Project report to determine the field relationship of the Beinn an Dubhaich Granite and its contact metamorphism of the surrounding Durness Dolomitised Limestone on the Isle of Skye

Daniel Webb

A research study project submitted to the University of Derby in partial fulfilment of requirements for the degree of Bachelor of

Science.

BSc (Joint Honours) Geology with Geography

2016

AbstractThe origin of the Beinn an Dubhaich granite has been debated for many years along with its contact

metamorphism with the surrounding Durness dolomitised limestone. Extensive research has taken

place to explain its origin and the result of the contact metamorphism. King (1960) and Whitten

(1961) suggested that the granite has a sheet-like form and that the irregular boundary was caused

by erosion through the granite sheet into underlying sediments. Raybould (1973) suggested the

granite is part of a shallow funnel-shaped intrusion”. Goulty et al (1996) suggested that the granite

is a steep-sided stock extending down to about 1km. Fieldwork was completed to produce a 6km²

geological map along the Beinn an Dubhaich granite and hand specimens were collected for the

production of thin sections which were analysed and interpreted. The granite contained minerals

such as quartz, plagioclase, orthoclase, hornblende altering to biotite and biotite altering to chlorite.

The Durness dolomitised limestone obtains more forsterite but less talc on arrival to the granite

intrusion. The steep granite contact shows it is a steep-sided discordant stock intrusion with the

integrated Durness dolomitised limestone acting as roof pendants. The stock intrusion of the granite

caused contact metamorphism of the dolomitised limestone. As a result, a sequence of new minerals

were formed with an increase in metamorphic grade; talc, tremolite, diopside and forsterite.

Fractures found within the dolomitised limestone are the reason behind the irregular boundary

between the granite and the dolomitised limestone.

i

Contents

Abstract.................................................................................................................................................i

List of figures......................................................................................................................................iii

List of Tables........................................................................................................................................v

Acknowledgements.............................................................................................................................vi

1 Introduction...................................................................................................................................1

1.1 Context of Report..................................................................................................................1

1.2 Aims and Objectives..............................................................................................................1

1.2.1 Aims...............................................................................................................................1

1.2.2 Objectives.......................................................................................................................2

1.3 Area of Study.........................................................................................................................2

2 Literature Review..........................................................................................................................4

3 Methodology.................................................................................................................................7

3.1 Fieldwork...............................................................................................................................7

3.2 Laboratory Work...................................................................................................................8

4 Lithologies....................................................................................................................................9

4.1 Beinn an Dubhaich Granite...................................................................................................9

4.1.1 Hand Specimen and Field Description...........................................................................9

4.1.2 Thin Section Description..............................................................................................10

4.2 Durness Dolomitised Limestone...............................................................................................16

4.2.1 Hand Specimen and Field Description...............................................................................16

4.2.2 Thin Section Description...................................................................................................16

4.3 Torridonian Sandstone.........................................................................................................20

4.3.1 Hand Specimen and Field Description.........................................................................20

4.3.2 Thin Section Description..............................................................................................21

4.4 Basalt...................................................................................................................................23

4.4.1 Hand Specimen and Field Description.........................................................................23

ii

4.4.2 Thin Section Description..............................................................................................24

4.5 Kilchrist Hybrids.................................................................................................................25

4.6 Pyroclastic Breccia..............................................................................................................27

4.7 Interbedded Sandstone and Conglomerate..........................................................................27

4.8 Quartzite..............................................................................................................................28

4.9 Limestone............................................................................................................................28

4.10 Limestone with Interbedded Sandstone...............................................................................28

4.11 Mudstone.............................................................................................................................29

4.12 Rhyolite................................................................................................................................30

5 Geological Structures..................................................................................................................31

5.1 Folds....................................................................................................................................31

5.2 Faults....................................................................................................................................32

6 Discussion...................................................................................................................................35

7 Geological History......................................................................................................................40

8 Conclusion..................................................................................................................................42

9 Reference List.............................................................................................................................44

10 Appendices..............................................................................................................................48

iii

List of figuresFigure 1.1 - A simplified map of the location of the Isle of Skye........................................................2

Figure 1.2 - A schematic that shows the location and size of the area of study...................................2

Figure 1.3 - A map showing the location and lithologies surrounding the Benn an Dubhaich granite .

