The Tectonic Evolution and Geochemistry of the Lewisian Complex of North Harris

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The tectonic evolution and geochemistry of the Lewisian Complex of North Harris

Stephen Richard Soldin

A thesis submitted for the degree of Doctor of Philosophy in the University of London

Geology Department Imperial College of Science and Technology London SW7

June l97$ Abstract

The Lewisian Complex of North Harris is composed of homogeneous acid gneisses derived from an early Scourian Complex which are intruded by basic and ultrabasic rocks of the Scourie Dyke Suite the whole being affected by Laxfordian deformation and metamorphism, events which dominate the observed history of the region. While four phases of Laxfordian deformation are seen only two (F2 and F3) are of major importance the former producing the predominant NW-SE trend of the gneissic banding, large scale fold structures in the Scourie Dykes and numerous minor structures throughout the region; and the latter being intimately associated with the late Laxfordian phase of granite and pegmatite production which is concentrated in the south and west of the area. Regional variations in Laxfordian strain can be suggested on the basis of the variable preservation of igneous features in the Scourie Dykes. The predominantly acid gneisses, although litholog- ically simple show gross variations in their mineralogy that can be mapped out and correlated with regional geochemical trends defined by trend surface analysis, such correlation leading to the suggestion that early litholo- gital variations of the complex are retained. Other petrological and geochemical features show the gneisses to be typical of amphibolite facies gneisses seen throughout the Lewisian; their characters do not suggest that they are retrogressed granulites. To the south and west a belt of granitic rocks is developed in which granites are produced by 'in situ' metasomatism and recrystallisation of the host gneisses. The area of granitic rocks correlates with a region of relatively high volatile contents of the gneisses, a during feature which is taken as evidence of high PH2O metamorphism. The gneisses of these areas are also texturally distinct from those elsewhere and provide further evidence of the concentration of late Laxfordian deformation in areas of granite production. The basic and ultrabasic rocks of the complex are predominantly derived by the deformation and metamorphism of Scourie Dykes. In regions marked by the high volatile contents of the gneisses simple amphibolites predominate while elsewhere clinopyroxene amphibolites are most common. On petrological and geochemical grounds the rocks may be divided into an uncommon suite of basic and ultrabasic noritic rocks, and basic (clinopyroxene- plagioclase) rocks which are the precursors of the majority of the amphibolites. Mineral chemistry of these rocks is used to provide further evidence of the preservation of igneous assemblages in some rocks, to study cryptic chemical variations in a layered ultrabasic complex at Maaruig, and to determine the PIT conditions of the amphibolite facies metamorphism to which the area was subjected. PIT estimates so obtained indicate a temperature of some 7500C at 6-7 kb. To my parents Acknowledgements

I would like to acknowledge the supervision of my work by Professor Janet Watson who, besides such supervision and the reading and correcting of long and sometimes 'ill-written' manuscripts also showed interest in the personal problems which inevitably occur during a four year period of research. Such kindness and consideration from one's supervisor seems rare and I thank her for both. For the XRF work I am indebted to Geoff Sullen without whose help the analytical work could not have been carried out; to Peter Watkins for his analyses of iron and sodium in my many samples; and to Robin Parker for help with the necessary computing techniques. I thank Paul Suddaby for assistance with the use of the micro-probe and Malcolm Frost for the many happy hours of work his computer programmes gave me, and for the light-hearted antagonism which assisted their passing. To my fellow postgraduate students, without whom the period of my research would have indeed been desolate, I particularly thank Rob Horsley and Dave Savage for discussions, John Wolff for being a friend In my need and Kate for her company and 'coercion' to help me put pen to paper and fingers to typewriter keys during the latter period of writing-up.

To my parents, to whom this thesis is dedicated, words are insufficient thanks for their constant encouragement and, lately, financial assistance...

Finally I acknowledge a grant from the Natural Environment Research Council. Contents

List of figures List of tables

Chapter 1 Introduction 1 Chapter 2 Structural History 21 Chapter 3 The gneisses: petrology and geochemistry 67 Chapter 4 The basic and ultrabasic rocks of the complex 139 Chapter 5 Mineral chemistry of the basic and ultrabaslc rocks - 225 Chapter 6 Conclusions 270

Bibliography 2$3 Appendices 295

Full contents lists are given at the beginning of each chapter. List of figures

Fig: PP 1.1 Location+ map of North Harris 4 l.lb Outline map of North Harris showing the chief topographic features referred to in the text 5 1.2 The geology of Lewis and North Harris 10 1.3 Schematic map of parts of the southern islands Outer Hebrides 14 2.1 Lewisian gneiss types 26 2.2 Possible F1 foliations in basic bands and lenses 29 2.3 Foliation trend map- 31 2.4 The distribution of basic and ultrabasic rocks 32 2.5 Schematic representation of the possible sequence of events during F2 deformation 34 2.6 F2 fold styles (a and b) 36 2.7 Interference structure in a thin basic layer, Gullaval Glas 37 2.8 Boudin types (a and b) 38 2,9 Diagram indicating the forms of boudinage with varying competence contrast 39 2,10 Mineral fabrics in amphibolites (a and b) 43 2.11 Examples of 'anomalous' foliations in basic layers 45 2.12 Zones of variable Laxfordian strain 48 2.13 F3 fold styles, fabrics and interference structures (3 pages) 51 2.14 Examples of discordances between amphibolite bands and the gneissic fabrics 56 2.15 F2 fold in amphibolite 58 2.16 Unusual F2 folds 59 2.17 Folded pegmatites contained within amphibolite layers 61 2.18 Small auartz vein cutting and offsetting an amphibolite band in grey gneiss 63 2,19 Late-stage features of tectonic activity 64 3.1 The distribution of granitic rocks and pegmatites 73 3.2 Distribution of gneiss types 74 3.4 A1 4 v, (Na + K) plot of hornblende compositions 78 3.5 Possible biotite-hornblende reactions 79 Fig: pp 3.6 Garnet in garnet-gneiss, Bunaveneader 82 3.7 Textural features in quartz grains (a,b,c) 84 3.8 Distribution of gneissose and other textural types 86 3.9 Relationship of K20 and Rb to Si02 in North Harris gneisses and granites 92 3.10 Scheme of K/Rb ratios according to Shaw (1968) 93 3.11 K/Rb plot for North Harris gneisses and granites 93 3.12 Compilation diagram showing the K/Rb ratios of gneisses and granites for North Harris..., the Uig Hills... and typical Lewisian granulites 94 3.13 AFM diagram, North Harris gneisses and granites 96 ._ 3.14 CKN diagram, ditto 97 3.15 Normative feldspar diagram, ditto 98 3.16 Variation diagrams for North Harris gneisses and granites (3 pages) 101 3.17 Variation diagrams for gneisses and granites from the Uig Hills 104 3.18 Relationships of various element ratios to modal hornblende, biotite and feldspar in gneisses 108 3.19 Comparison of North Harris gneisses and granites with granulite facies gneisses of Coll and Tiree (2 pages) 112 3.20 Concept of trend illustrated in two dimensions 115 3.21 Sample localities for use in trend surfaces 119, 3.22 Trend surface of selected. elements (4 pages) 120 3.23 Quartz-Ab-Or diagram, North Harris gneisses and granites 130 3.24 Normative feldspar diagram, ditto 131 3.25 Variation between K20, CaO, Na20, Rb, Sr and Ba in the granitic rocks of North Harris 132 3.26 Contoured distribution of volatile contents in the gneisses 134 4.1 General distribution of the basic rocks 143 4.2 Garnet being pseudomorphed by pyroxene- plagioclase aggregates, Rhenigadale 148 4.3 Layering in amphibolites 149 4.4 Hornblende-granulite (phototmicrograph) 151 4.5 Exsolved iron oxide in deformed hornblende 151 Fig: PP 4.6 Features of the noritic orthopyroxenes 155 4.7 Corona structures and mineral intergrowths in the noritic rocks 157 4.8 Clouded, twinned interstitial plagioclase from UB rocks at the top of the Maaruig series 162 4.9 Features of the UB rocks south of Tarbert 163 4.10 General form, lithology and mineralogy of the ultrabasic lens south of Tarbert 164 4.11 AFM diagram of the North Harris rocks 174 4.12 Alkali-silica diagram 175 4.13 The basalt tetrahedron 176 4.14 TiO2-K20-P205 discrimination diagram 178 4.15 Ti-Zr-Sr discrimination diagram 178 4.16 Variation diagrams for the North Harris basic and ultrabasic rocks (4 pages) 180 4.17 Sketch map of the noritic and associated rocks at Ardvourlie 186 4.18 Mafic pod within noritic rocks, Ardvourlie 187 4.19 Variations in whole rock chemistry within the noritic rocks, and between them and the marginal amphibolites 190 4.20 Sequence of rock types in the Maauig complex 192 4.21 Variations in mineral chemistry in the Maaruig ultrabasic rocks (2 pages) 194 4.22 Variations in whole rock chemistry of the Maaruig ultrabasic rocks (2 pages) 197 4.23 Map showing the suggested structure of the Maaruig UB complex 199 4.24 Graphs of element ratios drawn according to the methods of Pearce (4 pages) 205 4.25 Schematic representation of whole rock and mineral ratios in the Maaruig UBs 210 4.26 Schematic representation of whole rock and mineral ratios in the noritic rocks 211 4.27 Schematic representation of whole rock and mineral ratios in the amphibolites 213 4.28 Stability fields of tholeiitic assemblages 220 4.29 Tholeiitic assemblages at 1100°C and varying pressures 220 Fig; pp 5.1 Plot of end member compositions of pyroxenes and olivines 230 5.2 Relationship between MnO and Fo content in olivines 231 5.3 Olivine-opxn relationships in Maaruig UB and Tarbert UB rocks 231 5.4 Division of orthopyroxenes into metamorphic and igneous types on the basis of their composition 233 5.5 Relationship between Fe/Mg ratios of the pyroxenes and their host rocks 235 5.6 Relationships between Fs content and composition in clinopyroxenes (2 pages) 237' 5.7 Garnet compositions plotted in terms of their almandine-pyrope-grossular end members 240 5.8 Relationship between Fe/Mg ratios of garnets and amphiboles and their host rocks 241 5.9 Classification schemes for amphiboles 243 5.10 Element variations in amphiboles 246 5.11 Element variations in chromates 248 5.12 Element variations in ilmenites 251 5.13 Fe/Mg distribution in ferromagnesian minerals 252 .5.14 Element distributions between amphiboles and other ferromagnesian phases 252a 5.15 Element distributions between amphiboles and • plagioclase 256 5.16 P-T determination using cpxn-opxn 262 5.17 P-T determination using garnet-cpxn .263 5.18 Estimate of temperature using ilmenite-pyxns 265 5.19 Estimate of temperature 6using garnet-cpxn 265 5.20 Relationship between AI content of amphiboles and pressure 266 6.1 P/T fields for the metamorphic assemblages in Scourie Dykes of various regions 278a List.of tables

Table: pp 1.1 Structural correlations in the Outer Hebrides 15 1.2 Summary table of available age dates, Outer Hebrides ;16 2.1 Features of the Lewisian Complex of Lewis in relation to variations in Laxfordian strain 46 3. 1 Composition of hornblendek, plagioclase and biotite from three gneiss samples 77 3.2 Average compositions of some common crustal rocks 90 3.3 Comparison between biotite gneisses and hornblende-biotite gneisses from North Harris 90a 3.4 Composition of subsidiary gneiss types 90b 3.5 Interelement correlations in hornblende and biotite gneisses 107 3.6 Comparison between the gneisses of North Harris and the Uig Hills 109a 3.7 Average compositions of amphibolite facies gneisses 110 3.8 Average compositions of granitic rocks, from the Uig Hills 128 4.1 Averaged major and trace element analyses of the North Harris basic and ultrabasic rocks 168 4.2 Comparison of Scourie Dyke rocks of North Harris and elsewhere in the Lewisian 170 4.3 Composition of tholeiitic rocks of various provinces ' 171 5.1 Olivine compositions 229 5.2 Orthopyroxene compositions 229 5.3 Clinopyroxene compositions: hornblende- granulites and amphibolites 236 5.4 Garnet compositions 236 5.5 Amphibole compositions 244 5.6 Plagiocalse compositions 244 5.7 Ilmenite compositions 250 5.8 Summary of P/T estimates and their results 268 6.1 Comparative table of tectono-metamorphic events in North Harris, western Harris and Lewis 282 Chapter 1

Introduction

'No one but a blockhead ever wrote, except for money' Dr. Johnson

1 Chapter 1

Contents

1.1 The geography of North Harris 3 1.2 The Lewisian Complex- introduction 3 1.3 The Lewisian Complex of the Outer Hebrides 8 1.4 The geology of North Harris- introduction 17 1.5 Geochemistry: aims and results 18 1.6 Format of the thesis 19 Chapter 1

The work recorded in this thesis primarily involves a study of the petrology and geochemistry of the 'Grey Gneisses' and the basic and ultrabasic rocks of the Scourie Dyke suite which intrude them. Such a study is based on four months field work in which a general tectonic scheme was established and samples collected prior to extensive mineralogical and analytical work in London. In all aspects the work is a regional study- for example the geochemical data being used to establish gross regional variations in chemistry in the gneiss complex (see Chapter 3).

1.1 The geography of North Harris

The area of east North Harris (Fig:1.1) covers some 110 square kilometres and contains much excellent exposure, interspersed with lochs and peat bogs in which exposure is very poor. Such irregularity of exposure serves to create problems when sampling for geochemical purposes. Although of only moderate relief the area is one of the most rugged regions of the Outer Hebrides and includes the highest peak, Clisham (800m). Exposure on the flanks of the hills is often good due to glaciation whereas valleys are invariably filled with peat and the peaks of the hills frost shattered and poorly exposed. Glacial crags, often quite precipitous, have been developed on both north-facing slopes and in the head walls of corries but the area remains quite accesible if the route is care- fully chosen. Perhaps Macculloch's view (1819) of North Harris as a region " where unsurmountable rocks and impassable bogs alternately claim the mastery" is of a somewhat exaggerated style but it more than adequately conveys ones first impressions of the area!

1.2 The Lewisian Complex- introduction

While general accounts of the Lewisian Complex abound in various publications and papers I feel that some introduction to it is called for, if only to define the terms 3 7°W

57°55'N

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Fig:1.1 Location map of North Harris (stippled).

4 / Vj.ada\e •Iia Vigadalo s Ardvourlie Castle

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Fig:1.1b Outline map of North Harris showing the chief topographic features referred to in the text.

5 used in the rest of this work. The discussion will, however, be brief. The Lewisian Complex includes the banded gneisses, granites, metaigneous and metasedimentary rocks that form the Precambrian basement in the north-west Highlands of Scotland. The whole is characterised by having undergone several phases of metamorphism and deformation. Peach, et al studying the mainland Lewisian (1907) recognised that the complex was markedly heterogeneous and of complex history. It was only with the work of Sutton and Watson (1951) that the subdivision into a Scourian Complex and later Laxfordian Complex was suggested, a division based on the recognition of a swarm of basic rocks (the Scourie Dykes) intruded after the Scourian events and deformed and metamorphosed by the later Laxfordian events. This simple, but nonetheless crucial subdivision has remained with little modification ever since. The Scourian marks the period during which much of the Lewisian Complex was produced for little material- in the form of sedimentary or volcanic series- seems to have been added to it since that time. The original nature of the complex is, however, an enigma. It is possible that Scourian events affected an even older gneissic basement, or possibly a basement-cover sequence. Equally problematical is the question of how much of the present gneiss complex was derived from sedimentary and volcanic succesi.ons and how much from deep-seated plutonic rocks; certainly, metasedimentary relicts remain but their original extent is unknown. Also within the the complex are basic and ultrabasic rocks some of them showing evidence of layering, serpentinites, anorthosites and associated metasedimentary rocks all deformed and metamorphosed and forming units up to a few hundred metres in thickness and several kilometres in length. Further subdivision of the Scourian event has inevitably been suggested. The major metamorphic event of the period (at some 2800my), termed the Badcallian by Park (1970), represents the most widespread metamorphism producing granulite facies gneisses and high grade assemblages in metasediments towards the north and amphiboiite facies assemblages to the south. Later Scourian

6 events included the production of minor intrusions, pegmatites and metasediments (eg. the Loch Maree Series). Effects of late Scourian deformation are irregularly developed. For example, Evans et al (1974) proposed the 'inverian' episode to explain a zone of local retrogression and deformation of Badcallian rocks and structures in the Lochinver region.

