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Geochronological and Geochemical Constraints on the Lithospheric Evolution of the Arabian Shield, Saudi Arabia

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Geochronological and Geochemical Constraints on the Lithospheric Evolution of the Arabian Shield, Saudi Arabia Geochronological and Geochemical Constraints on the Lithospheric Evolution of the Arabian Shield, Saudi Arabia: Understanding Plutonic Rock Petrogenesis in an Accretionary Orogen. A thesis submitted for the degree of Doctor of Philosophy Frank Alexander Robinson B.Sc. (Hons) Department of Geology and Geophysics The University of Adelaide February 2014 Table of Contents Abstract vi Declaration viii Acknowledgments ix List of Figures x List of Tables xv Chapter 1: General Introduction 1 1.1 Granite Overview 1 1.2 Development of the A-type Classification 1 1.3 A-type Characteristics and Global Occurrences 2 1.4 Anorogenic Timing and Tectonic Significance 3 1.5 Petrogenetic Models for Generating Anorogenic Magmatism 4 1.6 The East African Orogen and its Relation to the Arabian-Nubian Shield 5 1.7 Arabian Shield Geological Overview 7 1.8 Aims 14 Chapter 2: Petrographic Constraints on Sampled Arabian Shield Plutons; Subdivision of A-type Suites 15 2.1 Introduction 15 2.2 Sample Selection 16 2.2.1 Adopted Classification Nomenclature 18 2.3 Granitoid Petrography 19 2.3.1 Makkah Suite (dm) 20 2.3.2 Shufayyah Complex (su) 23 2.3.3 Kawr Suite (kw) 25 2.3.4 Abanat Suite (aa) 31 2.3.5 Malik Granite (kg) 33 2.4 Petrography Summary and Discussion 35 Chapter 3: Arabian Shield Pluton Geochronology; Subtle Changes in a Homogeneous Juvenile Mantle 40 3.1 Introduction 40 i 3.2 U-Pb Geochronology 41 3.2.1a Makkah Suite (dm): Gabbro-Diorite 41 3.2.1b Shufayyah Complex (su): Tonalite 43 3.2.1c Jar-Salajah Complex and Fara’ Trondhjemite (js): Granodiorite 46 3.2.1d Subh Suite (sf): Rhyolite 48 3.2.1e Kawr Suite (kw): Granodiorite 50 3.2.1f Al Hafoor Suite (ao): Alkali-Granite 53 3.2.1g Najirah Granite (nr): Granite 55 3.2.1h Wadbah Suite (wb): Alkali-Granite 57 3.2.1i Ibn Hashbal Suite (ih): Alkali-Granite 59 3.2.1j Ar Ruwaydah Suite (ku): Granite 61 3.2.1k Haml Suite (hla): Quartz-Monzonite 63 3.2.1l Kawr Suite (kw): Alkali-Granite 65 3.2.1m Idah Suite (id): Alkali-Granite 67 3.2.1n Al Khushaymiyah Suite (ky): Quartz-Monzonite 70 3.2.1o Malik Granite (kg): Granite 72 3.2.1p Admar Suite (ad): Syenite 74 3.2.1q Al Bad Granite Super Suite (abg): Alkali-Granite 76 3.2.1r Al Hawiyah Suite (hwg): Granite 78 3.2.1s Mardabah Complex (mr): Syenite 81 3.2.2 Arabian Shield U-Pb Geochronology Summary and Discussion 83 3.3 Hafnium (Hf) Isotopes 88 3.3.1 Western Shield 90 3.3.2 Eastern Shield 90 3.3.3 Nabitah Orogenic Belt 93 3.3.4 Hafnium Isotope Summary 95 Chapter 4: Geochemical Constraints on Arabian Shield Plutonic Rocks 97 4.1 Introduction 97 4.2 Whole Rock Major and Trace Element Geochemistry 98 4.2.1 Island Arc and Syncollisional Granitoids (IA+Syn) 99 4.2.2 Nabitah and Halaban Suture Granitoids (NHSG) 104 4.2.3 Post-Orogenic Perthitic Granitoids (POPG) 111 4.2.4 Anorogenic Aegirine Perthitic Granitoids (AAPG) 119 ii 4.3 Whole Rock Isotope Geochemistry 125 4.3.1 Island Arc and Syncollisional Granitoids (IA+Syn) 126 4.3.2 Nabitah and Halaban Suture Granitoids (NHSG) 129 4.3.3 Post-Orogenic Perthitic Granitoids (POPG) 134 4.3.4 Anorogenic Aegirine Perthitic Granitoids (AAPG) 137 4.3.5 Shield Volcanics 139 4.3.6 Isotope Geochemistry Summary 142 4.4 Arabian Shield