PETROLOGY AND GEOCHEMISTRY OF LEWISIAN PEGMATITES AND GRANITES, N.W. SCOTLAND by Graham John Cunningham B.Sc.(Lond.) A thesis submitted for the degree of Doctor of Philosophy in the University of London Royal School of Mines June, 19 81 Imperial College London 1 ABSTRACT Late Scourian pegmatite dykes and Laxfordian granite sheets and pegmatites intruded into the lithologically and structurally 'heterogeneous Lewisian gneisses of N.W. Scotland display wide variations in geochemistry and mineralogy reflecting their varied origins and evolutionary histories. Laxfordian granites emplaced into the Laxford shear zone, Sutherland, have high Ba and Sr, relatively low K, Rb, Th and U and distinctive rare-earth element patterns which indicate their origin by partial melting of the depleted Scourie granulites. The Outer Hebrides granites of the same age may have originated by partial melting of undepleted amphibolite-facies grey gneisses. The compositions of pegmatites associated with the granites are rationally explained by the partition of major and trace elements between crystals, liquids and/or fluids. Pegmatites which occur independently of the granites show comparable variations in their geochemistry, often evolving towards low-temperature, fluid dominated, garnet- muscovite pegmatites. The Late Scourian pegmatites are similar in their geochemistry to the mainland Laxfordian granites, and record high crystallisation temperatures !L 2 (800°C) and oxygen fugacities (log fn = 10 ). 2 Metasomatic reactions occurring at higher (700 C) and lower (500°C) temperatures between acid pegmatites and basic host rocks caused strong leaching of Ca from the basic rocks, with the introduction of K, resulting in the formation of reaction zones whose chemistry is similar to many semipelitic metasediments. Diffusion-controlled structures are important on a small scale, but infiltration of a pegmatite fluid provided efficient transport over larger (up to several cm) distances . Intrinsic diffusivities of elements were of less importance than the nature of mineral/fluid reactions and the influence of such factors as rock porosity and tortuosity to the relative transport of components. LIST OF CONTENTS Abstract ! List of Contents 3 List of Figures 6 List of Tables ...... 1]L Acknowledgements Chapter 1 INTRODUCTION 1.1 Aims of the thesis . ^ 1.2 The Lewisian Complex 1.2.1 The Lewisian assemblage ^5 1.2.2 Structural development 19 1.3 Scope of the thesis 25 Chapter 2 GEOLOGICAL RELATIONSHIPS AND PETROGRAPHY 2.1 Relationships between the acid intrusive rocks and Lewisian structure 29 2.1.1 General distribution . 29 2.1.2 The Laxford Front . 29 2.1.3 South Harris 33 2.1.4 Discussion and extension to the rest of the Lewisian yj 2.2 Description of granites and pegmatites ... ^ 2.2.1 Laxfordian granites and related pegmatites 42 A. Laxford Front ^ B. Harris and Lewis 2.2.2 Other Laxfordian pegmatites: Northern and Southern types ^ 2.2.3 Late Scourian pegmatites ^ Chapter 3 GEOCHEMISTRY OF THE GRANITES AND PEGMATITES 3.1 Introduction 59 3.2 Chemical variations 60 3.2.1 Laxfordian granites and related pegmatites . 60 4 3.2.2 Northern-type and Southern-type pegmatites 75 3.3 Discussion 77 3.3.1 Major geochemical features of the granites 77 3.3.2 Late-stage processes in the granites, and pegmatite evolution . 94 3.3.3 Independent Laxfordian pegmatites . 109 Chapter 4 PHASE CHEMISTRY 4.1 Introduction 114 4.2 Laxfordian granites and related pegmatites. 116 4.2.1 Feldspars 116 4.2.2 Ferromagnesian and oxide minerals . 122 4.3 Laxfordian independent pegmatites 127 4.3.1 Feldspars ..... 127 4.3.2 Ferromagnesian and oxide minerals: Northern-type assemblages 131 4.3.3 Ferromagnesian and oxide minerals: Southern-type assemblages 136 4.4 Late Scourian pegmatites 151 4.4.1 Feldspars . 151 4.4.2 Ferromagnesian and oxide minerals . 164 4.5 Distribution of trace elements in pegmatite minerals . ....... 173 4.5.1 Ba, Sr and Rb 173 4.5.2 Rare-earth elements 178 4.6 Conclusions 186 Chapter 5 METASOMATISM 5.1 Introduction 190 5.1.1 Scope of the study: metasomatic reaction between basic host rocks and acid pegmatites 190 5.1.2 The development of metasomatic structures 191 5 5.2 Leenish, Isle of Barra: metasomatic reaction zones associated with small acid veins .... 198 5.2.1 Geological relationships 198 5.2.2 Petrography 202 5.2.3 Mineral composition and estimation of physical conditions 207 5.2.4 Chemical mass transfer 2.15 5.3 Garry-a-Siar, Benbecula: metasomatic zones associated with a large acid pegmatite .... 231 5.3.1 Geological relationships and petrography 231 5.3.2 Chemical mass transfer 243 5.3.3 Processes governing mass transfer . 253 5.4 Conclusions . 264 5.4.1 Processes in metasomatism 264 5.4.2 Extent of chemical modification .... 