All structures and rocks of Scourian age are cut by the intrusive basic rocks of the Scourie Dyke Suite. These intrusions are regarded as dykes emplaced in a more or less single swarm and range from simple tholeiitic intrusions to peridotites, norites and other rock types. Sutton and Watson (1951) regarded them as being emplaced into relatively stable and brittle crust, but other authors (eg. O'Hara, 1961; Park and Cressweil, 1972) have suggested they were intruded into hot and even plastic rocks. It seems likely that the dykes were emplaced over a considerable period of time and both conditions of intrusion could possibly be satisfied from region to region. The subsequent deformation and metamorphism of the Scourie Dykes has led to the recognition of a distinction between the earlier Scourian and later Laxfordian episodes. Areas in which the dykes are not substantially affected are regarded as portions of the Scourian Complex set within the reworked Laxfordian Complex. Such massifs occur on all scales and have been the subject of much further study.

The Laxfordian episode involved extensive metamorphism and deformation of the early members of the complex, remobilisation of parts of which gave rise to local development of granites and migmatites in the later Laxfordian. Subdivision of the Laxfordian events have been suggested based on metamorphic or structural criteria but it may in general terms be regarded as a period of predominant amphibolite facies metamorphism in association with several phases of deformation, with reworking of the early complex in ail areas other than the relict Scourian massifs mentioned above. Locally the Laxfordian episode culminated in the production of granitic rocks.

7 1.3 The Lewisian Complex of the Outer Hebrides

The Outer Hebrides, se.perated from the mainland by the Minch, are composed almost entirely of rocks of the Lewisian Complex whose history is broadly similar tõ that of the mainland. What follows is a brief account of some of the major, regional features, an account based on previous research in the Outer Isles.

Early visitors to the Outer Isles in the 19th century included Macculloch, Murchison, Geikie and Heddle. Macculloch (1819) refers to the Hebrides as a "country of gneiss" and his remarks concerned both Harris, where he recorded "limestone" and other such rock types, and Lewis. He was the first to .record the presence of "argillaceous schists" on the east of the island, later to be recognised as crushed rocks in the Outer Isles Thrust. Murchison and Geikie (1861) record the dominant NW-SE strike of the gneiss foliation, several occurences of limestone and a granite at Dalbeg, Lewis. Heddle (1878) referred to Harris as "... almost entirely composed of hornblende gneiss... It is certainly the oldest of all known rocks in Scotland." Much of his record concerns the lithologies of the South Harris Complex. Peach and Horne (1913,1930) visited the islands with a view to comparing them with the Lewisian of the Scottish mainland. They described the gneisses- biotite) biotite-hornblende and hornblende gneisses- and later acid rocks and commented on the general absence of quartzo-pyroxene gneiss, the existence of some flaggy granulites and the occurence of crushed rocks in eastern Lewis. Dougal (1928) noted that the Uig Hills consisted of red microcline granite and recorded the presence of a belt of crushed rocks from Barra to Lewis and the occurence of several masses of basic rock- including that at Maaruig in North Harris (see Chpt.4).

The single most important contribution to the geology of the Outer Hebrides was that of Jehu and Craig who studied the islands from Barra to Ness publishing their 8 work in five papers from 1923 to 1934. Their account of North Harris was published in 1934 and included a map of Lewis and Harris (Fig:1.2) on a scale of 1cm. to 1 mile and in which many of the important features described in this work are recorded for the first time. In their North Harris paper they describe basic pods and lenses as the earliest portions of the complex, regarding the more leucocratic gneisses as having intruded them at a later stage. The extreme uniformity of the biotite gneisses is noted. Three paragneiss occurences are shown in east North Harris and some basic and ultrabasic rock types are described in detail. The extent of granites and pegmatites in west North Harris and the Uig Hills is indicated as is that of the belt of crushed rocks on the eatern side of the island.

Dearnley (1962) was the first to correlate events in the history of the Lewisian of the Outer Hebrides with those of the Scottish mainland. He recognised a suite of basic intrusions similar to the Scourie Dykes and hence introduced the Scourian-Laxfordian concept to the Outer Isles. In these basic rocks he saw evidence - for an early Laxfordian granulite facies metamorphism and later retrogression to amphibolite facies, a view not shared by this author (Chpt.4). He suggested the existence of three broad structural zones- a central zone between South Uist and the Sound of Harris exhibiting dominantly Scourian features and correlated directly to the Scourian block on the mainland after a displacement of some 70 miles along a postulated Minch fault, and northern and southern zones with predominantly Laxfordian features. In a second paper, on the South Harris complex, Dearnley (1963) described it's general lithology and structure and established the chemistry of several of it's components. For the igneous rocks he suggested a sequence of ultrabasics-gabbro-anorthosite-tonalite all derived from an original tholeiitic magma by differentiation, and regarded the complex as a possible plutonic centre related to the ultrabasic and basic intrusions of the Scourie Dykes in the Lewisian Complex.

He recognised relict textures and mineral assemblages dating from a "pre-Laxfordian metamorphism and folding" in rocks which he considered to precede the igneous complex. Much work has since been carried out on the complex- much of it of a geochemical nature- and the complex has come to be regarded as Scourian in age and subjected to several metamorphic and tectonic episodes.

The granite-migmatite.complex of South Harris and west North Harris was studied by Myers (1968,1971a) who established the existence of a region within this complex in which migmatisation had taken place in Scourian times although it is the later Laxfordian events whose effects now predominate (see Table 1.1). He established sequences of pegmatite and granite production, described the development of 'in situ' granitic units and the relationship between granite formation and the tectonic development of the complex. He also established the form of the complex as a broad dome of migmatisation, roofed by country rock gneisses. In general the sequences and structures Myers records are duplicated in east North Harris but on a somewhat smaller scale.with only locallised granite development (Chpt.3). The extension of the complex into the (Jig Hills is being studied by geologists of the Institute of, Geological Sciences who are also carrying out geochemical work on the granitic, gneissic and basic rocks of the region, data which has been made available to me by Drs.D.Fettes and D.Smith, and which I gratefully acknowledge. Myers (1970) also suggested a scheme for the subdivision of the gneisses based on the form and composition of the gneissic banding. This scheme is described and discussed in chapters 2 and 3.

The basic and ultrabasic rocks of the Scourie Dyke Suite have been studied by a variety of researchers. As stated above Dearnley (1962) first recognised the basic rocks as being Scourie Dykes similar to those of

11 the mainland. In subsequent papers (1968,1973) he describes features of deformed dykes and presents geochemical data of typical examples. Watson (1968) working on the metadolerites of Great Bernera in west Lewis obtained an E-W trend as the original orientation of the dykes, a trend which is therefore similar to that characteristic of dykes of the mainland. Myers (1968, 1971) suggested that zones of abundant dyke fragments could be interpreted in relation to large Laxfordian folds oriented on NW-SE lines, such zones providing the only evidence of large scale Laxfordian structures in west North Harris. Using such a distribution he concl- uded that the original orientation of the intrusions was on NE-SW lines, an orientation somewhat at variance with that of the undisturbed dykes of the mainland. All such features are duplicated in east North Harris (Chpt.2) and lead to a similar conclusion; it would therefore seem that in the deformed Scourie Dykes of North Harris in general there is evidence of a set of intrusions whose original orientation was somewhat different to those of the mainland. Further to this conclusion is the division of the Scourie Dykes of east North Harris into two sets of intrusions on petrological and geochemical grounds.

Davies, et al (1975) in their discussion of the Lewisian of northern Lewis describe areas of relatively low Laxfordian reworking, a conclusion partly based on the preservation of igneous features and intrusive contacts in the Scourie Dykes. They also describe structural features of the gneisses which give credence to their suggestions (see Chpt.2). Other regions of low Laxfordian deformation relating to various structures in the Lewisian of the Southern Isles were noted by Coward, Francis and Graham (all 1969) in PhD theses. Low deformation areas are recorded while elsewhere intense Laxfordian deformation- in four phases- associated with amphibolite facies metamorphism and local late Laxfordian granite formation is much in evidence. These features were discussed again by Coward et al (1970) and Coward (1973a) in which the structure of the Southern

12 Isles was shown to incorporate large-scale antiforms seperated by pinched synforms (fig:l.3) related to viscosity contrasts between the drier and thus more viscous rocks of the east- where pre-Laxfordian structures and mineral assemblages are preserved, metasedimentary relicts are rare and pyroxene gneisses common- and wetter, less viscous rocks of the west- where metasediments are common, early features rarely preserved and the gneisses exhibit amphibolite facies assemblages. In the areas of low Laxfordian strain recorded on the eastern side of South Uist Scourian features are retained (Coward, 1973b). These areas occupy rounded antiforms and are only substantially deformed by the F3 phase of Laxfordian deformation. Such large scale structures and areas in which Scourian features are preserved are not seen in east North Harris and the sequence of structural events is somewhat different (Table 1.1).

Finally, metasedimentary rocks are described in various areas by Coward et al (1969), Watson (1969) and Davies et al (1975). The former distinguish between calcareous, psammitic, pelitic and semi-pelitic metasediments, types which include within them both biotite and hornblende gneisses of "uncertain affinities". Little evidence of metasediments exists in east North Harris.

Thus far nothing has been said of the ages of the events outlined above. Table 1.2 (based mainly on Dickinson, 1974) lists the ages obtained in the Outer Hebrides using a variety of lithologies and methods. Many of the pegmatites exhibit a late Laxfordian age, as do the gneisses but some Scourian pegmatites are included. The Grey Gneisses generally yield a Laxfordian age3Scourian zircon and lead-lead dates have been obtained, giving further evidence of an early complex.

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F 1g : 1 . 3 Schematic map of parts of the southern islands, Outer Hebrides illustrating the arrangement of Laxfordian F3 antiforms and synforms in relation to other tectonic and metamorphic features. (From Coward, et at, 1970). GENERAL HISTORY SOUTiEi ISLES Sib and West NORTH HARRIS LEWIS NORTH HARRIS Local development of pegs Both local and large- Small scale development Large scale development of F3 structures, small

io and granites with their ks scale development with only. granites and pegmatites. scale; flat-lying it oo n Minor granites associated F3 structures. r associated structures. F4 structures present F3 structures are main folds.

Gra phase of the regions. and pegs'. seen chiefly in west.

Large scale folds in the F3 very large folds with Major folds seen in the F2 structures are F2 structures dominate, , N Sthn. Isles evidenced by pinched synforms (due to gneisses of South Harris predominant, small, widespread and on all scales, largest outlined m the gneisses; suggested contrast between 'dry'roaks and suggested by the F1 structures (Lax') m elsewhere by patterns of in east and'wet'in west) Scourie Dykes in W.Harris identified, by gneisses and Scourie N ā Scourie Dyke fragments.agments. F These era the largest of Dyke fragments. Areas of 2 only divided in south DIA Suggested Scourian set in F1 the F2 structures, which varying Laxfordian strain a north and south reworked are otherwise small and are suggested. F1 folds AXFOR L ā Laxfordian complex. widespread, seen but uncommon, small. Amphibolite facies with Early complex not well Little published. Early 8mphibolite facies metamorphism throughout the

ic Laxfordian, culminating in a period of granite granites, etc at it's h preserved. Suggested two complex preserved in east des

rp o and pegmatite production (onvarious scales). Two- climax. Again two-phase , o metamorphic phases in with amphibolite met'ism m is phase metamorphism discounted, 'granulites metism met'ism discounted. Late ta ep Laxfordian (granulite in reworked complex of

Me early; amphibolite later) west, suggested as due to local 'hot-spots'. greenschist retrogression. Recognition of Scourie Seen throughout the area. Not recognised i the 5th Scourie dykes of Scourie Dykes very, abund-;.

Dykes led to similarity Mineralogy suggested to be Harris complex, but every- various types seen, ant, everywhere deformed.

ta. of events with mainland associated with style of where else. Various types Their degree of def- by F2, but some with KES e with Soourian/Laxfordian deformation, and assemblages. Outline ormation is used to relict features in areas episodes, etc. Variety of Suggested E-W trends, large folds in W.Harris. indicate areas of low of 'low' Lax' strain. tion, E DY I

bu 'dyke types a]o similar. Basic and ultrabasic types. - Basic and ultrabasic Laxfordian strain. various types originally, i UR

tr Basic and ultrabasic types rocks present. Original Original H-N trend (UB, basic and noritic). SCO is Original NESW trend. D Original trend ENE-ENE. NE-SWtrend suggested. suggested. Scourian structures No Scourian structures Occasional areas in which Such areas of low Lax' Some suggested Scourian d can be suggested apart ian Laxfordian strain was low deformation are seen in structures in the gneiss in areas of low Lax' show preserved Scourian the east; no large early of both areas (F1 phase). strain- folds and from the early gneissic eserve

Scour features; uncommon, Also in Sth.Harris. lineations. foliation. Pr structures seen. Metasediments, metaigneous Metaseds' and igneous Relicts of sed' and other Relict sed' and other Isolated metaigneous

relicts locally preserved- rocks preserved in the rocks widespread. Chief rocks. Some evidence rocks, some sed' (?)

tures particularly in the large 'dry' gneisses of the east. occurence in the South of Scourian amphib- gneiss. Region seems dian c ui r complex of South Harris. Harris Complex. olite facies met'ism. generally devoid of fo U tru Scourian rocks and strua- s Lax — +

e tares due to massive ks

Pr Lax' reworking. oo r r Table 1.1 Structural correlations in the Outer Hebrides. c ► Locality Rock Material Method Date Ref. Roneval pegmatite Ksp K-Ar 1140± 70 1. Chaipaval pegmatite Ksp K-Ar 1110- 70 1. Shillay pegmatite Ksp K-Ar •10604 70 1. Shillay pegmatite Ksp Rb-Sr 1655± 60 2. Harris pegmatite Ab Rb-Sr 2504 90 2. Chaipaval pegmatite Mu Rb-Sr 15704 40 3. Chaipaval pegmatite Mu Rb-Sr 16104 35 3. 1440± 40 Northton schist Bi Rb-Sr 3. Northton schist Bi Rb-Sr 15004 40 3. Sletteval pegmatite Bi Rb-Sr 1530± 30 3. Sletteval pegmatite Bi Rb-Sr 14504 30 3. Rodil marble Ph Rb-Sr 1510+ 40 3. Chaipaval pegmatite Mu K-Ar 1570± 80 3. Chaipaval pegmatite Ur U-Pb 14901 35 4. Loch a'Sgurr pegmatite Ur U-Pb 1565+ 100 5. Chaipaval pegmatite Mo U-Pb 1565± 100 5. Barra pegmatite w.r. Pb-Sr 2535± 50 6. Barra ultrabasic Ho K-Ar 2585 6. Harris granites w.r. Rb-Sr. 1700± 34 7. Harris granites Zi U-Pb 1715+ 10 7. Harris grey gneiss Zi U-Pb 27704 10 8. Harris grey gneiss Zi U-Pb 1715± 60 8. South Uist pegmatite w.r. Rb-Sr 25604 80 9. Lewis basic dyke w.r. K-Ar 2440± 60 10. Harris grey gneiss Zi Rb-Sr 2690± 140 11. Harris grey gneiss Zi Pb-Pb 26404 120 11.

