Provided courtesy of the British Geological Survey. Fettes, D. J., Mendum, J. R., Smith, D. I. and Watson, J. V. 1992. Geology of the Outer Hebrides. Memoir of the British Geological Survey, Sheets (solid edition) Lewis and Harris, Uist and Barra (Scotland).
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Geology of the Outer Hebrides: Memoir for 1:100 000 (solid edition) (Scotland)

(Front cover)

(Rear cover)

(Frontispiece)

Contents

Title page and preliminaries. Geology of the Outer Hebrides: Memoir for 1:100 000

Other publications of the Survey dealing with this district and adjoining districts

Books

Maps

Preface

Summary of geology

Chapter 1 Introduction

History of research

Physical features

Summary of the geology

Geological terminology

The problem of the Scourie dykes

Do the Younger Basics represent more than one age of intrusion?

Is the Younger Basic suite of Scourie Dyke age?

Miscellany

Place names

Spelling of place names in the Outer Hebrides varies considerably. This memoir conforms to those given in the Ordnance Survey Second Series 1:50 000 maps. National Grid references are given in the following forms: [NR 1234 5678] and [NR 123 567] Rarely the form [NR 12 56] is used.

Mineral abbreviations

The following abbreviations have been used in mineral assemblages in the text:

Normative minerals

The following abbreviations have been used in discussion of normative minerals

Chemical analyses

Thin section numbers

Use of the prefix meta-

Radiometric age dates

Amphibole formulae

Chapter 2 Lewisian Complex: introduction and quartzofeldspathic gneisses

Introduction

Lithology

Lewis and Harris

Uist and Benbecula

Barra and the southern isles

Note on the distant isles

Petrography

Geochemistry

Metamorphism

Chapter 3 Lewisian Complex: metasedimentary and metavolcanic gneisses

Lithology

Langavat Belt

Leverburgh Belt

Sound of Harris metasediments

North Rona metasediments

Petrography

Areas other than South Harris

Langavat Belt

Leverburgh Belt

Geochemistry

Chapter 4 Lewisian Complex: older basics and ultrabasics

Early basics of Barra

Banded basics of the Uists and Benbecula

Field aspects

Petrography

Geochemistry

Conclusions

Banded basics of the Northern Isles

Older ultrabasic complexes

Northern isles

Langavat Belt

Sound of Harris

Chapter 5 Lewisian Complex: the nature of the early gneisses

Chapter 6 Lewisian Complex: Late-Scourian intrusions

Metadiorites and microdiorites

Field aspects of minor intrusions

Field aspects of major intrusions

Petrography of the minor intrusions

Petrography of major intrusions

Granites

Field aspects, Barra

Field aspects, North Uist

Field aspects, Lewis

Petrography, East Barra

Petrography, South Barra

Petrography, North Uist

Pegmatites

Geochemistry

Microdiorites

Conclusions

Chapter 7 Lewisian Complex: the South Harris Igneous Complex

Previous work

General lithology

Major intrusive bodies

Metagabbros

Meta-anorthosite

Metanorite

Metadiorite

Metatonalite

Minor intrusive bodies

Ultramafic rocks of the Leverburgh Belt

Hornblendites

Mafic gabbro dykes and sheets

Hornblende-plagioclase pegmatites

Anorthosite-ultramafite 'breccias'

Anorthositic gabbro dykes and trondhjemite pegmatites

Late potash-rich basic dykes

Pegmatites

Structure

Pre-DL1 event–metasediments

Primary foliation/banding in the igneous bodies (dL1)