267 5.4.3 Products of metasomatism 270 Chapter 6 DISCUSSION AND CONCLUSIONS 6.1 The problem of the granite and pegmatite source 274 6.1.1 Geochemical and mineralogical evidence ......... 274 6.1.2 Structural evidence . 276 6.1.3 The pegmatite environment 278 6.2 Conclusions 291 References 297 Appendix 1 Tables of rock analyses 313 Appendix 2 Methods of analysis 324 6 LIST OF MAPS AND FIGURES MAPS Map 1 Distribution of Lewisian rocks 16 Map 2 Parts of Sutherland 30 Map 3 Parts of Lewis and Harris 34 Map 4 The Southern Outer Isles : a. General map 39 b. The Leenish peninsula, Isle of Barra . 40 FIGURES 2.1 The Laxford granite sheets at Rubha Ruadh, Sutherland 43 2.2 Pegmatite in a Laxford granite sheet 43 2.3 Breakdown of clinopyroxene to amphibole and sphene in a Laxford granite . 43 2.4 Xenoliths in Laxfordian granite sheets, Beitarsaig, N.W. Harris 47 2.5 Ptygmatically-folded pegmatites, Colluscarve, N.W. Harris . 47 2.6 Laxfordian pegmatite cut by Laxfordian leucogranite, Mangersta, S.W. Lewis ... 50 2.7 Style of zoning in part of the Chaipeval pegmatite, S. Harris 54 2.8 Kyanite in a Late Scourian pegmatite, Scourie, Sutherland ........... 54 3.1 Laxfordian granites: major element geochemistry . 61 a. Major elements plotted against Si02. 61 b. Bar graphs of Si02 concentrations . 62 c. The granites plotted in the system Qz-Or-Ab-An 63 7 d. AFM diagram 64 e. Normative corundum or diopside plotted against SiC^ 64 3.2 Laxfordian granites: trace elements plotted against SiC>2 67 3.3 Variation of trace element concentrations between coexisting granite-pegmatite pairs . 69 3.4 REE pattefrns of the Laxfordian granites . 72 * 3.5 Variation of Eu:Eu and Ce^:Yb^ in the Laxfordian granites ....... 74 3.6 Ca plotted against (a) Ba and (b) Sr for various Laxfordian acid rocks 76 3.7 USGS G-2 normalised trace element diagrams. (a) Laxfordian granites, (b) Lewisian gneisses . 78 3.8 Trace element models for (a) Laxford and (b) Outer Hebride* s granites 84 3.9 Variation of Eu:Eu , Ce^Sn^, and REE and Tb :Yb with Si0 in the KTN NM 2o Laxford granites . 96 3.10 Geochemical variations across the Laxford granite sheets 97 3.11 Possible crystallisation paths of the granites in the system Oz-Or-Ab-An 100 * 3.12 Variation of Eu:Eu , Ce^Sn^, REE and Tb„:Yb with SiO~ in the Outer N NM 2 Hebrides granites . 104 3.13 Cogenetic pegmatites from Loch Laxford and the Outer Hebrides plotted in the system Qz-Or-Ab-An . , . 105 3.14 USGS G-2 normalised trace element diagram: the Northern-type and Southern-type pegmatites 110 8 4.1 Laxfordian granites: feldspar compositions . 121 4.2 Laxford granites: amphibole compositions 125 4.3 Biotites from Northern-type and Southern-type pegmatites 134 4.4 Garnets from Southern-type pegmatites .... 142 4.5 Comparison of garnet compositions from Southern-type pegmatites with Green's (1977) data 143 4.6 Muscovites from Southern-type pegmatites . 149 4.7 Distribution of Fe and Mg between muscovite and garnet in two Southern-type pegmatites 149 4.8 Phase relations between muscovite, potash feldspar and sillimanite with varying pH . 150 4.9 Estimated bulk feldspar compositions from Late Scourian pegmatites 156 4.10 Celsian- and anorthite-content of Late Scourian alkali feldspars plotted against estimated temperature 160 4.11 Ternary feldspar solvi determined by Seek (1971) 162 4.12 T-fo2 conditions estimated for Late Scourian pegmatites 169 4.13 Exsolution textures and compositions of Fe-Ti oxide minerals from Late Scourian pegmatites 171 4.14 Distribution of Ba, Sr and Rb between pegmatite feldspars 177 4.15 REE patterns for pegmatite feldspars and garnets . 180 4.16 Distribution of trace elements between alkali feldspars and plagioclases plotted against ionic radius 184 9 5.1 Effects of combined diffusion and infiltration metasomatism 196 5.2 Zoned aureoles at Leenish, Isle of Barra . 200 5.3 Aureole widths plotted against vein widths at Leenish 201 5.4 Coarse perthites from an acid vein, Leenish . 204 5.5 Microscopic features of a zoned aureole, Leenish 205 5.6 Estimates of (P,T) based upon mineral assemblages at Leenish 210 5.7 Section across a zoned aureole from Leenish . 218 5.8 Estimated molar compositions of traverses across a zoned aureole 219 5.9 Variations of oxides across a zoned aureole normalised to the metadolerite composition 221 5.10 Al-fixed and Si-fixed oxide variation across a zoned aureole 226 5.11 Metasomatic reaction zones at Garry-a-Siar, Benbecula 233 5.12 Relationships between size of spherical diffusion structures and distance from pegmatite contact . 234 5.13 Microscopic features of the Garry-a-Siar reaction zones 237 5.14 Transition from poikiloblastic to clear biotites 239 5.15 Variation of plagioclase composition across the reaction zone 242 5.16 Major element variation across the reaction zone 245 5.17 Variation of trace elements across the reaction zone .