1. Holmes et al, 1955 2. Smales et al, 1958 3. Giletti et al, 1961 4. Bowie, 1962 5. Bowie, 1964 6. Francis et al, 1971 7. van Breeman et al, 1971 8. Pidgeon at al, 1972 9. Lambert et al, 1970 10. Lambert et a1, 1970 11. Moorbath et al, 1975

Table 1.2 Summary table of available age dates, Outer Hebrides (After Dickinson, 1974)

16 1.4 The geology of North Harris- introduction

As will be described in more detail in chapter 2 little direct evidence of the nature of a pre-Laxfordian complex is retained in North Harris and the effects of Laxfordian tectonic and metamorphic events therefore predominate. Areas of very low Laxfordian strain in which Scourian structures are preserved, such as have been described elsewhere in the Outer Hebrides, are not recognised and evidence of the nature of an early complex lies in the scattered occurences of Scourian agmatites, pegmatite veins, minor folds and basic-ultrabasic rocks none of which are very common. This early complex was cut by a suite of basic intrusions- the Scourie Dykes In North Harris they are remarkably abundant and variations in the degree of their disruption and deformation give a possible indication of variations in Laxfordian strain throughout the region. The original orientation of many of these dykes is considered to have been NE-SW. After emplacement of the Scourie Dykes the region suffered metamorphism at amphibolite facies. Associated with this metamorphic episode, which extended throughout the period, were several phases of deformation of which F2 was the most pervasive and during which the strong NW-SE fabrics, so typical of the region, were developed. Locally granulite facies assemblages were produced in the basic rocks but such assemblages are not preserved in the gneisses. The late Laxfordian is characterised by metamorphism at low amphibolite facies during which migmatisation, with the production of granites and pegmatites, was locally intensely developed. Contemporaneous with and spatially related to this phase of granite production folds on E-W lines and with somewhat variable style were developed. The close of the tectono-metamorphic history of the region was marked by mild cataclasis, the emplacement of N-S quartz veins and some greenschist retrogression in the extreme south-east of the region.

17 1.5 Geochemistry: aims and results

Little in the way of systematic analytical work had been carried out on the gneisses and granites of the Outer Hebrides, nor on the Scourie Dyke rocks and it eas thought that such a body of information would prove useful in itself. Since the start of the work it has become apparent that I.G.S. have also been carrying out analytical work, chiefly on the granitic rocks but including also gneisses and basic rocks, and there has been a considerable exchange of analytical data. With regard to east North Harris itself a regional study of the geochemistry was made in an attempt to explain gross regional variations in lithology observed in the field and also with a view to studying the poss- ibiity that geochemical changes were associated with variations in Laxfordian deformation.

Study of the basic and ultrabasic rocks of the Scourie Dykes was carried out for similar reasons; that is to add to existing information and to study specific problems. Both whole rock and mineral chemistry was studied in these rocks.

The gneisses have been found to show gross lithological and chemical variations that can be directly correlated, and some 'interelationship of chemistry and deformation have been observed (certain areas of high Laxfordian deformation having relatively high volatile contents in the gneisses). In addition to these special features the gneisses showed 'typical' features of amphibolite facies gneisses and are very similar in composition to Grey Gneisses recorded elsewhere, both in the Outer Hebrides and on the mainland. The Scourie Dyke rocks yielded very interesting geochemical data. On the basis of petrology and geochemistry the rocks have been divided into two groups, the first tholeiitic (with clinopyroxene) and the precursor of the majority of the amphibolites in the region, and the second noritic and including the layered ultrabasic of Maaruig. Mineral chemistry gave a basis for PIT

18 estimations which showed that metamorphic conditions throughout the area were quite constant, and thus by Implication that the variations in the rsineral assemblages of both the Scourie Dykes and the gneisses were

due to variations in PH2O (or some similar factor) rather than metamorphic conditions.

The full details of this analytical work are given in chapters 3,4 and 5.

1.6 Format of the thesis

While covering a large number of topics the thesis contains only six chapters of which the first is the present introduction and the last a brief summing-up of results. Thus the remaining four chapters are rather long and of complex structure. For this reason a contents list is given at the beginning of each chapter and should be referred to to obtain the general structure and sequence of topics in the chapter. Generally division of the longer chapters into sections labelled by letters is 'overprinted' by a sequential numbering of headings and sub-headings, the latter involving a third subscript. Thus one might have:

SECTION A: The price of stuffed moles 1.1 Source of moles 1.2 Cost of stuffing 1.2.1 Labour 1.2.2 Materials 1.2.3. Taxation SECTION B: Results 1.3 Aesthetics 1.4 etc. and so on through the whole chapter Such a scheme was adopted to avoid the highly complicated

numbering schemes of some theses. t

Chapter 2 deals with the structural history of the region and is intended to be both a general account of the structure and a 'background' of information given before the petrological and geochemical data is presented. Chapter 3 discusses the petrology and chemistry of the gneisses and includes a shorter section on the granitic rocks. Chapters 4 and 5 concern the petrology and whole rock chemistry and mineral chemistry respectively of the basic and ultrabasic rocks of the region.

All diagrams, maps and photographs are classed as figures and numbered sequentially through the chapter. Tables are referred to as 'Tables' and also numbered sequentially. After the bibliography appendices are given of analytical methods and geochemical results. These include the bare minimum of data, norms and so on not being included in an attempt to save both time and space.

20 Chapter 2

Structural History

Round and round the rugged rocks The ragged rascal ran. Anon.

Chapter 2

Contents Introduction 23 Section A: Pre-Laxfordian structures 25 2.1 The gneisses 25 2.2 The Scourie Dykes 27 Section B= Laxfordian Structures 28 2.3 F 1 structures 28 2.4 F2 structures 30 2.4.1 Major structures 2.4.2 Minor structures (i) Folds (ii) Boudinage (iii) Shear zones 2.4.3 Fabric development (i) The gneisses (ii) The Scourie Dykes 2.4.4 Large scale variations in Laxfordian strain 2.4.5 Conclusions 2.5 F3 structures 49 2.5.1 Introduction 2.5.2 Structures 2.6 Dyke deformation: orientation and competence contrast 55

2.7 Late structures 60 2.7.1 F4 structures 2.7.2 Quartz veins 2.7.3 The Outer Hebrides Thrust Zone 2.7.4 Jointing 2.7.5 Tertiary dykes

2.8 General conclusions 65

22 Chapter 2

Introduction

The region is made up of biotite, and hornblende- biotite 'Grey Gneisses' whose origins lie in Scourian episodes of deformation and metamorphism of which little evidence now remains. The rocks of the early complex were cut by basic rocks of the Scourie Dyke suite and all rocks were metamorphosed and deformed during the Laxfordian period. In the late Laxfordian episodes gra nites and pegmatites were produced, locally in North Harris but on a large scale in the migmatite complex to the west, the development of which was accomp- anied by recrystallisation an d deformation of the host rocks.

The tectonic history revealed in the region is dominated by Laxfordian events which may be divided into four phases of deformation of which only two were of major improtance. Metamorphism was contemporaneous with all phases of deformation as a continuous process, at amphibolite facies grade. The final important product of metamorphism was the local development of granites and pegmatites, principally in the west of the area. Pre-Laxfordian features are poorly preserved. The most obvious early structural feature now remaining is the gneissic banding which, as is described below, owes its origin to both Scourian and Laxfordian deformation the Scourian component having been flattened and rein- forced by new Laxfordian fabrics. Early folds are probably represented in the region but positive identification of such structures is always difficult relying as it does in these terrains on cross-cutting relationships between foliations, pegmatites and basic rocks of the Scourie Dyke suite. Indeed the rocks of this suite are of vital importance in elucidating the relative ages of the structural events of the Lewisian in general. It is generally assumed that the Scourie Dykes were emplaced more or less as one sequence of intrusions cross-cutting the earlier rocks and fabrics

23 and being themselves deformed and metamorphosed by Laxfordian events. A note of caution is nevertheless required when using amphibolites as 'time-markers' for the positive identification of Scourie Dyke material is often difficult and, as is described in section 2.4 below, the Scourie Dykes of this region show interesting variations in orientation and deformation when compared with those of the mainland which suggests the possibility of two phases of intrusion.

Separating structural events is based on grouping the structures they produce on the basis of the following criteria: a) style b) orientation of structural elements c) relationships between large and small structures d) interference of one structure by another. Valid assumptions concerning age-relationships can only be made when several of these criteria have been fulfilled and even then misinterpretation can occur. Style is particularly notorious being often quite variable, even in the products of the same phase of deformation (eg. the various styles of F2 or F3 folds shown below). Orientation can be equally troublesome with a wide variation of original trends being produced in the same episode and with the reorientation of structures by later events. Given a region of sufficient size- and hence some degree of structural homogeneity- it is possible to collate all available data and produce a cogent history of events, and such is the case presented below.

The chapter is intended to provide a general account of the structure of the area and is not therefore presented as a detailed structural analysis. It also serves as a general account of some of the important features of the geology of the region ref„fered to in the more detailed petrological and geochemical studies of the later chapters.

24 Section A: Pre-Laxfordian structures

2.1 The gneisses

By far the most important early structure is the gneissic banding much of which is described as pre- Laxfordian for the following reasons: a) Scourie dykes are seen to cut a pre-existing banding; although actual discordances are uncommon they do occur throughout the region. b) Early Laxfordian structures are superimposed on a pre- existing banding whilst later events involve the production of new Laxfordian components in the gneiss fabric. c) The gneisses have yielded an age of 2700 my (Pidgeon and Aftalion, 1972) using zircons from a sample at Bunaveneader.

The banding is defined by a compositional layering between basic and felsic bands and pegmatite layers. Myers (1968) divided the gneisses of western Harris into several types on the basis of the nature of this banding a division shown in fig:2.1. Also in this diagram (which is taken from Myers, 1970) are trends of evolution connecting gneiss types that could be followed during deformation. As a descriptive scheme it has considerable value but unless the relative ages of the fabric components can be evaluated it can be used merely as a guide to the form of the gneisses of the early complex and to the effects of deformation on these rock types. For example there is little doubt that the agmatite and hornblende pod gneisses point to the existence of early basic and ultrabasic layers (originally either intrusions or volcanics) but so toamust some of the amphibolite layers, now indistinguishable from the Scourie Dyke amphibolite layers.

Small scale pre-Laxfordian structures may also be present but remain unrecognised due to a lack of conv- incing evidence as to their age, small irregular structures which do occur being generally classed as early Laxfordian (F1 ). To the west such folds were classed as Scourian by Myers (1968) and it is therefore likely that structures of both ages are present in D

• A. .LE A.

•-- — r-r7rrig-r1-71-177r 7-7V "."7.—,"7" •

ca.

— • - — ve.mvesstir..W.: •zia. Ca — •

• v. F ■ .* H ii. A •—• D C iv. 0 K

The main pre-Laxfordian gneiss types of Western Harris; A. foliated basic agmatitc gneiss. F. biotitc-bandcd gneiss. B. amphibolitc-banded gneiss. G. weakly biotitc-bandcd gneiss. C. amphibolite-striped gneiss. H. massive biotitc-foliated gneiss. D. amphibolite-lens gneiss. J. weakly amphibolitc-banded gneiss. E. pcgmatiic-banded gneiss. K. hombicndite-pod gneiss.

Fig:2.1 Lewisian gneiss types (from Myers, 1970).

26 both areas.

The picture thus emerges of a banded gneiss complex which contained deformed intrusives/volcanics and pegmatites prior to the emplacement of the Scourie Dykes. The existence of such a complex leads to the suggestion of relatively high grade metamorphic conditions during its formation, with the production of banded gneisses,and pegmatites and the deformation and metamorphism of early basic rocks but the details of these events have been obliterated by the Laxfordian episodes. In Chapter 3, on the basis of mineralogical and geochemical variations over the region as a whole, original lithological variations are suggested.:

2.2 The Scourie Dykes

The Scourie Dykes are themselves pre-Laxfordian in age, intruding the rocks of the early complex prior to Laxfordian events. Their petrology and geochemistry are described in Chapters 4 and 5 but as their distribution and general form are governed by Laxfordian deformation these features are discussed in the present chapter. Their distribution and fabrics are discussed in section 2,4.3 and general considerations regarding their deformation in section 2.6. Originally tholeiitic they now exhibit predominantly metamorphic assemblages and textures, original igneous material being preserved in only a few localities in the east and north-east of the region. Deformation has resulted in the obliteration of the original orientation and form of these intrusions and the loss of discordant contacts. It seems likely that they were emplaced as dykes and, as discussed below, were intruded as a swarm with an approximate NE-SW trend, a trend at some variance with that of the mainland

27 Section 6: Laxfordian deformation

Laxfordian tectonic activity may be subdivided into four episodes, of which the second and third are of major importance; the degree to which these events may be regarded as separate entities is rather debatable. The main features of the four phases are as follows:

F1 irregular folds; rare fabrics; poorly represented F2 main fold phase on all scales, NW-SE trend of axial planes with predominant SE plunge. Strong planar fabrics are developed i n all rock types E-W oriented folds correlated directly with areas of granitic rocks. Some recrystallisation of earlier fabrics; little new fabrics F4 minor N-S buckles; infrequent and of little import' Late stage cataclasis.

2.3 structures F1

A phase of dubious significance, little now remains of the first structures but some scattered small, irregular folds. These structures are often rather open and lacking in fabrics. Notable exceptions are seen, fig:2.2 showing somewhat diagrammatically the occurence of possible F1 foliation in several amphibolite lenses. Of the three only (a) is unequivocal, an early foliation being folded by F2 deformation. The other examples could relate to F2. deformation as is described below (2.4.3). These examples occur in widely separated exposures and as they are defined by amphibolite facies minerals it seems that at least partial amphibolitisation of these basic rocks took place during F1 . The lack of F1 fabrics marks a distinction between this region and that of the Southern Isles (Coward, Francis, Graham; all 1969 PhD's) where the foliation of the Scourie Dykes is produced primarily during the first identifiable phase of Laxfordian deformation. Possible F1 folds are involved in interference structures with F2 folds in the Tarbert region.

28 Fig:2.2 Possible Fl foliations in basic bands and lenses. 2.4 F2 structures

The F2 episode was the major Laxfordian tectonic event of the region. It produced innumerable minor folds, major folds outlined by the Scourie Dykes, strong planar fabrics in all rocks and was responsible for all the major trends in the region as seen today.

2.4.1 Major structures

The foliation trend map of fig:2.3 was produced by plotting the strikes and dips of banding and foliation of the gneisses on a map and extrapolating trend lines on the basis of the plotted data points. The foliation Is oriented generally NW-SE, dipping to the SW with little variation. Large-scale fold structures are uncommon although some do occur in association with folded Scourie Dykes while others may be inferred from closure of the foliation-banding in certain areas. A map showing the distribution of pods and lenses of basic rocks (regarded as derivatives of the Scourie Dykes) is shown in fig:2.4- the actual distribution is given in fig:4.1. On this map the areas in which basic rocks are abundant are outlined and, as may be seen, these regions seem to exhibit the effects of large scale folding. No attempt was made to contour the distribution of basic lenses by methods such as that suggested by Lisle (see Myers 1971) and the map thus represents the results of 'in situ' field observation rather than any statistical assesment. The trails of dyke fragments are discordant to the gneiss banding and the overall trend of the enveloping surfaces is NNE-SSW. The orientation of the axial traces suggested in the map is NW-SE and field evidence- particularly that of the larger folded basic masses such as that at Ardvourlie and Maaruig (see Chapter 4)- points to a general, moderate south-easterly plunge of the fold structures and a south-westerly dip of axial planes.

• The presence of large fold structures in the basic rocks with a general absence of structures of similar 31 Fig:2.4 The distribution of basic and ultrabasic rocks: the great majority occur within the stippled zones. (see also Fig:4.1).

32 size in the gneisses leads to interesting speculation on the orientation of both the gneissic banding and, particularly, the dykes prior to their deformation. In 19g:2.5 a greatly simplified sequence of events in the deformation of Scourie Dykes and gneissic banding by F2 deformation is suggested based on the following features and assumptions: a)' The gneisses lack many large-scale folds; the predominant trend of the banding is NW-SE b) The distribution of basic rocks shows evidence of large-scale folds oriented on NW-SE axial planes. c) F1 deformation was minor; the structures of the area are principally due to F2 deformation d) The original orientation of the dykes was more or less parallel to the direction of maximum compressive stress to allow the production of the large folds of f ig :2-4. From these considerations, in fig:2.5 it is suggested that- prior to Laxfordian deformation— the Scourie Dykes were aligned approximately NE-SW with a high angular discordance to the gneissic banding (whose exact form and orientation are unknown). Subsequent deformation led to folding and finally flattening and boudinage of the dykes with the production of bands and lenses concordant with the gneissic banding. This banding, on the other hand, was subject to transposition and flattening and the imposition of new Laxfordian fabrics, few folds being produced.