DL2 event

dL3 event

dL4 and later events

Metamorphism

Synthesis

Chapter 8 Lewisian Complex: Corodale Gneiss

Petrography

Geochemistry

The age of the Corodale Gneiss

Chapter 9 Lewisian Complex: Ultrabasic Complexes

Northern Isles

Southern Isles

The age of the ultrabasic rocks

Chapter 10 Lewisian Complex: Younger Basics

Field aspects

'Cleitichean Beag' dykes

Metadolerites

Petrography

Tleitichean Beag' dykes

Metadolerites

Mineralogy

Amphibole

Garnet

Pyroxene

Scapolite

Feldspar

Geothermometry

Geochemistry

Crystallisation sequence

Petrogenesis

Initial recrystallisation

Laxfordian deformation and metamorphism

Summary

Age of intrusion

Relationships of dyke types and ultrabasics

Conclusions

Chapter 11 Lewisian Complex: Late-Laxfordian granites and pegmatites

Uig Hills-Harris granite complex

Field relationships

Petrography

Geochemistry

Granite genesis

Other granite bodies

Pegmatites

Late-stage deformation in the granitic rocks of Lewis and North Harris

Chapter 12 Lewisian Complex: Late-Laxfordian minor intrusions

Benbecula

North Harris

Lewis

Chapter 13 Structural history

Large-scale structures

Mesoscopic structures and fabric systems

Gneissic banding

Tectonite fabrics

Effects of ductility contrasts

Scourian structures

The earliest structural developments (dS1)

The main gneiss-forming event (dS2)

Late-Scourian structures

Shear zones

The Younger Basic Suite and first Laxfordian events

Early Laxfordian structures (dL2)

Later Laxfordian structures (dL3 and dL4)

Laxfordian strain

Zones of low strain

Zones of moderate strain

Zones of high and very high strain

South Harris Complex

Inverian shear zones (dL4)

Chapter 14 Regional variations of structure

The Southern Isles

Barra and adjacent isles

South Uist

Benbecula

North Uist

The Northern Isles

Central South Harris and North Harris

Central Lewis

North-east Lewis

The steep belt of north-west Lewis

The Sound of Harris and Southern Harris

Chapter 15 The Outer Hebrides Thrust Zone

Previous work

Rock types

Pseudotachylite

Gneiss with marked 'cataclastic' fabric

'Mashed' gneiss'

Mylonite

Pseudotachylites

Petrography

Mode of occurrence and origin

Pressure–temperature conditions of pseudotachylite formation

Stress levels: theoretical considerations

Mylonites

Distribution

Mode of formation

Petrography

Mylonitic fabric orientation

Folding

Late-stage alteration

Age of the Outer Hebrides Thrust Zone

Chapter 16 Post-Lewisian minor intrusions

Caledonian

Appinite

Permo-Carboniferous

Quartz-dolerite dykes

Camptonite-monchiquite dykes

Tertiary

Division into swarms and petrological types

Crinanite dykes

Crinanite-teschenite sills

Olivine-bearing dolerite and basalt dykes

Olivine-free dolerite and basalt dykes ('tholeiites')

Chapter 17 Post-Lewisian sedimentary rocks

The Stornoway Formation

Contact relationships

Lithology and origin

Jurassic rocks of the Shiant Isles

Chapter 18 Summary of pre-Caledonian geological history

1 The supracrustal sequence (?2900 Ma)

2 The early-Scourian event

3 The igneous sequence

4 The main Scourian event

5 The Late-Scourian intrusives

6 The Late-Scourian deformation

7 The Younger Basic intrusives

8 The South Harris Igneous Complex

9 The main Laxfordian event

10 The Late-Laxfordian granites

11 Late-Laxfordian deformation

12 Thrusting

Appendix Mineral analyses

(Appendix Table 1) Amphiboles

(Appendix Table 2) Garnets

(Appendix Table 3) Pyroxenes from the Younger Basics

References

Figures, plates and tables

Hand specimen photographs from the BGS Systematic Series

(Frontispiece) Castlebay and Heaval (383 m), Barra. The feature running upwards from the building on the right marks the position of a thrust within the Outer Hebrides Thrust zone (D 2936)

(Cover photograph) Eaval from Baymore, Grimsay (Benbecula). The upper part of the scarp and the dip slope to the east are formed of resistant pseudotachylite-gneiss breccia. This marks the position of the Outer Hebrides Thrust in southern North Uist. (D03197)

(Rear Cover) Geology of the Outer Hebrides