Granitoid Classification/Tectonic Discrimination 143 4.4.1 Post-Orogenic Perthitic Granitoids (POPG) and Anorogenic Aegirine Perthitic Granitoids (AAPG) 143 4.4.2 Island Arc and Syncollisional Granitoids (IA+Syn) and Nabitah and Halaban Suture Granitoids (NHSG) 148 4.4.3 Western Shield and Nabitah Suture Mafic Endmembers 154 4.4.4 Geochemistry Summary and Discussion 157 4.4.4a Subdivision of Arabian Shield A-type Granitoids 159 4.4.4b Differentiating Contaminated and Enriched Mantle 163 Chapter 5: Insights into Mantle Source from Zircon Geochemistry 167 5.1 Introduction 167 5.2 Island Arc and Syncollisional Granitoids (IA+Syn) 168 5.2.1 Major Element Geochemistry 169 5.2.2 Trace Element Geochemistry 170 5.3 Nabitah and Halaban Suture Granitoids (NHSG) 175 5.3.1 Major Element Geochemistry 178 5.3.2 Trace Element Geochemistry 180 5.4 Post-Orogenic Perthitic Granitoids (POPG) 186 5.4.1 Major Element Geochemistry 186 5.4.2 Trace Element Geochemistry 187 5.5 Anorogenic Aegirine Perthitic Granitoids (WPG) 189 5.5.1 Major Element Geochemistry 190 5.5.2 Trace Element Geochemistry 191 5.6 Zircon Geochemistry Summary 193 iii Chapter 6: Discrete Crystallisation Ages within Granitoid Zircon Populations 195 6.1 Introduction 195 6.2 Identification of Distinguished Zircon Morphologies 196 6.3 Identification of Non-Gaussian Probability Distributions 200 6.4 Discrete Ages Using Isoplot Software 203 6.5 Establishment of Three Discrete Zircon Ages 204 6.6 Relationships between Machine Error and Age Separation 207 6.7 Additional Evidence for Discrete Ages and Discussion 214 Chapter 7: Arabian Shield Post-Orogenic and Anorogenic Granitoids; Petrogenetic Mechanisms for Distinct Contaminated and Enriched Mantle Sources 216 7.1 Introduction 216 7.2 Geochemical Characteristics of Contaminated Granitoids 217 7.3 Magmatic Pulsing in a Contaminated MASH Zone 222 7.4 Subtle Differences in Primitive Syenites; Two Mantle Sources 228 7.5 Enriched Mantle Within Plate Granitoids; Key to Economic Deposits 233 7.6 Within Plate Granitoids; Products of Lithospheric Delamination 236 7.7 Generation of Crustal Melts 238 Chapter 8: A Tectonic Synthesis of the Arabian Shield; Implications for Gondwana Assembly 241 8.1 Geochronological Overview 241 8.2 Island Arc Magmatism (950-750Ma) 243 8.3 Syncollisional Magmatism (730-636Ma) 250 8.4 Post-Orogenic Magmatism (636-600Ma) 251 8.5 Anorogenic Magmatism (<600Ma) 258 8.6 Evidence for Final Gondwana Assembly 265 8.7 Subtle Changes in Homogenous Juvenile Mantle Melts 270 8.8 Concluding Remarks 277 References 280 iv Appendices Appendix a Analytical Techniques 290 Appendix 1 Sample Catalogue from the Arabian Shield 303 1.1 Midyan Terrane 303 1.2 Jiddah Terrane 303 1.3 Hijaz Terrane 308 1.4 Ha’il Terrane 320 1.5 Ad Dawadimi Terrane 324 1.6 Tathlith Terrane 333 1.7 Afif Terrane 337 1.8 Asir Terrane 342 Appendix 2 Zircon U-Pb Geochronological Data 353 Appendix 3 Zircon Cathodoluminescence Images 396 Appendix 4 Zircon Hafnium Isotope Data 420 Appendix 5 Whole Rock Major and Trace Element Data; Ferrous Iron Determination 444 Appendix 6 Whole Rock Sm-Nd and Sr Isotope Data 468 Appendix 7 Zircon Rare Earth Element Geochemistry 477 v Abstract The Arabian-Nubian shield reflects the complex interplay between juvenile oceanic and continental arc fragments accreted during the final stages of Gondwanian super continental assembly. To date, much of the geochronological and geochemical data from the Arabian Shield, Saudi Arabia, is absent or poorly constrained and