The suggestion that the original orientation of the Scourie_ Dykes was NE-SW is at some variance with the orientation of Scourie Dykes on the mainland whic are of ESE-WNW trend. Myers (1968) studying the gneisses of west North Harris came to the same conclusion as to the original orientation of the dykes, for similar reasons, and it seems, therefore, that in North Harris there is evidence of a suite of Scourie Dykes of differing orientation to those of the mainland and elsewhere in the Outer Hebrides.

33 SW —~ F--- NE

a).Trend of Scourie Dykes discordant to original gneissic banding.

b) Deformation causes transposition and flattening of the gneissic banding and folding of the basic 'layers'.

Fig :2.5 Schematic representation of the possible sequence of events during F2 deformation.

34 2.4.2 Minor structures a) dol ds

Minor folds of this phase occur throughout the region. On the whole they show considerable uniformity of style and orientation, being typically isoclines whose axial traces are oriented NW-SE, and whose plunge is to the,SE. Some variations in style are to be seen, however, examples being shown in fig:2.6 but there is no systematic distribution of such variations. Their relative age is generally easily determined by virtue of their abundance, style and orientation and also by their effect on the Scourie Dykes (figs:2.6, 15 and 16) and by the various interference structures between succesive fold phases (fig:2.7).

) Boudinage

While 'classic' boudinage is not a common feature In the region the Scourie Dykes show evidence throughout the area of flattening, attenuation and disruption in 'boudinage-like' phenomena.(fig:2.8). The form of boudinage depends on the degree of flattening and on the competence difference between the enclosed and enclosing layers, fig:2.9 (from Ramsay, 1967) showing the general form that boudins could be expected to take given varying degrees of flattening and viscosity contrast. The most common boudins in the basic rocks of North Harris are somewhat squat lens- shapes suggesting a relatively large competence contrast between them and the gneisses; less commonly longer, thinner forms are seen suggesting a somewhat lower competence contrast. The two types, with the former predominant, are widespread. (see also section 2.6). c) Shear zones

Small shear zones are infrequently seen in the gneisses and some basic rocks. In the Tarbert region, where the majority of examples are to be found, some quite large shear zones- up to 10 metres across- are visible in quarries and road cuttings. In these zones the gneisses suffered cataclasis with grain-size 35 Fig:2.6 F2 fold styles

a) F2 fold in amphibolite banded gneiss b) F2 fold with limb pegmatite. Note offset along peg'.

36 Fig:2.7 interference structure in a thin basic layer ) Gullaval Glas. The effects of all three phases of Laxfordian folding can be suggested in this structure. i f

(a)

(b)

Fig: 2.8 Boudin types Of the two, type (a) is perhaps the most common, both in size and style.

38 B

II' Ī . ii, IlY _ i. I.

' 111 • ir. ~'Y' I'~ j~

:a: .V.~• p•i r,l.

I I 4 ina

( I •I,+;~~ii~ j. 999 .Y I 11'I'~) . IPI IJ ,d. IIII~ prP~l~

The progressive development of boandiage. The competent bands I, 2, 3, and 4 are arranged hi decreasing order of coinperence, and hand 4 has the same properties as the matrix.

Fig:2.9 Diagram indicating the forms of boudinage with varying competence contrasts. (From Ramsay, )967).

39 reduction and the production of quartz 'ribbons' (see section 3.5); occasionally highly flattened isoclinal folds are associated with these shears. The zones parallel the foliation and banding of the gneisses and seem_to be rather discontinuous along their length. Few large shear zones of this type are seen outside the Tarbert region and they seem to be a rather local development, possibly related to the high water content of the gneisses of this area (Chapter 3).

2.4.3 Fabric development

The development of fabrics during F2 deformation was everywhere intense but such development was greatly controlled by the rock type in which it took place. Hence the fabrics of the gneisses and of the basic rocks are described separately.

a) The gneisses

The strongest fabric element of the gneisses is the planar component of foliation (defined by Hobbs et al, 1976, as "..any surface in metamorphic rocks be that due to composition, oriented minerals, grain size variations, etc.) within which is incorporated a locally strong linear component. As suggested earlier some of the foliation was derived by transposition of pre-existing fabrics but the intensity of F2 deformation was such that much must have been produced as a new Laxfordian foliation. In areas of late Laxfordian granites recrystallisation causes the reduction of F2 fabrics but completely massive rocks are rare.

The coarsest fabric elements of the gneisses are represented by 1ithologica1 variations- amphibolite layers, pegmatites and so on- as are schematically shown in fig:2,1, the coarsest of all being produced by the basic layers and lenses derived by the deformation of Scourie Dykes.

40 On a somewhat smaller scale .the gneisses throughout the region exhibit a foliation defined most frequently by the preferred orientation of biotite. A strong fabric element it serves to emphasise the lithological banding and becomes the only planar fabric in the more homogeneous gneisses. As may be seen in fig:2.3 the foliation has a strong NW-SE trend, the predominant dip being to the SW, although this latter can be somewhat variable in both direction and degree. in the extreme north and north-west of the area.

A strong linear component, almost invariably defined by the preferred orientation of both individual quartz grains and quartz aggregates (although hornblende - lineations also occur), is seen throughout the region. Locally such fabrics may be intensely developed and it Is likely that this represents original variations in its production rather than the effects of later recrystallisation. In the region as a whole, however, planar fabrics predominate. b) The Scourie Dykes

Basic layers and lenses derived from the Scourie Dykes show little evidence of having developed fabrics during F1 , the few examples that do having already been described. Thus by far the most important fabric development in these rocks took place during F2 deformation. Not all the basic rocks exhibit fabrics; rocks with relict igneous textures and mineralogies were obviously untouched by such processes but lenses and bands of amphibolites whose assemblages are metamorphic and which appear to have been deformed may also be fabric-free. Whether this feature is due to a lack of fabric development originally or to subsequent loss by recrystallisation is difficult to assess. As in the gneisses, the fabrics are generally a combination of a strong planar foliation which incorp- orates a weaker, but occasionally strong, linear element. Sometimes fabrics are strictly axial planar, developed only at fold hinges, while in other cases the whole of a body can develop a foliation. In larger units foliation is commonly only produced at the margins 41 leaving a massive core of amphibolite or, possibly, relict igneous material (see Chapter 4). In such cases fabric development can be traced from the original igneous texture to foliated amphibolite. Typical stages in this development are outlined below: a) The original texture is preserved, sometimes with a relatively unchanged original igneous mineralogy b) Static metamorphism may create mottled rocks in which the original texture is mimetically preserved. Such is the origin of many of the 'sub-ophitic' textures commonly reffered to.in the Scourie Dykes, although such textures may also be produced as metamorphic textures. c) Development of a weak fabric with elongate minerals, or streaked-out mineral aggregates (from b) aligned in the foliation. Both mafic and felsic minerals can produce such streaks (fig:2.10). d) Strong fabrics developed. More amphibole is present and original igneous mineralogy and texture is lost. Segregation of the minerals into bands may occur. e) Fabric fully developed; texture may coarsen if rock suffers recrystallisation. The mineralogy is now that typical of an amphibolite (hornblende-feldspar).

One folded basic layer in which such textural variations are traced is found just south of Ardvourlie in the north-east of the region. Here a core of orthopyroxene-plagioclase rocks with relict textures is surrounded by a variety of amphibolite types with a range of textures varying from coarse mottled metadolerites to strongly foliated simple amphibolites. The foliation dips at some 30-35° to the SW and is axial-planar, the overall fold structure itself being strongly asymetric with a sub-vertical northern limb and gently dipping southern limb, the shallow dip creating a large area of outcrop of these rocks.

42 (a) Feldspar streaks in amphibolite

(b) Hornblende 1 ir+eat ion in amphibolite, Clett Ard

Fig:2.10 Mineral fabrics in amphibol ices

43 In a region just east of Tarbert some rather unusual fabrics are devloped, generally in thin concordant layers. Examples are given in fig;2.11 in which can be seen foliations set oblique to the margins of the layers, an orientation thought to be due to shearing of the basic rock during fabric development. In another example the contact of the gneiss and amphibolite shows 'intermixing' with small blebs and lenses of amphibolite incorporated in the gneiss. These features are ref/erēd to again in section 2.6 for they suggest that in this area at least some of the Scourie Dykes behaved as the incompetent member during deformation. They may represent yet another feature relating to the high volatile contents of the.gneisses of the Tarbert area. All fabric development of the basic rocks relates to F2 deformation, a fact which seems to distinguish North Harris from the Southern Isles where the fabrics were developed in the first recognised phase of Laxfordian deformation, and only modified by later events.

2.4.4 Large scale variations in Laxfordian strain

Elsewhere in the Outer Hebrides- notably in northern Lewis and parts of the Southern Isles- distinctions are made between areas of only modest and areas of high Laxfordian reworking of the early complex. The areas - - - -'- _ _ * - R .. _~ _ The areas of relatively low Laxfordian strain are characterised by the retention of Scourian fabrics and structures, preservation of discordances between Scourie Dykes and the gneissic banding and the retention of early textures and mineralogies of both the gneisses and the Scourie Dykes. Table 2.1, taken from Davies et al (1975), is based on studies of the Lewisian of northern Lewis and gives the general features of the three zones of progressively increasing Laxfordian reworking that are seen in the region. When applied to North Harris it is evident that some of the features- notably those concerning the Scourie Dykes- can be used to define areas of possible low Laxfordian deformation. Bearing in mind

44 sv • (eaJe WOJj • I~sOW) spueq iseq u suoi~E iiOJ ,5no eiuoue, 4 0 saLdwex: tl'Z:G!d Areas of low strain Areas of moderate strain Areas of high strain

Grey gneiss and supracrustal belts I Pre-Laxfordian migmatitic struc- Pre-Laxfordian structures distorted Pre-Laxfordian structures strongly tures and veins well preserved distorted

2 Mesoscopic shape-fabric LYS, Mesoscopic shape-fabric shows Mesoscopic shape-fabric S> L, large pre-Laxfordian component varying types and relationships large Laxfordian component

3 Anhydrous mineral assemblages As in areas of low strain Anhydrous mineral assemblages locally preserved in rocks of supra- very rare. amphibolite facies crustal belts, amphibolite-facies assemblages dominant assemblages dominant

Scourie dyke suite 4 Intrusive habit often clearly recog- Intrusive habit locally recognisable, Intrusive habit seldom recognis- nisable, many intrusions obviously a few intrusions discordant able, discordant contacts vers rare discordant and steep -

5 Primary plagioclase, pyroxene, Primary minerals rarely preserved, No remnants of primary minerals, (olivine) in centres of large meta- mimetically-preserved primary tex- remnants of primary textures very dolerites, primary subophitic textures widespread only in inner sub- rare tures mimetically preserved in zone many intrusions .

6 Anhydrous metamorphic assem- Anhydrous assemblages in centres Anhydrous assemblages not rec- blages preserve a centres of many of metadolerites common only in orded metadolerites inner sub-zone

7 Widespread amphibolitisation As in areas of low strain, most Amphibolisation almost universal, intrusions foliated most intrusions foliated

Gneisses and Scourie dykes 8 d2 folds open, associated fabrics d2 folds tight to isoclinal, associ- d2 folds tight to isoclinal, associ- weak ated fabrics of variable F.rength ated fabrics usually strong

9 Dominant textures reflect recrystal- As in areas of low strain, but later As in areas of moderate strain lisation during and after d2 reductions of grain-size important in some regions

Table:2.1 Features of the Lewisian complex of Lewis in relation to variations in Laxfordian strain. (From Davies, et al; 1975) .

46 that the region as a whole shows evidence of considerable Laxfordian structural and metamorphic activity and an absence of preserved Scourian structures and lithologies, It nonetheless possible to define three structural zones (fig:2.12) based on the following features of the Scourie Dykes:

Zone 1: relatively low Laxfordian reworking in which Scourie Dykes occur as large, continuous folded units with some preservation. of original igneous mineralogies and textures and the preservation of discordant contacts Zone 11: moderate Laxfordian reworking in which the dykes are now more disrupted, lack igneous minerals- but may have mimetically preserved textures. The typical assemblage is that of ciinopyroxene-amphiboiite. Zone 111: relatively high Laxfordian reworking in which the basic rocks occur as small lenses and bands, all concordant to the gneissic. banding. Simple amphibolites predominate in the assemblages. The area (around Tarbert) also shows other structural features such as shear-zones, unusual fabrics in basic rocks and no large folds in the gneisses and all may relate to the high volatile contents of the gneisses in the area (see Chapter 3).

Throughout all zones the gneisses show no variations which could correlate with the features of the basic rocks, for anhydrous assemblages are not seen- even in zone 1-,strong planar fabrics are everywhere dominant and only Laxfordian structures are found.

2.4.5 Conclusions

The F2 phase of deformation was the most important of the Laxfordian episodes in that it alone affected the complex and all its constituents as a whole generating

47 Fig:2.12 Zones of variable Laxfordian strain (see text for details)

Areas of relict igneous mineralogies in Scourie Dykes

48 pervasive fabrics and a range of structures on all scales from the major folds outlined by the trails of basic rocks down to microscopic fabrics. So common are the structures produced that their age is rarely in doubt. Areas of variable Laxfordian reworking can be suggested on the basis of features seen in the Scourie Dykes, a large region of moderate reworking containing both relatively low deformation areas in which original igneous features are preserved and a region of high deformation in which a variety of structural features are possibly created by a relatively high water content In the gneisses.

2.5 F structures

2.5.1 introduction

The late Laxfordian episode was marked by the production of granites and pegmatites in the west and south of the region (see fig:3.l). The fullest development of.these granitic rocks is seen in the migmatite complex described by Myers (1968,1970,1971) which extends from South Harris, across west North Harris and into the Uig Hills. In east North Harris the granites are local 'in situ' bodies apparently produced by the progressive metasomatism and recrystallisation of the host gneisses to give a weakly foliated granite gneiss or granite s.s. The granites are found in association with areas of flat-lying gneisses- chiefly in the hills of Clisham and Tomnaval and at Tarbert- structures which may represent the largest F3 structures of the region. The distribution of the F3 structures correlates directly with the areas of granitic rocks and are thus concentrated in the west and south. This relationship is one they share with the F3 structures of western 'Harris which were shown by Myers (1968) to be more fully developed in the granite-migmatite area than in the grey gneiss to the east.

49 2.5.2 Structures A

The chief product of this period of tectonic activity are small folds (up to lm in amplitude) with steep axial planes, axial traces oriented E-W, and with variable plunge. They are somewhat variable in style (fig:2.13) and often incorporate pegmatites. On a larger scale- up to 20m or so- the F3 :folds are geniculate structures with the same orientation; folds of this style are best developed at Tarbert. In fig:2.8 such a structure is seen to refold F2 and possible F1 folds in a thin Scourie Dyke amphibolite. The largest structures are thought to be those producing the flat-lying gneisses of Clisham and Tarbert. Nowhere else in the region are such areas of flat-lying gneisses seen and the spatial correlation with the granites would suggest some relationship between the two. Perhaps the simplest explanation of these areas is that they represent local, large monoclinal folds, developed-.on steep axial planes, within the otherwise south-westerly dipping gneisses. F3 fabrics are rarely developed. Occasional axial-planar fabrics are produced by biotite but such developments are quite variable. Of interest in this connection, however, is the observation that the gneisses within the region of granitic rocks show textural features suggesting that deformation was concentrated in that area. Outside this region coarse gneissose textures are the norm suggesting some degree of post-F2 recrystallisation.(see section 3.5).

In the migmatite complex of western Harris Myers describes an F3 history that is much more complete than that recorded in east North Harris. The granites occur both as 'in situ' and intrusive masses; the contact between the granitic rocks and the host gneisses can be mapped out and the transition between them studied; pegmatite phases can be better differentiated and F3 structures are better developed. However , the overall pattern for both regions is much the same with a stronger development of F2 in east North Harris.

