegirine give temperatures around 500°C somewhat lower than expected for felsic magmas that are consistent with the rock textures that imply subsolidus growth of aegirine. Quartz δ18O values of 8.4 to 9.2‰ indicate peralkaline granite magma with an average δ18O value of 8.6‰. A number of unusual silicates are present in the granite dykes e.g. the Zr-silicates: eudialyte, dalyite and vlasovite, and a Na-Ti silicate, narsarsukite. The presence of Na-bearing accessory minerals is likely related to the peralkalinity of the dyke magmas that inhibits zircon formation. Strongly negative ԐNd (<-10), high initial 87Sr/86Sr and high δ18O (average 8.6‰) compared to the expected value for a felsic magma produced entirely by fractional crystallization of a mantle- derived magma (~6.5 ‰) is consistent with a substantial crustal component in the dyke magmas. Because the country rock does not have high δ18O values (average 7.7 ‰), it is suggested that the peralkaline granite dykes formed by partial melting of an alkali- metasomatised gneiss followed by fractional crystallisation. Table of Contents Plagiarism declaration Abstract 1. Introduction ................................................................................................................................ 3 1.1 Previous work in Western Dronning Maud Land, Antarctica ................................................ 3 1.2 Regional geology ....................................................................................................................................... 4 1.3 A review on the origin of A-type granites....................................................................................... 8 1.4 Previous studies on the felsic dykes of the Straumsvola Complex ...................................... 9 1.5 Aims of this study .................................................................................................................................. 11 2 Major rock types of the Straumsvola Complex ............................................................ 12 2.1 Field relations ......................................................................................................................................... 12 2.2 Dykes in Hand Specimen .................................................................................................................... 15 2.3 Petrography ............................................................................................................................................. 18 2.3.1 Dykes of the Straumsvola Complex ................................................................................................ 18 2.3.2 Country rock gneisses ......................................................................................................................... 33 2.3.4 Nepheline syenites ................................................................................................................................ 37 3. Methods ...................................................................................................................................... 39 Analytical techniques .......................................................................................................................... 39 3.1. Electron Microprobe ............................................................................................................................ 39 3.2. XRF ............................................................................................................................................................... 39 3.3. Trace elements ....................................................................................................................................... 40 3.4. Radiogenic Isotopes.............................................................................................................................. 42 3.4.1. Pb .................................................................................................................................................................. 42 3.4.2. Sr and Nd ................................................................................................................................................... 42 3.5. Stable Isotopes ........................................................................................................................................ 46 3.5.1. Oxygen Isotopes ...................................................................................................................................... 46 4. Results ........................................................................................................................................ 47 4.1. Mineral Chemistry ................................................................................................................................. 47 Pyroxene .................................................................................................................................................................. 47 Unusual Minerals ................................................................................................................................................. 48 4.2 Major element geochemistry ............................................................................................................ 55 4.3 Trace elements ....................................................................................................................................... 57 4.3.1 Rare Earth Elements ............................................................................................................................. 68 1 4.4 Isotopes ...................................................................................................................................... 71 4.4.1 Oxygen Isotopes ..................................................................................................................................... 71 4.5 Radiogenic Isotopes.............................................................................................................................. 73 4.5.1 Comparison with other elements: ................................................................................................... 75 4.5.2 Comparison with other isotopes: Pb .............................................................................................. 79 5. Discussion ................................................................................................................................. 81 5.1. Major element variation ..................................................................................................................... 82 5.2. Trace elements ....................................................................................................................................... 87 5.2.1. Trace element variation in the dykes ............................................................................................ 87 5.2.2. The effect of the unusual silicates on the composition of the dyke magma ................... 88 5.3. The REE ..................................................................................................................................................... 92 5.3.1. The significance of eudialyte ............................................................................................................. 93 5.4. Oxygen Isotope constraints on magmatic evolution at Straumsvola ............................... 94 5.5. Radiogenic Isotopes.............................................................................................................................
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