extrapolated from neighbouring Middle Eastern and African countries. Little attention has been paid to the petrogenesis and tectonic significance of the plutonic rocks pursuant to lithospheric orogenesis. A total of 137 samples from 26 geological units were collected from the Midyan, Hijaz, Asir, Tathlith, Afif, Ad Dawadimi and Ha’il terranes with particular emphasis on accretionary suture zone and within plate setting relationships. Extensive data bases are constructed using zircon U-Pb geochronology and Hf isotopes to evaluate Gondwanian significance and whole rock major and trace element geochemistry, Nd, Sm, Sr isotopes and zircon geochemistry to determine their petrogenetic properties. These parameters provide new insight into changing mantle conditions beneath collisional sutures (Yanbu, Nabitah and Halaban) and within plate asthenospheric upwelling. 19 granitic units are subdivided into metaluminous, peraluminous and peralkaline groups that possess distinguished island arc (~950-730Ma), syncollisional (~<730-636Ma), post tectonic (~<636-600Ma) and anorogenic (<600Ma) U-Pb geochronology. These magmatic phases represent accretionary cycles initiating from the dismantlement of Rodinia, closure of the Mozambique Ocean and final Gondwana amalgamation. Evidence for final assembly is recorded at ~525Ma (Najd fault reactivation) which is now the youngest dated magmatism in the Arabian-Nubian Shield and warrants repositioning of the regional unconformity at ~542Ma. Emplacement of sampled Arabian Shield classic A-type post-tectonic and anorogenic granitoids falls into three categories: 1) Intrude sutures immediately following collision which contain extensive mafic cumulate fractionation and N-MORB affiliation. 2) Plate boundary juxtaposed suites without obvious mafic cumulates, but vi posses contaminated N-MORB geochemistry. 3) Within plate granitoids isolated from plate boundaries and also without obvious mafic cumulates, but with a distinctive enriched (OIB) like asthenospheric mantle source. All categories produce similar felsic endmembers, but contain isotopically distinct mantle source. These are differentiated using a newly developed geochemical scheme (contaminated and enriched mantle granitoids) that is successfully applied to regional Arabian-Nubian examples. The diachronous Nabitah Orogenic Belt symbolises collision and subduction between western oceanic and eastern continental terranes that was terminated by the appearance of category 1 post-tectonic granitoids. This long lived (~50Ma) granitic magmatism contains mingling textures, discrete crystallisation ages, distinguished zircon morphologies and isotopically less juvenile mafics that geochronologically and geochemically reflect magmatic pulsing from a contaminated lower crustal MASH zone. The transition from N-MORB like mafics to isotopically enriched granitoids (isotopically similar to category 3 suites) reflects subduction magmatism followed by slab tear and asthenospheric influx. Conversely, the appearance of category 3 anorogenic plutons is characterised by widespread, tightly constrained (<10Ma) magmatism that is geochemically enriched, economic and symbolic of lithospheric delamination and asthenospheric (OIB like) upwelling. Differences between category 1, 2 and 3 zircon geochemistry constrain further contaminated and enriched mantle source behaviour that produces similar felsic products from distinguished petrogenetic processes.
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