50 Fig:2.13 F3 fold styles, fabrics and intereferende structures. Pegmatites are shown in black.

51 (a) F 3 fold with weak axial planar Fabric

(b) Highly irregular F3 Folds in pegmatite/gneiss

Fig 2.13 F3 Fold styles and interFerence structures (see over.)

52 (c) F3-F2 interference structure

Fig:4.13 Continued.

53 2.6 Dyke deformation: orientation and competence contrast

After describing the structures developed in the Scourie Dykes and their value in chronology, defining structures and distinguishing possible regions of variable F2 deformation it is perhaps valuable to describe, in retrospect, the deformation of the Scourie Dykes more thoroughly. Controlling factors in dyke deformation are: a) application of sufficient stress b) suitable orientation of the dyke with regard to the stress system c) contrasting material properties between the dyke and its host rock. Of these three only (a) i s_ a major prerequisite, the others controlling the type and style of the structures produced only if (a) is satisfied, that the Scourie Dykes of the area are deformed shows that it was.

Estimation of the original orientation of the dykes is problematical. The original form of the intrusions is unknown and they could have been irregular in both orientation and form, deformation has obliterated most discordances between the dykes and the gneissic banding and those discordances which remain are only of the order of lO-200 (fig:2.14). The Scourie Dykes of the mainland take the form of parallel dyke-like intrusions with an ESE-WNW trend and it is tempting to.postulate a similar form and orientation for the dykes of the Outer Hebrides. Such is indeed the case in certain areas of the Outer isles (eg. Great Bernera, Watson 1968) but for the reasons given earlier it is thought that, prior to deformation, the dykes of North Harris had a NE-SW or NNE-SSW trend cutting quite strongly across the gneissic banding. Whether the dykes with this trend (which is suggested by Myers for western Harris and by Drs.Fettes and Smith for North Uist, pers,conm.) were emplaced at one and the same time as the dykes elsewhere is not certain, but there is no reason why it should not be the case.

55 0

(b)

Fig :2.14 Examples of discordances between amphiboiite bands and the gneissic fabrics. Note the small folded pegmatite in (a)

56 The third factor to be considered is that of competence contrast between the basic rocks and their host gneisses, absence of which would have led to homogeneous deformation and an absence of folds and other structures even though the rocks would have suffered considerable strain. The presence of folds, 'S A~ boudinage and so on~a competence contrast did exist between the basic rocks and the gneisses and from the style of the folds and boudinage it seems that the basic rocks were the more competent of tF two. In fig:2.15 for example, a thin folded amphibolite layer is enclosed by gneisses which exhibit extreme extension and flattening at the fold hinge and one is left with little doubt as to which was the more competent layer during their deformation. Such an extreme degree of difference in fold styles is not a common feature, however, and estimation of the competence contrast becomes more difficult. In an attempt to make such an estimate dip- isogons may be used, drawn on photographs of the fold profile. for several reasons the method is faced with difficulties: a) the original shape of the layer is unknown b) very few examples are suited to such analysis, many folds having suffered later deformation (fig:2.16x). In the fold of this figure, for example, different competence contrasts were obtained at almost every inflexion point while interference structures such as is shown in fig:2.16b obviously cannot be used. Boudinage allows for rather more satisfactory solutions as it is possible to suggest the shape of boudins that would be expected with a given degree of flattening and competence contrast,(fig:2.14). As was described above the stumpy lens-shaped boudins of the region suggest a moderately high competence of the basic rocks for any degree of flattening and their wide distribution suggests that the basic rocks were invariably more competent than their hosts. However, there are examples of structures in which this is demonstrably not the case. As described above (fig:2.12) certain thin basic layers develop a foliation oblique to its margins, a fabric which suggests that the basic rock- acting as the incompetent layer- suffered 57 (b)

Fig:2.15 F2 fold in amphibolite. Note the extreme attenuation of the gneisses at the fold crest (2 views).

58 ;7 __ "`", ðj,.+

♦ i_ J 1

• I J

(a)

(b)

Fig:2.16 (a) Irregular F2 fold, Clett Ard (b) Interference structure in amphibolite

59 shearing during fabric development. These and other such features are somewhat locallised in an area around Tarbert and may be related to the other unusual features of that area. A more common feature is that shown in fig:2.17 where a pegmatite vein cutting a basic layer is buckled while being deformed homogeneously in the gneisses (whose composition it must approximate to and with. which the competence contrast will be low). Such a feature suggests the behaviour òf the basic layer as the least competent of the three rocks involved, assuming that buckling can only occur in:.the more competent material. Such a view is disputed (eg. Watterson,1968) and it may well be that such an interpretation is incorrect. To sum up, therefore, the Scourie Dykes of the region seem to have behaved as the more competent material during deformation, with a few, notable, exceptions. This is in contrast to the interpretation of similar features in the Southern isles by Coward (1969) and Francis (1969) :

2.7 Late structures

A11 events post-dating the F3 episode of deformation and granite production are here classed, albeit arbit- rarily, as 'late{.

2.7.1 Fri structures

In the waning phases of the Laxfordian broad flexures were developed with some 10 or 15m amplitude, oriented approximately N-S and with shallow plunge to the south. They occur chiefly in the west and south of the region (areas which remained tectonically active in late Laxfordian times) but are never a common feature. They affect all pre-existing structures but without causing any important regional effects. Myers (1968) reports them as quite frequently developed structures in west North Harris.

60 (a)

(b)

Fig:2.17 Folded pegmatites contained within amphibolite layers.

61 2.7.2 Quartz veins

Throughout the region thin, lenticular quartz veins are seen infilling fractures and cutting across all structures and fabrics (fig:2.l8). These predominantly N-S oriented quartz veins can be quite large- up to lm in width- and may involve some displacement. They frequently exhibit strong fiaser fabrics; highly deformed examples exhibit a blueish colour and are very distinctive. There seems on this evidence to have been a very late cataclastic phase confined almost solely to the quartz veins and probably contemporaneous with their formation.

2.7.3 The Outer Hebrides Thrust Zone

Whilst east North Harris is considerably removed from any outcrop of the thrust its effects are more pervasive than one at first realises. The photographs of fig:2.19 show two examples of featuresgenerated by the thrust. The first shows N-S, E-W joints in a basic lens, associated with 'flinty- crush' gneisses. Both are cut by thin pseudotachylyte veins. The second shows a pegmatite crushed between lenses of amphibolite, movement of which was caused by the thrust, the whole set again in 'flinty-crush. Such features are sporadically developed throughout the region, crushed gneisses and thin pseudotachylyte veins being the commonest features. The latter are particularly common at the contacts of basic lenses and gneisses. Such features commonly occur in lenses or bands oriented N-S but lenses of pseudotachylyte and crushed rocks do occur within and parallel to the foliation, presumably as a line of weakness.

2.7.4 Tertiary Dykes

Thin Tertiary dykes- usually only lOcros or so thick- occasionally with jointing perpendicular to their margins are seen throughout the region. Larger dykes may be porphyritic with augite phenocrysts in an augite- labradorite matrix. One particular example above Tarbert is almost im wide and creates a zone of nornfelses and saussaritis- 62 Fig:2.18 Small quartz vein cutting and offsetting an amphibolite band in grey gneisses

63 (a) Block jointing in amphibolite. Also present were pseudotachylite veins.

(b) Crushed pegmatite (between two amphibolite lenses)

Fig:2.19 Late-stage features of tectonic activity, relating to the Outer Isles thrust.

64 ation in the gneisses it intrudes for up to a metre on either side. Tertiary dykes are not recorded on the maps in this work.

2.7.5 Jointing

Throughout the Outer Hebrides occur N-S, E-W joints. In mapping they come to be ignored unless of some size and involving displacements, and they are not shown on the maps. Aerial photographs show them most clearly and much of the present topography owes its form to the larger examples. An obvious major fault occurs at Tarbert where an imbricate and reddened smash zone some 100m wide is exposed in road cuttings. The fault produces the very marked topographic features of east and west Loch Tarbert and extends at least as far as Scalpay (R.Sibson,pers,comm.) where it cuts Tertiary dykes.

2.8 General Conclusions

The main features and conclusions of the preceeding chapter are here summarised, and will be referred to again in the final chapter.

a) Scourian events, although poorly preserved, involved the production of a high-grade gneiss complex with lithological banding, pegmatites, basic and ultrabasic intrusions or volcanics and a variety of structures. All such features have suffered extensive modification by Laxfordian events.

.b) The early complex was cut by basic and ultrabasic intrusions of the Scourie Dyke suite. Their original form and orientation is obscured by deformation but a general NE-SW trend is suggested on structural grounds. Such a trend is at variance with the Scourie Dykes of the

65 mainland and elsewhere in the Outer Hebrides but a similar trend was suggested in western Harris by Myers (1968) and in North Uist (Drs.Fettes and Smith). It may be that both trends were established in on ly one set of intrusions- as in west Greenland for example.

c) Laxfordian tectonic activity can be divided into four phases of which the first and fourth were of little importance. The apparently simple structure of the region is due to intense deformation during the F2 episode in which old fabrics were transposed and superimposed by new Laxfordian fabrics.

d) The major Laxfordian episode was the second phase (F2). Strong planar fabrics were produced throughout the region; the gneisses were flattened and the Scourie Dykes folded, disrupted and flattened. Little evidence of large-scale structures is seen in the gneisses but the distribution of the basic rocks suggest large scale folds. During the deformation of the dykes they acted as the more competent material. Deformation was more intense in tip Tarbert region, a feature possibly related to relatively high water contents of the gneisses in that area. Elsewhere local, low deformation zones are highlighted by the presence of large, folded basic layers many of which retain either igneous textures, or minerals, or both. e) The last major phase of deformation (F3) was accomp- anied by the production of granitic rocks in the west and south of the region. F3 structures and textures are directly correlated with these areas.

66 Chapter 3

The gneisses: petrology and geochemistry

tFor a stone of stumbling and for a rock of offence'

Isaiah $:14

Chapter 3

Contents Introduction 69 Section A: Field relations and petrology 3.1 Lithological banding 70 3,2 Gneiss types and their distribution 71 3.3 Granititc rocks 75 3,4 Mineral assemblages 76 3.5 Textures of the gneisses 83 3.5.1 Textural types 3.5.2 Distribution of the textural types Section B: Geochemistry of the gneisses 3.6 Sampling 87 3.7 Aims of the geochemical study 88 3.8 General chemistry of the gneisses 89 3.8.1 Comparisons with other crustal rocks 3.8.2 Average compositions of the North Harris gneiss types 3.8.3 K/Rb ratios 3.8.4 Variation diagrams 3.8.5 Geochemical evidence for the origin v. of the gneisses 3.9 Comparison with other Lewisian gneisses 109 3.9.1 The Uig Hills 3.9.2 Other Outer Hebrides gneisses 3.9.3 Mainland gneisses Section C: Regional trends in geochemistry 3.10 Trend surface analysis 111 3.10.1 Method 3.10.2 Statistical tests of trends 3.10.3 Problems in trend surface analysis 3.11 Geochemical distributions in North Harris 118 3.12 Discussion and conclusions 125 Section D: The granitic rocjks 3.13 General chemistry of the North Harris granites 127 3.14 Chemical evidence for the origin of the granitic rocks 129 Section E: General conclusions 137

68 Chapter 3

introduction

The 'Grey Gneisses' of North Harris are a rather homogeneous group of banded acid gneisses with subsidiary granitic rocks and rare basic and metasedimentary gneisses. The mineral assemblages are .typical of the amphibolite facies. They may be divided on a basis of the type of banding they exhibit, on their lithology or on their mineralogy, these latter two showing systematic variations throughout the region. The simplest division is on a mineralogical basis; the gneisses falling into either biotite- or hornblende-biotite types. Some chemical distinctions can be made between the two types which allow correlation of regional patterns in geochemistry defined by trend surface analysis with their distribution. Such patterns suggest the preservation of pre-existing lithological variations in the early complex. Granitic rocks,formed by 'in situ' metasomatism of the host gneisses in areas in which the gneisses show evidence of relatively high volatile contents, occur in a N-S belt in the west of the area. Late-stage Laxfordian deformation was concentrated in this region, a feature evidenced by the textural variations developed in the gneisses. Comparisons between the gneisses and granitic rocks of North Harris and elsewhere in the Outer Hebrides and the mainland show the former to be typical of grey gneisses developed throughout the Lewisian. Sampling of the gneisses was extensive; a total of 230 thin sections have been cut and studied and of these samples 105 were subjected to major element analysis and $0 of these used for trace element analysis, the data being presented in the appendices. The chapter is divided into four sections the first describing the petrology of the gneisses, the second giving details of the chemistry of the rocks as a whole

69 but also incorporating details of the variations in chemistry of the main groups. Section C describes the regional geochemical variations as defined by trend surface analysis (the method of which is outlined). The , granitic rocks are discussed in section D and a summary of the important conclusions comprise the final section, section E. a Section A: Field relations and petrology

The gneisses of east North Harris are typical of the banded grey gneisses seen throughout the Outer Hebrides. They are principally acid gneisses, little basic or ultrabasic gneiss being seen, and include local development of granitic rocks. They differ from the rocks of west North Harris in that the development of such granitic rocks is very much restricted.

3.1 Lithological banding

The most obvious feature of the gneisses is a compositional banding created by the incorporation in the acid gneisses of varying proportions of mafic and pegmatitic material in bands, lenses and pods. Myers (1968) attempted to classify such gneisses in west North Harris on the basis of the type and composition of the banding, his scheme being shown in fig:2.1 of the previous chapter. He then used the various sub-divisions as bases from which to suggest trends in the evolution of the gneiss types during Laxfordian deformation. In east North Harris such a scheme was found useful as a field ctarssification and showed the area to consist predominantly of amphibolite-banded and 'striped gneiss and biotite-banded end foliated gneiss whose distributions are discussed below (section 3.2). Many of the more regular amphibolite bands are believed to represent deformed rocks of the Scourie Dyke suite while the less regular bands and lenses couldfr qually well represent older basic material or even be products of segregation during metamorphism and it is often extremely difficult to differentiate between these possible sources. Where

70 The distributions of the acid gneisses and granitic rocks are shown in fig:3.1. In this map areas In which granite gneiss and pegmatite are abundantly produced are delineated but it must be noted that even in these areas the host gneiss Invariably remains predominant. As can be seen granitic rocks occur chiefly in the west and south with local pegmatite- rich areas to the east of the main belt. s On a mineralogical basis the acid gneisses may be divided into just two groups: a) biotite gneisses b) hornblende-biotite gneisses Other mineralogical variants can then be regarded as 'unusual'; in fact, pyroxene is seen in only two local ites and garnet in only one. Biotite is ubiquitous defining the foliation of these rocks and producing a compositional banding in its own right. Hornblende, on the other hand, although a common constituent of the gneisses as small aggregates of crystals or in hornblende-rich mafic bands, is an important constituent of the gneisses only in certain areas in which hornblende-biotite gneisses are generally more abundant and mafic banding more obvious. An attempt has been made to map out such variations with the results shown in fig:3.2 in which regions to the south and south-east, aroun Loch Maaruig and in the north were seen- to have a greater frequency of hornblende-biotite gneisses and mafic bands. The map itself was produced by plotting field observations and sample details which were then contoured by making estimates of the predominance of one lithology over another; when there was little to choose between them a 'neutral zone' (ie. of mixed lithology) was outlined. As discussed below this distribution of lithologies suggests the preservation of original compositional variations in the early complex and correlates with the results of the regional geochemical study described in section C.

P pP P\ P

P P Pp p P P IF P / Q PP Rp, P pQp Q_ R P PP- P P p PP

-P P- P' P P- P F t p

p p P Pi P P ?

P P P PPP P P P R PP PP ft. P pp P 4)s p p p Abundantt-- pegs, p 0 PP and S P p - P p small gronriites~ P PP S pPP P P PP P 5 5 P p IIPP C p pP Distribution or granitic rocks P Granites/granite gneisses O V - Pegmatite, 20% and over v 00 Boundary of max. granitic rocks wr er Boundary of Tarbert complex

i=ig:3.1 The distribution of granitic rocks and pegmatites Scale is approx. 1" to one mile.

73 Fig:3.2.T Distribution of gneiss types

regions of predominant hornblende-gneiss

(intermediate regions mixed more or less equally)

regions of predominant biotite-gneiss

74 3.3 Granitic rocks

Fig:3.l shows the present exposures of granitic rocks and pegmatites to be concentrated in a north- south belt in the western half of the area. Small granitic bodies are quite common in the north of this belt but the most intensive development of granitic rocks is seen at Tarbert. Here a core of granite and granite gneisses is flanked by pegmatite-rich acid gneisses (cf. Dearnley, 1962), in places the gneisses are left as 'rafts' in rocks whose pegmatite content is some 80 or 90%. Smaller granite bodies occur within the flanks of the complex. Interpretation of the outcrop- topography relationships shows the form of these granitic rocks to be that of an irregular dome covered by acid gneisses which, only a short vertical distance from the granitic rocks, show surprisingly little effects of their proximity to the complex apart from the presence of some coarse, irregular pegmatites. in the mountains of Clisham and Tomnaval, within the area of flat-lying gneisses described in Chapter 2, lie sheets of granitic rocks which are particularly well exposed in the corrie wall on the south side of Clisham. They form a broad outcrop on the west flank of the hills (figs3.l) and are composed of granite gneisses and coarse pegmatites. Outside the main belt of granitic rocks small areas in which pegmatites are abundant occur in three or four localities. From the general features of the distribution of granitic rocks it seems that an irregular 'surface of migmatisation' exists at shallow depths under the acid gneisses, local swells and domes in such a surface giving rise to the local pegmatite-rich areas and granite gneisses shown on the map. The form of this underlying complex is thus similar, though smaller in scale, to the dome of granites and migmatites described by Myers (1968) in west North Harris.

75 3.4 Mineral assemblages

In view of the relatively simple mineralogy and absence of obvious metamorphic reactions between phases it is intended simply to give a brief account of each mineral, together with the few analyses made on hornblendes. and feldspar, and to end with a discussion of the variations in modal composition.

3.4.1 Hornblende

Deep green, occasionally green-brown in colour, hornblende frequently occurs as large, well-formed crystals aligned in the foliation, or as clusters of small grains, or as scattered porphyroblasts. When concentrated it makes up the more mafic bands of the gneisses. When strained the mineral fractures and iron oxide is exsolved (fig:3.3). r~ Hornblende was analysed by electron-microprobe in three samples the analyses being given in table 3.1 and graphically i n the A 1 4 v. ( Na + K) plot of f i g : 3.4 . The relationship between hornblende and biotite is discussed in the following section.

3.4.2 Biotite

Biotite occurs in all gneisses in amounts varying between 10 and 25%. It has a .variety of forms, occuring as laths which define a strong foliation and are either scattered, concentrated in bands or occur as stringers around feldspar porphyroblasts. it is pleochroic from pale to dark yellow-brown. Inclusions are uncommon and consist of occasional iron oxides or quartz stringers along the cleavage.

The possible age relations between biotite and hornblende are obscure. Modal compositions suggest an inverse relationship between the two which would suggest a reaction between them. Evidence for such a reaction is, however, slight. The photomicrograph of fig:3.5 shows biotite enclosing flecks of hornblende

76 316 258 256 blot hblnd hblnd plag hblnd plag SiO2 37.12 42.50 42.57 62.04 42.44 62.13 TiO2 3.57 1.11 1.19 - .97 - A1 203 17.59 11.77 11.31 24.22 11.16 25.21 Fe0 19.70 20.48 19.19 - 19.86 - Mn0 .14 .29 .31 - .29 - Mg0 9.89 7.71 8.66 - 8.63 - Ca0 - 11.78 11.81 5.45 11.64 5.97 Na20 - 1.13 1.09 8.50 .99 8.32 K20 9.48 1.48 1.33 .20 1.37 .16 Cr205 - - .06 - .05 - Total 97.50 98.26 97.51 100.40 97.39 101.79 Si__" 6.01 6.39 6.78 10.96 6.78 10.84 Ti .44 .13 .14 - .12 - Al 3.36 2.21 2.12 5.04 2.10 5.19 Fe 2.67 2.72 2.56 - 2.66 - Mn .02 .01 .04 - .04 - Mg 2.38 1.83 2.05 - 2.06 - Ca - 2.01 2.01 1.03 1.99 1.12 Na - .35 .34 2.91 .31 2.81 K 1.96 .30 .27 .04 .28 .04 Cr - - .01 - .07 - 16.85 15.97 16.32 19.99 16.34 19.99 Ab% 73.07 70.95 An% 25.83 25.06 Or% 1.10 3.99

Table 3.1 Composition of hornblendes, plagioclase and biotite from three gneiss samples. Oxides given in weight percentages Formulae (program FERRIC): 24 oxygens- hornblende/biotite 32 oxygens- plagioclase

77 Tschermakite Pergasite 2.0

/ / / / / .. ~ -__,. / ,...-- . --•.., LO / -' I C Eden ite A1 4 / i / i atoms / / -'

0 Tremolite 0.5 1.0 (Na + K)atoms

Fig:3.4. Al v. (Na+K) plot of hornblende compositions.

78 (a) Biotite (secondary ?) enclosing hornblende and magnetite fragments.

hbind

biot

t hbind

blot

(b) Seeming reaction between biotite and hornblende, both involving the exsolution of magnetite fig.3.5 Possible biotite-hornblende interactions

while fig:3.Sb and c suggest that biotite and opaque minerals were produced from the hornblende which encloses them. Thus even the few cases of reaction between the two phases that are seen provide conflicting evidence. In the majority of samples the two minerals coexist with no evidence of reaction and, as is discussed below., it seems that chemistry is the chief control over the formation of hornblende gneisses. a

3.4.3 Plagioclase

Typically oligoclase and frequently anti-perthitic plagioclase forms a constant proportion of the rocks varying only between 42 and 45%. Gneisses whose plagioclase is andesine occur in three areas: Ardvourlie in the north-east, Glen Trollamaaruig in the south-east and Bunaveneader in the south-west. All three areas are unusual in that they contain large and continuous

ark Scourie Dyke rocks and as such were classed as relatively low deformation areas in Chapter 2. Table 3.1 includes plagioclase analyses made in two samples. They have an average composition of Or:Ab:An = 1:72:27 with an Ab/An ratio of 2.8, values close to those obtained by optical methods.

3.4.4 Orthoclase and microcline

Potash feldspar is always subordinate to the plagioclase being generally only 5-10/ofthe modal composition, and even in the granites only achieving a maximum of 20% of the total assemblage. Perthites are common in a variety of forms, often quite coarsely developed. Alteration and sericitisation are not frequent, except in gneisses in the extreme south-east of the area where low-grade retrogression is evidenced.

Microcline is seen in less than 10% of the samples and generally occurs in small grains as an accesory phase. Occasional large porphyroblasts are seen. Where microcline does occur the rock invariably exhibits a coarse'gneissose texture and the presence of the mineral seems to bear no relationship to the chemistry of the

80 rock in which it occurs. Its production seems to have been favoured in gneisses that did not suffer late Laxfordian cataclasis in regions of granite production.

3.4.5 Quartz

A ubiquitous phase, quartz tends to be rather over-

I looked but it forms 20-25% of the rock and exhibits a wide range of textural features varying from coarse grains with undulose extinction to much deformed grains or clusters of sub-grains, as described in section 3.5.

Accesory_phases

3.4.6 Pyroxene-

Diopside, rarely seen, occurs in gneisses in the south-east and east near Rhenigadale and on t'he west side of Loch Seaforth. In these rocks small rounded grains of diopside are scattered through the gneiss and show partial retrogression to amphibole. The thin. veins of pyroxene gneiss reported by Jehu and Craig (l931-) at Ardvourl ie were not found.

3.4.7 Epidote

A common accesory throughout the region, it becomes important only in the extreme south-east, in an area of low-grade retrogression. Cross-cutting veins of epidote occur in both gneisses and metabasites of this area. (cf. Myers, 1968) .

3.4.8 Allanite

Infrequent, as small red-brown grains with typical radiating cracks in.the surrounding feldspar. In the pegmatites it becomes a more common mineral.

3.4.9 Apatite

A common accesory, occasionally occuring in some abundance. Generally it forms small, rounded grains. 81 a

a Fig:3.6 Garnet in garnet-gneiss, Ceinn an Ora Quarry, Bunaveneader.

The garnet is chloritised along the fractures

82 3.4.10 Iron ore

Although quite common as an exsolved phase in deformed ferromagnesian minerals, particularly hornblende, iron oxides are generally scarce as a seperate mineral species even as an accesory.

3.4.11 Garnet

Reddish almandine garnet is seen in only one

locality, a garnet gneiss (metasedimentary ?) in Ceinn an Ora quarry at Bunaveneader. The rock is a strongly banded hornblende-biotite gneiss in which the minerals are seen to be rather altered the garnets chloritised; and the feldspar sericitised (fig:3.6).

3.5 Textures of the gneisses

In thin section studies it becomes obvious that the minerals making up the gneisses vary in their reaction to deformation in a systematic way quartz being by far the most susceptible of all phases to deformation. Further, it was clear that using the features of the deformed quartzes one could sub-divide the textural features of the rocks into a relatively few groups which show systematic variations in distribution through the region. These divisions and their significance are now discussed below.

3.5.1 Textural types

The rocks may be subdivided on the following criteria: a) gneissose textures (fig:3.7a)- coarse grained equi- g ranular rocks in which the quartz shows undulose extinction while the other phases are undeformed. Microcline occurs only in rocks of this group, although it is not always present. In some examples the biotite forms stringers around the feldspar porphyroblasts, b) quartz develops one or other of two forms. In one (fig:3.7b) it forms long sinuous grains while in the

83 (a)Undulose extinction in large grains 7

(b) Elongate, sigmoidal quartzes with marginal/central sub-grains. Note position of the extinction bands

Fig:3.7 Textural features in quartz grains (see text).

84 other (fig:3.7c) the production of small sub-grains from the original crystal is seen, usually these two types are developed together, although strictly speaking they are produced by different processes in different conditions. As these textures develop and become more intenses the other phases become involved, with fracturing of hornblendes (and exsolution of iron ores), kinking and comminution of biotite and fracturing and recrystallisation of the feldspar. c) in the most extreme form of deformation- that of the shear zone- all minerals become deformed and recrystallised and the quartz grains form long ribbons, far too large to have been produced by the deformation

4 of single crystals which suggests either coalescence of grains or grain growth during deformation. This texture is, by its very nature, locallised. Mylonitic gneiss generated by the Outer Hebrides Thrust show typical mylonite textures with pseudotachylyte veining. They represent cataclasis at a very late stage in the history of the region.

3.5.2 Distribution of textural types

The map of fig:3.8 shows the distribution of gneisses exhibiting gneissose (unshaded) and other textures and from it one can clearly see that the gneisses in the east of the region have predominantly gneissose texture while those in the west, in th a areas of granite and pegmatite production, in fact, exhibit the more deformed and varied textures of (b) above (shaded area). This marked correlation shows the extent to which the effects of late Laxfordian (F3) deformation was concentrated in the granitic areas while the gneisses outside these areas became at least partially recrystallised to give the coarser gneissose textures.

85 M

Fig:3.8 Distribution of gneissose and other textural types. Gneissose textures are developed in the unshaded region of the map,

86 Section B: Geochemistry of the gneisses

X-ray fluorescence analysis (see appendix A) was used to analyse for the major elements and a variety of trace elements (Rb, Sr, La, Zr, Ni, Ba and P) in 88 samples. The elements Th and Y were analysed for 27 of these samples, and major elements alone in a further 17 samples. The localities of these samples and full •analytical results are given in the appendices. It is worth stating clearly at this stage that the final quality of the analyses left a great deal to be desired in spite of repeated analyses and duplication of samples. It has since been discovered that this was due to problems of water absorbtion by the pressed glass disc a problem inherent in and uncorrected for in the method of preparation followed at Imperial College. For this reason not all data was used, the analyses being presented in the appendix being the final, accepted information. Further details of this problem are given in appendix A.

3.6 Sampling

Although the range of rock types and mineralogies is small their distribution is markedly heterogeneous on outcrop scale and sampling was a difficult task. Systematic sampling based on the grid system of the 1:10000 base maps was found to be impracticable, the regularity of the sampling being offset by natural obstacles (crags, lochs and so forth). A more random method was therefore necessarily employed in which samples were collected according to the degree of exposure and accesibilty from area to area. At each locality an attempt was made to collect a representative sample, a decision inevitably leading to bias as such a sample would invariably be an acid gneiss, the final choice then being between biotite or hornblende- biotite gneiss, or some similar decision. Such samples invariably contained clots and streaks of mafic material but further mafic gneisses were collected in order to asses their chemical features. 87 Sample size varied, being the result of a clash of interests between samples large enough to be representative while being small enough to carry in some numbers. Careful/ choice of material provided something of an answer. A considerable number of samples were collected and many of these were analysed. The sheer number of samples collection of B involved results in a more representative material and, hopefully, thereby, more valid conclusions.

3.7 Aims of the geochemical study

A detailed geochemical stūdy of the grey gneisses was felt justified for several reasons; a) little geochemical data existed for the grey gneisses of the Outer Hebrides, although what little there is (eg. Skinner,1970; Sheraton, 1970 and Sheraton eLal, 1973) is frequently alluded to in the literature for the sake of comparisons. it was felt that a more systematic study would provide a body of useful data which would do more than supplement that already in existence. b) In the context of a regional study an attempt could be made to correlate gross iithological and petrological with geochemical trends. c) The possibilty of chemical variations between the gneisses of 'low' deformation zones and elsewhere could be investigated, and indeed some correlations could be made.

It has also been possible to make some comparison with geochemical data on the gneisses and granites of the Uig Hills thanks to analyses provided by Drs. D,Fettes and _ D.Smith of I.G.S. (Edinburgh) which are grtefully acknowledged. The actual data are not presented here but are used in many of the diagrams which follow.

88 3.8 General chemistry of the gneisses

In much of the discussion which follows the gneisses are regarded 'en masse' regardless of their mineralogical and field variations outlined in the previous sections. Variations in chemistry between the groups is introduced later in this section and discussed in greater detail in seperate sections below. r

3.8.1 Comparisons with other crustal rocks

The average compositions of several crustal rock types as estimated by Taylor (1964,1966) are given in table 3.2. From this data comparisons can be made betweeen the compositions of the North Harris gneisses and granites (tables 3.3 and 3.4) and 'typical' crustal material, granodiorites and granites. As can be seen the gneisses compare quite closely with a typical granodiorite in i2 terms of both their major and trace element chemistries, a classification which befits the position of the gneisses on the AFM diagram of fig:3.14, while the granites are almost 'typical' in their composition the chief discrep- ancies being those of Ba and Sr.

3.8.2 Average composition of the North Harris gneisses

The average compositions of the various gneiss types are given in tables 3.3 and 3.4. Differences between the groups are slight, the most striking being shown by the granitic and sheared gneisses and by the more basic gneisses._ The former show relatively high Si, K, Rb and Ba and low total Fe, Mg, Ca, P and Sr. Such variations are mirrored by the sheared gneisses where, however, Ba is lower and Sr and Na higher. Such variations are what one might expect in rocks whose chemistry has been affected by influx of the more mobile components of metamorphism and are discussed in more detail in section D. The more basic gneisses show features that are the inverse of those characterising the granitic and sheared

89 andesite crust grano- granite basalt diorite Si02 60.1 60.3 66.9 71.2 48.9

1102 .7 1.0 .6 .5 1.8 A1 203 17.2 15.6 15.7 14.7 15.7 Fe0 6.1 7.2 3.8 3.2 10.7 MgO 3.5 3.9 1.6 .6 8.7 CaO 7.1 5.8 3.6 2.0 10.8 Na20 3.3 3.2 3.8 3.5 2.3 1(20 1.3 2.5 3.1 4.2 .7

Ba 430 500 600 250 Sr 300 440 260 375

Zr " J 150 140 120 110 Cr 105 - 10 200 Rb 85 110 150 25 Ni 80 15 4 160 Y 33 35 40 25 Th 10 14 18 2 La 40 P 920

Table 3.2 Average compositions of common crustal rocks (from Taylor, 1966)

90 a, b. t-test F-test 5102 69.71 (4.8) 68,10 (2.6) * * Ti02 .38 (.16) .38 (.12)

A1203 15.23 (.83) 15.40 (•51) Fe203 1,95 (•65) 1.09 (.92) FeO 1.02 (.61) 2.20 (1.1) * )Ino .03 (.02) .05 (.02) *

MgO •78 (.33) 1.20 (.51) * t CaO 3.05 (.76) 3.95 (.64) * Na20 4.20 (..50) , 4.10 (..52) K20 2G40 (1.3) 1.80 (.49) * Total: 99.31 99.10 Sr 399 (99) 359 (ioo) Bb 85 ( 22) 7Ō ( 22) * La 34 (17) 26 (12) Zr 173 (45) 154 (25) * Ni 18 ( 9) 22 (12) Ba 681 (328) 624 (328) P 1342 (436) 1357 (486) Tb.. 6 ( 2) 6 (3) Y 6 (2) 8 (4) n•29 n-18

Table 3.3 Comparison between biotite gneiss (a.) and hornblende-biotite gneiss (b.) compositions. Major elements as percent oxides and trace elements in parts per million. The numbers in parentheses are standard deviations. In asterisk in either the t-test or F-test columns indicates a differenoe between the elements concerned to 99.95% significance.

90a a. b. c. d. 8102 71.44 (2.2) 70.44 (1.8) 69.31 (2.3) 64.63 (.91) TiO2 .29 (.11) .27 (.11) .34 (.09) .53 (.23) A1203 14.79 (.82) 15.59 (.60) 15.30 (.62) 15.43 (.74) Fe203 .96 (.63) 1.43 (.56) 1.24 (.63) 2.70 (1.5) Fe0 1.29'(.70) .86 (.34) 1.57 (.73) 3.36 (.61) MnO .02 (.01) .03 (.01) .04 (.02) .07 (.02) Mg0 i56 (.34) .50 (.36) .94 (.49) 1.77 (.39) Ca0 2.16 (.94) 2.83 (.70) 3.42 (.82) 4.62 (.57) Na20 3.44 (.27) 4.68 (.71) 4.20 (.46) 4.05 (.55) K20 3.99 (1.7) 2.81 (1.2) 2.12 (1.1) •1.58 (.21)

Total: 99.72 99.84 99.31 98.69

Sr 290 (90) 478 (229) 369 (79) 339 (112) Rb 150 (39) 87 (29) 79 (27) 62 (20) La 27 (6) 20 (14) 30 (13) 32 (15) Zr 160 (54) 139 (27) 161 (37) 135 (23) Ni 21 (14) 9 (4) 19 (9) 28 (14) Ba 854 (305) 576 (116) 632 (255) 506 (140) P 976 (407) 1054 (763) 1378 (405) 1346 (395) Th 8 (2) 6 (2) 6 (5) F 6 (1) 7 (2) 7 (1)

n=11 a= 7 n=18 a= 7

?able 3.4 Composition of various subsidiary gneiss types: a. granite gneisses b. sheared gneisses c. microcline-bearing gneisses d. 'low silica gneisses• (ie. with less than 65% Si02)

The numbers in parentheses are standard deviations.

90b rocks. Si, K, 8a and Rb are relatively low and total Fe, Mg, Ca and Ni higher, both with respect to the bulk of the gneisses. In keeping with their average composition these rocks are the only gneisses to show appreciable normative diopside, a feature typical of the more basic gneisses throughout the Lewisian.

3.8.3 K/Rb ratios a

It is now well documented that both amphibolite facies terrains and the 'average continental crust' are relatively enriched in Rb and K when compared to granulite terrains, and show lower K/Rb ratios (Heier and Thoreson, 1971 ; _Drury, 1974; S igh i nol f i , 1969, 1971) . Fig:3.9 shows the relationship of both K20 and Rb to silica (Si02) in the North Harris gneisses, The granitic rocks are obviously much enriched in both K and Rb relative to most of the gneisses but they do not differ markedly from the gneisses in K/Rb ratio (gneisses 272, granites 309) as is discussed below.(section 0). A major work on K/Rb ratios was presented by Shaw (1968) in which he established three trends: a) a 'Main Trend' of K/Rb = 200-300 along which the majority of rocks fall (MT in fig:3.10) b) a high K/Rb trend of oceanic tholei ites (0T) c) a low K/Rb trend of pegmatitic and hydrothermal rocks (PH). Following Shaw's methods and plotting log Rb against log K% for North Harris gneisses and granitic rocks (fig:3.11) shows that they cluster on the main trend with an average K/Rb ratio of some 250. In the compilation diagram of fig:3.12 the Uig rocks are compared to the North Harris gneisses and granites. As can be seen the Uig gneisses plot on the main trend but the granitic rocks extend off along the low ratio pegmatite-hydrothermal field. In the aplogranites (see section 0) the average K/Rb ratio of 75 which is very low indeed. Also on this diagram is shown the granulite field of high K/Rb ratios, comparison with which serves to emphasise that the K/Rb ratios of the North Harris rocks are typical of those of the amphibolite facies.

91 200 t

• gneisses + granitic rocks

ppm Rb

100-

060 ?0 80 % Si02

60 ?0 80 % Si02

Fig:3.9 Relationship of 1C20 and Rb to S102 in North Harris gneisses (The symbols are used in all such diagrams)

92 10

.01

.001 2 t 1.0 10 100 1000 ppm Rb Fig :3, 10 Scheme of K/Rb ratios according to Shaw (1968). (See text)

Fig:3.11 K/Rb plot for North Harris gneisses and granites (point$ and crosses). The dashed line encloses the ratios of basic and ultrabasic rocks from the region

93 i 1

10 100 1000 ppm Rb Fig:3.12 Compilation diagram showing the X/Rb ratios of gneisses (c) and granites (Gr) for north Harris- thick lines- and the Dig Hills- thin lines. 'i' indicates the field of porphyritia and aplogranitos from the Uig Hills. Typical Lewisian granulites are shown by the dashed line marked •Granulite' (from Lambert, it a1 1976).

94 Various authors have suggested mineralogical control of the K/Rb ratio in metamorphic rocks (eg Collerson, 1975; Shaw, 1968) indicating possible influences of biotite, hornblende and plagioclase. In fig:3.18 several ratios- including K/Rb- are plotted against modal hornblende, biotite and feldspar in the gneisses and the correlation of K/Rb with biotite and hornblende is a striking feature, the ratio exhibiting no such s .relationship with feldspar.

3.8.4 Variation diagrams

Figures:3.13, 3.14, and 3.15 show the variations in chemistry of the North Harris gneisses on AFM, CKN and normative feldspar digrams respectively. Data for the Uig rocks (represented by the solid line) are presented for comparison. The gneisses are shown as dots and the granites by crosses in these and further diagrams. The AFM diagram shows all gneisses to have typically calc-alkaline affinities, the rocks falling along the granite-granodiorite section of such a trend. The CKN plot, as expected, shows a strong division between the gneisses and the granitic rocks due to the overall increase of K20 in the latter. The range of the distribution of granitic rocks in the diagram is created by the irregular correlation of K20 and Na2- 0, while K20 ne g;ltive, and Ca0 show strong positi=ve correlation (see fig:3.25). This division between the gneisses and granites Is mirrored in the normative feldspar diagram, and is again due to the higher K20 of the granitic rocks. From this diagram the average normative Ab:An ratio of the gneisses is approximately 69:31 (Ab/An= 2.2) while the actual composition is Ab:An= 73:27 (Ab/An= 2.7). This discrep- ancy is due to the fact that all available K20 is calculated as orthoclase in the CIPW norm whereas, in reality, some is incorporated in biotite thereby affecting the Ab/An ratio.

95 R

. • .

• + . • • •

. • . .• • •

. • ;+..

• Fig:3.13 AFM diagram North Harris gneisses and granites gneisses granites + Data from the Uig Hills (IGS data, unpublished) is shown in outline.

96 t

Fig:3.14 CKN diagram North Harris gneisses and granites Symbols as in fig~3.13

97 Fig:3.15 Normative feldspar diagram North Harris gneisses and granites Symbols as in fig.3.13

98 3.8.5 Geochemical evidence for the origin of the gneisses

It was stated above that the average composition of the gneisses of North Harris was close to that of granod-, iorite and that when plotted in the AFM diagram they follow a 'typical' talc-alkaline trend. The problem exists, however, of whether this necessarily implies derivation by metamorphism of an equivalent suite of • . purely igneous rocks or whether a mixed assemblage of igneous and sedimentary rocks was involved. Lack of much obviously metasedimentary material in the region renders impossible resolution of the problem by field observation alone. Bowes (1971) studied the granulite facies gneisses of the •Kylescu Group near Scourie on the Scottish Mainland. Analysing 55 samples representative of the gneisses he compared them to Archaean volcanic rocks described by Baragar and Goodwin (1969) from the Superior and Slave provinces of the Canadian Shield. To make the comparison he plotted both major and trace elements against a differentiation index first suggested by Larsen (1938). The original index was calculated as 1/3Si02 + K20 -(Ca0 + Mg0 + Fe0) but for the work of Baragar and Bowes (and the present author) iron is not included in the calculation. Although Bowes' diagrams are not given here his conclusion was that "The chemical composition of the pyroxene and hornblende granulites of the Kylescu Group indicates that these rocks form an igneous suite or part of an igneous suite", and again ".. the granulites are interpreted as representing a metamorphosed volcanic assemblage." He goes on to suggest basalts, andesites dacites and rhyolites, or their pyroclastic equivalents, as the original members of the suite. The data could equally well refer to the plutonic equivalents of such rocks such as was suggested by Holland and Lambert (1975). The North Harris gneisses would only correspond to the more acid portion of such a sequence and, in the absence of recognisable metasedimentary or metavolcanic rocks would seem likely to be metamorphosed plutonic rocks. 99 Fig:3.16 shows the major elements (as weight percent oxides) and selected trace elements of North Harris gneisses and granites plotted against the modified Larsen index while fig:3.17 does the same for the Uig rocks (major elements only). A:decrease in mafic element concentrations and increase in Si02, K20, and Rb with respect to increasing Larsen index is evident in both sets of data. Again, the data from the two sets of rocks are closely comparable although the North Harris gneisses incorporate slightly more basic gneisses and hence have some data points with lower indices. That the trends reflect igneous features is suggested by comparisons with similar plots of actual igneous series (eg Larsen, 1931) and it would seem that the diagrams can best be interpreted on that basis. Two points must,°- however, be borne in mind: - a) the possibility that some sedimentary or volcanic rocks were incorporated in the North Harris gneisses remains an open question b) as will be described in _Chapter 4 where similar plots are employed (ie: elements versus some form of di fferen- tiaion index) metamorphism can sometimes be shown to obliterate original igneous trends rather than preserve or reinforce them. Caution is therefore called for in the interpretation of such diagrams for metamorphic assemblages.

3.8.6 Chemistry of the biotite and hornblende-biotite gne i sses

Having described the main variations of gneiss types and outlined.the general variations in their chemistry it is now opportune to consider the geochemical distinctions between the two chief groups of gneisses in North Harris, the biotite gneisses and the hornblende-biotite gneisses. The average compositions of the two groups are given in Table 3.3, together with the results of two simple statistical tests used in an attempt to determine whether any significant differences exist between them. An asterisk in either the student's-t or F-test column signifies a difference between the groups for that particular element to a minimum 99.5% confidence level. 10 •

• ' 74 • • • + + 4 + • . 4. 72 •

• • + • :•. • • 70 • • • •• •. • • • • : ,F • • • + 68 . • • • • • . - • • 66

• • • • • .64 • • • Si02% •

• 6 • • Ti02 9~

• • • • • •• • .4 . -. •• • • .• •. ••• • 4 • • •• • •;• • • • • ~ • • • • . ++ • + .2 • + + 4

17 • A1 203 q •• • •• 16 a • • •.•.-F . ••• 1. + • •.•• • • • .••. . + • 15 • • •• • • + + 14

• • • • . Fe203 • . 2 • • •• + .• ; • •• •• + • • • • • • • • • • . • • 4 + • 14 16 18 20 22 24 26

1 /3Si + K -(Ca + Mg)

Fig:3.16 Variation diagrams for North Harris gneisses and granites; oxides plotted against a modified Larsen Index.

4 - Fe0 % 3 • • •• • • • + - y y y 2 , •• .•• • • • , • + T + ♦. • ••♦•• _ • y

2.5 •• • 2.0 % Mg 0 q • • 1.0 •••••:• •+ • • • + • •• • +•/F 4 + t + +; + - • • • 5 • ,. • 4 - C a 0 % ....•. .•

• ..•+4$. • . • - • . 3 ♦• •, • •• •.+ y y •• + + + 4- 2 L yy y • T + + + +

5.0 • ' • Na20 % r- • • , •••• •;, • . • + ♦ + • + 4.0 • + • + + + + • + • • • + + 3.0 6 + t - • # 5 _

• ++ • + 3 •• ••..• + •.+ .•• . . 2 _ .. • •: .• • . • .• • • • • •.•, •; • • • , • •. .• •••♦+ ••• 1 1( 20 % - I . 1 t ) l 14 16 18 20 22 24 26

Fig:3.16 continued 102 200 :- + + 1 60 + • 120 + + .•• • .. 4 . 80 • .- • . • . • . • •••. . •

•• . . . 40 . Rb .

• . 40 Ni .. • . 30

20 • • i 10

b~ 1400

. _ • 8a 1200

+ +

• . . 1000 + I.

800

.=r •

. . 600 + . . 400

. 14 16 18 20 22 24 26

Fig:3.16 continued Trace elements (in ppm) 103 0O M

• + ' •} • • • o •'•• • O A O w O'r 0. •. 4 • O • Y , ao ;• • ū 4 i N • •ti • • f

'0N

i • • • • N • • • • • • 4 °• • ••• • ° • • • • • • • • • N • • • • • , N

• • • • • ° • °° • •• A • • • • • • • • • • •• •• •• ON • • • • • •

• • • • •

M N O tn O O U' M ? [V — to .7 M N — 0 •D u1 N O N N

• gne i sses- •granites

O M

.. .. . '• • • O O •• °• •• O_cv S N M • — •- ♦ • O

•.'~ • • s • • t • •

I. • • •

• • • • • • • •• •• ° • • i• ° ° • N ~ • • • • - • • • N

• • • • • • • •• • • .° • • • • • • • • O N • • • • • • • • •

• • • • '

• • • •

'.0 • •• . • • • I • , , • . • • , , !• ° . , , • M . a •O .o•O 'O ~ '.0 N N .o

Fig:3.i7 Variation diagrams for gneisses and granites from the Uig Hills (data provided by I.G.S., unpublished) Oxides plotted against a modified Larsen Index.ndex.-

104

Student's-t was obtained by the standard formula:

t- X1 - X2 . sp 1 + 1 n1 n2

2 with sp== (n1 -1).S1 + (n2-1).S22

I nl + n2 - 2

S = standard deviation X = mean n number of samples subscripts 1 and 2 indicate sample populations

The value of t so obtained is checked in tables for the desired confidence level. The F-test is applied to the sample variance (standard deviation, squared) and is defined as: »g

F = (S1 ) where S1 is the larger variance (S2)

Again, the value of F is compared to values in tables, obtained at specific confidence levels; if the value is not significant there is no evidence to suggest that the samples are drawn from two populations with differing variances.

Student's-t suggests differences in the two groups for the elements: Si02, MnO, MgO, CaO, K20 and Rb while the F-test established significant variation in: Si02, FeO, K20 and Zr

Using these results Section C, below, attempts to correlate regional variations in geochemistry to the gross distribution of lithologies described earlier. Such contrasts between biotite and hornblende- biotite are to be expected on the basis of data from geochemical work on other Lewisian gneisses. For example Sheraton (1970, 1973) presents analyses of hornblende and biotite gneisses suggesting differences in Fe, Mg, Ca and Cr (greater in the hornblende gneisses) and Si, K, Rb, Zr and Ba (all reduced).

The relationships between mineralogy and chemistry were studied within each group using a correlation program (GCOR, Imperial College Computer Library). Data for the two groups were subjected to interelement correlation and the results are given in Table 3.5. • The intention in this study was to compare the correlations in each group and relate the differences in these correlations to variations in mineralogy of the two gneiss types. Thus, for example, the presence of hornblende would seem to be associated with the correlation between Ti and Fe, Mn and Ca, features absent in the biotite gneisses. The results, however, are rather ambiguous due to the wide range of substitutions possible in the ferromagnesian minerals (see, for example, Moxham 1965).

A further correlation between mineralogy and chemistry, and this time a more profitable one, was shown between modal mineralogy and various element ratios (fig:3.18). In this diagram the modal percentages of hornblende, biotite and feldspar are plotted against K/Rb, Rb/Sr Ba/Rb and K/Ca. As may be seen the ferromagnesian minerals exercise control over all the ratios while they appear to be fairly constant in the feldspar. K/Ca shows only a strong negative correlation with hornblende. The existence of these relationships leads to the inclusion of these ratios in the regional study of Section C.

Finally, an attempt was made to correlate the modal proportions of the various minerals directly to composition but without success, a result possibly due to the relatively small number of modal analyses available (28).

106 Hornblende-gneisses Biotite-gneisses ( n a 19) ( n 29) Si (-) Ti, Fe, Mn, Mg, Ca Si (-) Ti, Fe, Mg, Ca, Na Ti (+) Fe, Mn, Mg, Ca Ti (+) Mg Fe3+ (+) Mg Al (+) Ca, Na Fe2+ (+) Mn Fe2+ (+) Mg Mn (+) Mg, Ca Mn (+) Ca Mg (+) Ca Mg (+) Ca, (-) K Ca (+) Na, (-) K Na (-) K Ni (+) Fe Sr (+) Ca, Na, (-) K Ba (-) Ca Rb (+) K Ba (+) K Zr (+) Ti ~F P (+) Fe, Mn, Mg, Ca

Table 3.5 Interelement correlations in hornblende and biotite gneisses. Only a summary of the correlation matrix is presented here, showing the nature of the correlation (+ or -) for values of correlation at 99.99% significance.

107 20 2k/Rb Lorob1.ad•

k/Rb r♦ Rb/Sr • Rb/Sr borab1 n d. 10. •.. •.' • • . • i i • .

10 10

600

X/IIb bistits 20 400 Ds/Rb biotit.

Rb/Sr btoUt• es/rb - • . x/Rb .4 Rb/Sr O 10 • • 2o0 • . • • • - S .R • • • • .

10 20 30 1d 20 10 20

600

[/IIb fsldspsr 1.0

400 [/Cs borabl.ad.

E/Rb •• .4 - Rb/Sr feldspar • • • x/C+ •• 200 Rb/Sr • • • . .2- •

~ SO 60 40 So Fig:3.18 Relationships of various element ratios to modal hornblende, biotite and feldspar in gneisses 3.9 Comparisons with other Lewisian gneisses

Having established the overall geochemical features of the North Harris gneisses comparison can now be made with gneisses of other areas, both in the Outer Hebrides and on the mainland. Data used for comparative purposes are given in Tables 3.6 and 3.7. a

3.9.1 The Uig Hills (IGS data)

Using student's-t and the F-test described earlier- the results of which are shown in Table 3.6- it may be seen that differences exist in the Si02, MgO and CaO the North Harris gneisses being, apparently, somewhat more basic in overall composition. (The differences in CaO appear to be shown systematically in all analytical data of the two sources- ie, gneisses, granites and basic rocks- and it is therefore believed that these differences are due to analytical procedure). To all intents and purposes the gneisses correlate closely in their major elements, a feature which comes as no surprise when one considers the overlap of the two in the variation diagrams. The trace element concentrations, however, differ quite markedly between the two regions most being present in much higher concentrations in the Uig rocks (excluding Ni and P), see Table 3.6. Such constant variation would suggest that the environment in which the Uig gneisses were metamorphosed differed considerably from that in North Harris, a difference which seems due to the position of the gneisses in a region of large scale granite production.

3.9.2 Other Outer Hebrides gneisses

Skinner (1970) working in the Outer Hebrides analysed only four samples from east North Harris and a further • 22 from Lewis and Harris. His results are given in Table 3.7 (averages from his PhD thesis). In both major and trace element concentrations the analyses of grey gneisses closely with those of east North Harris but with slightly lower total iron .and higher Sr and Ba. a. b. t-test F-test S102 69.08 (2.51) 68.12 (2.6) • TiO2 .38 (.14) .34 (.11) £1203 15.31 (.73) 15.23 (.71) Fe203 :1.03' (.77) .82 (.42) • • Fe0 1.81 (.83) 2.21 (.75) Ma0 .04 (.02) .05 (.02) MgO .93 (.46) 1.40 (.66) * • CO 3.40 (.84) 3.31 (.95) • Na20 4.16 (.50) 4.33 (.47) K20 2.15 (1%1) 2.38 (1.1) Total: 99.32 98.30

Sr 383 (101) 634 (220) • • Rb 79 (25) 113 (52) • • La 31 (16) 73 (12) * • 4 163 (39) - Ni 20 (11) 27 (18) • Ra 658 (287) 930 (513) P 1357 (436) 1114 (326) • • Tb 7 (3) T 6 (4) 9 (5) Cr - 64 (33) n=64 n=21

Table 3.6 Comparison between the gneisses of North Farris '(a.) and the Uig Hills (b.). Numbers in parentheses are standard deviations. An asterisk in either the t-test or F-test columns indicates a significant difference for that element (99.5%)

109a a. b. c, d. e.

Si02 68.27 67.58 68.23 68.22 64.9 TiO2 ..21 • .33 .33 .34 .43 A1203 14.96 15.19 14.98 16.04 16.0 Fe203 1.48 1.76 1.61 1.94 2.24 FeO 1.16 1.28 1.25 1.47 2.05 Mn0 .04 .05 '.04 .04 .05 MgO 1.00 1.28 1.19 1.44 1.8 Ca0 3.58 3.22 2.57 3018 3.39 Na20 3.96 4.15 3.80 4.90 4.2 K20 2.68 2.58 3.73 2.09 2.28 Total: 97.41 97.63 97.83 99.66 97.34 Sr 444 487 419 580 382 ( Rb 61 I 75 141 74 85 La 32 26 Zr 135 185 221 193 169 Ni 10 19 13 20 35 Ba 802 829 870 713 626 P 746 1134 1364 1473 1039 Th 9 7 6 11 10 Y 8 5 6 7 7 n= 3 n=22 n=18 n=39 n=26

Table 3.7 Average composition of amphibolite facies gneissest a. North Harris b.Lewis and Barris c.South Harris migmatite complex (a,b, and c from Skinner,1970) d. Gneisses from the Laxfordian complex, Rhiconich (Sheraton,1973) e.Amphibolite facies gneiss, Coil and Tiree (Drury,1974)

110 Comparison with the gneisses from the migmatite complex of South Harris analysed by Skinner shows that the biotite gneisses of North Harris somewhat resemble them in composition while the hornblende-biotite gneisses have higher total Fe, Mg and Ca. The trace elements correlate quite closely apart from Rb, Ba and Th which are all higher in the migmatites.

•3.9.3 Mainland gneisses

An overall comparison shows the North Harris gneisses to have a chemistry which is typical of amphibolite facies Lewisian gneisses. Fig:3.19 shows several diagrams in which the granulite facies gneisses of Coll and Tiree (Drury, 1974) are compared to the gneisses and granites of North Harris.• The latter occupy the field. in which amphibolitised granulites also plot and as such it is impossible to determine whether or not the North Harris rocks were once granulites.

Comparisons with the average composition of Laxfo- rdian gneisses from the mainland shows the North Harris rocks to be similar in both major and trace element chemistry (differing markedly in only Sr) to rocks from the type area at Laxford Bridge.

Section C: Regional trends in geochemistry

As discussed above lithological and mineralogical variations exist in the gneisses, chief amongst them bEing the distribution of hornblende-biotite gneisses and the occurence of granites and pegmatites. The geochemical•evidence already outlined reveals CA% Ee re•nce5 subtle but significant1 between biotite and hornbiende- biotite gneisses and suggests various relationships between mineralogy and chemistry. Having thus established such variations an attempt to correlate them became necessary. To do this the chemistry was examined on a regional basis and the

111

10 +

0 00 • ~O •0 K~ 0

10 30 50 70 90 110 K/Ba

a 60

40 0 o 0 Ba/Rb 20 . • •• ;.:• '= + .: + + i 0

1.0 10 100 1000 Rb ( ppm) 1 og .

1.0

.75 +

Rb /Sr .50

.25 . o. O0• o 0

0 10 100 1000 Rb ( ppm) 1 og .

Fig:3.19 Comparison of North Harris gneisses (.) and granites (+) with granulite facies gneisses of Coll and Tiree (o) (After Drury, 1974). 112 20 15 Th ppm 10 • • •• '0 • • ••••0 5 •• • o. 0 " •• 0 0 • oao 0 .". it '" 0 1000 .,. • , 0 • 0 .. Sr.·ppm • 0 500 • ' . • • .... ., .. ~...... ~.:. ~~. • t .. ..- ...... :,. •+ •• • + 0 .4 ." ·0 • 0 .3 • • ., P20S% • • . 2 ·0, • "...., 0 0 ...... ;. ~:.. . • 1 I:··.... ~. :: .- • 'l. • ... _ ". + • + +.- ~ 0 200 400 600 800 1000 K/Rb

Fig:3.19 continued

113 .; distribution of the various element concentrations and ratios compared with the lithological and mineralogical distributions. Trend surface analysis was used to . obtain the geochemical trends and some aspects of this technique are therefore described before. turning to an interpretation of the results.

3.10 Trend surface analysis . (See Davis, 1973, from which the following is taken) As trend surface analysis is discussed in several textbooks and papers only an introduction to the method will be given here. The computer program used in the analysis was KW1KR8 written°by Esler etal (1968) and reported in the Kansas Geological Survey Computer Contributions Bulletin No. 28.

3.10.1 Method

Trend surface analysis is used to establish trends in spatially arranged data, be that data chemical or otherwise. Regional patterns may be established in which local fluctuations are smoothed-over, or the method can be used to detect local fluctuations on the regional pattern, this latter being dependant on the availability and distribution of data. In this work it was desired that local effects be contoured-over leaving a regional trend that could, perhaps, be interpreted with respect to mineralogical variations and so on. Fig:3.20 (from Davis, 1973) illustrates this concept of regional trend and local variation. in all cases the heavy line indicates the regional trend while the local deviations from that trend produce positive and negative residuals. As can be seen, the more complex the regional trend becomes the better it fits the data until, theoretically at least, it fits the data exactly and the residuals are lost. To estimate a regional trend multiple regression analysis is carried-out on the data, each observation being fitted to a polynomial function in which the geographical coordinates are an integral part. For

114 a~

(a) (b)

(C) (d) F 1 g : 3 . 20 Concept of trend illustrated to two dimensions. (a) Collection of original data points and the line on which they lie. (b) Straight-tine trend fit to the observations. (c) Parabolic trend. (d) Cubic trend. Shadings represent positive and negative residuals from the trends.

(From Davis, 1973) .

115 example a linear trend surface is represented by an equation such as:

ybo + 1 + b2X2

in which the observation, y, is regarded as a linear function of a constant value (bo related to the mean of the data) plus an EW coordinate, b1 , and a NS coordinate, b2. The three unknowns necessitate three simultaneous equations the solution to which gives the coefficients of the best-fitting linear trend surface, best-fit being determined by least squares as in regression analysis.. program.KWIKR8 is one such program which deals with one, two and three-dimensional data (ie. graphs, maps and block diagrams) and will determine the coefficients of trend surfaces up to the seventh degree, it produces line printer output listing, as desired, statistical data, maps of the original data points and residual values, contoured maps of the trend surface and residuals, etc.

3.10.2 Statistical tests of trends

Output from KWIKR8 includes a variety of statistical coefficients used to estimate the value of the trend surface. These are: a) total variation in the data (the sums of squares) b) mean and standard deviation of the data c) variation explained by the surface (as a percentage of the total variation) d) coefficient of correlation (typical "''r' of regression analysis) e) the F-ratio and degrees of freedom between the regression and the residuals f) the coefficients of the polynomials describing both the regression and the residuals Using a combination of these data it is possible to see how far a particular surface explains the variation in the data and the significance of such a correlation.

116 An important test is concerned with whether or not surfaces of succesively higher degrees yield succesively greater significance in the analysis. This is a problem discussed by Chayes (1970) who decided that, in most cases, only low order surfaces should be used as, frequently, higher order trends are not significantly different from lower ones and succeed only in producing more complex contour maps. In preference to Chayes' method, but employed for the same reasons, Davis' (1973) test was applied to some of the surfaces produced by KWIKR8. The test involves the following procedure: a) finding the difference in the sums of squares due to regression of the higher polynomial minus that of the lower b) di-viding this difference by the difference in regression degrees of freedom, giving the mean square of the regression c) dividing this mean square by the mean square due to derivation from the higher polynomial d) If the resulting P-value is insignificant (determined by tables) nothing has been gained by fitting the higher degree polynomial. In general it was found that little significance could be attached to the higher order surfaces and hence, generally, only 3rd and 4th order surfaces have been used. These orders were usually the first to show any significance at all (determined by the regression coefficient, r) in the trend surfaces.

3.10.3 Problems in trend surface analysis

As already mentioned Chayes suggested that errors in trend surface analysis are not generally guarded against. The main sources of error are therefore outlined: 4 a) It has to be assumed that samples are collected with out bias from, ideally, a normally distributed populat- ion. Neither assumption can be tested. b) The number of observations should be as large as possible and as an absolute minimum should exceed the number of coefficients in the polynomial equation (up to 36 in a seventh degree surface; all maps are based on a

117 minimum of 60 data points) or the results are invalid. Equally statistical tests cannot be faithfully applied in small samples. c) The lack of data points at map boundaries can create edge effects in which extrapolated values can become enormous. it is good practise to have a 'buffer-zone' at the edge of the map created by the presence of data in excess of the map area. This is even more important If sample numbers are low and the map area large. d) A well scattered arrangement of data points, preferably random and unclustered, will produce more acceptable results. Sample localities for the trend surface analysis are given in fig:3.21.

3.11 Geochemical distributions in Worth Harris

The results of the trend surface analysis, applied to the elements and element ratios which were earlier shown to differ between the biotite and hornblende- biotite gneisses are given in fig:3.22. Each diagram includes a title giving the element concerned, the degree of the surface employed, a correlation coefficient (r) and the percentage of the total variation explained by the regression (%VER). The table below shows the anticipated variations in chemistry of the hornblende-biotite gneisses with respect to the biotite gneisses:

Hornblende biotite gneiss Biotite gneiss

Mn0 Si02 MgO greater than K20 greater than FeO biot. gneiss► Rb hbind—biot gneiss CaO Zr

K/Rb Rb/Sr , greater than lower than Bey/Rb biot.gneiss K/Ca hbind—blot gneiss

Possible lower Fe/Mg in biotite gneisses

From the distribution maps (figs:3.l, 3.2 and. 3.22) and this table of variations it can be seen that the

118 • 136 .135

318. 317 256 316. 338 • 333 • 26. 60. .332 63 • 58. 36.E 250 • .263 55 • .54 44. .244 •261 .53

15 .45 363 • 74 'ō3 .73 128 • •127 83. .129 126 .120 .150 .238 240 . .239 .90 140. 113. 117. .305 215. .I14 .115 . 242 140 . • 96 284••7 .163 233. .209 236. .273 274. .290 .268 .156 153.

Fig:3.21 Sample localities for use in trend surface analysis

119 Si02 K 2 0

4th degree surface 3rd degree surface

r= .56 %VER= 32 r .58 %VER= 33

Fig:3.22 Trend surfaces of selected elements Some statistical information is given with each map: r correlation coefficient %VER- percentage of total variation explained by the regression Oxides in wt% Trace elements in ppm.

120

Fe 0 Fe203