..............................................................................................................................................................3

Figure 2.1 - An image showing the position of Skye in the Late Cambrian........................................4

Figure 2.2 – (A) a plan view and (B) a cross section of a shallow funnel-shaped intrusion................6

Figure 4.1 – A photo showing jointing in the Beinn an Dubhaich granite...........................................9

Figure 4.2 – A thin section photo showing a perthitic texture in the orthoclase minerals.................11

Figure 4.3 - PPL thin section showing alteration of plagioclase to sericite and hornblende to biotite

............................................................................................................................................................12

Figure 4.4 – A sketched version of figure 4.3 ...................................................................................12

Figure 4.5 – XPL thin section showing alteration of plagioclase to sericite and hornblende to biotite

............................................................................................................................................................13

Figure 4.6 – A sketched version of figure 4.5 ...................................................................................13

Figure 4.7 – PPL thin section showing biotite alteration to chlorite..................................................14

Figure 4.8 – A sketched version of figure 4.7....................................................................................14

Figure 4.9 – XPL thin section showing biotite alteration to chlorite.................................................15

Figure 4.10 – A sketched version of figure 4.9 .................................................................................15

Figure 4.11 – A thin section of the dolomitised limestone (NG 61127 18486) in PPL and XPL......18

Figure 4.12 – A sketched version of 4.11...........................................................................................18

Figure 4.13 - A thin section of the dolomitised limestone (NG 61425 19153) in PPL and XPL......19

Figure 4.14 – A sketched version of figure 4.13................................................................................20

Figure 4.15 – A field photo showing the Torridonian sandstone with quartz veins .........................21

Figure 4.16 – A thin section photo of the Torridonian sandstone in PPL and XPL...........................22

Figure 4.17 – A thin section sketch of figure 4.16.............................................................................23

Figure 4.18 –.Basalt from a dyke containing phenocrysts of 2-3mm................................................23

Figure 4.19 – A thin section photo in XPL showing the phenocrysts present in basalt.....................24

Figure 4.20 - A thin section photo in PPL showing the phenocrysts present in basalt......................25

Figure 4.21 – A field photo showing the jointing present in the Kilchrist hybrids............................25

Figure 4.22 – A thin section photo of the Kilchrist hybrids under cross polarised light...................26

Figure 4.23 – A thin section photo of the Kilchrist hybrids under plane polarised light...................26

Figure 4.24 – A field photo showing the range in size of clasts in the interbedded sandstone and

conglomerate......................................................................................................................................27

iv

Figure 4.25 – A field photo showing jointing that appears in the quartzite.......................................28

Figure 4.26 – A field photo showing the limestone with interbedded sandstone..............................29

Figure 4.27 – A field photo showing the mudstone...........................................................................29

Figure 4.28 – A field photo showing the jointing in the rhyolite.......................................................30

Figure 5.1 – A field photo which shows an s-fold in the schistosity on the dolomitised limestone. .31

Figure 5.2 – A field photo which shows an m-fold in the schistosity on the dolomitised limestone.32

Figure 5.3 – A digital elevation model of the study area...................................................................33

Figure 5.4 – A map showing the location of the Torridonian and Cambrian foreland......................34

Figure 6.1 – A graph showing the temperature against distance from igneous contacts...................36

Figure 6.2 – Triangular diagrams illustrating the progressive decarbonation in the sequence of new

minerals..............................................................................................................................................37

v

List of TablesTable 6.1 – Minerals that appear in the groups of skarn zones……………………………………38

vi

AcknowledgementsI would like to take this opportunity to thank my parents for supporting me financially, allowing me

to take part in the field excursion to the Isle of Skye. Along with my parents I would like to thank

my brother for proof-reading the project report and providing useful feedback. I also want to

sincerely thank Prof. Hugh Rollinson for his guidance and support throughout the course of

producing the project report. The analysis and interpretation of the lithologies could not be

completed without the production of thin sections; therefore I would like to thank the lab technician,

Matthew Hunt. As well as this I would like to thank him for taking photos of my thin sections. Last

but not least, I would like to express my gratitude, towards Jonathan Cummins and Liane Baiao for

offering their support and guidance.

vii

1 Introduction

1.1 Context of Report

The Beinn an Dubhaich granite has been the subject of profound research for over a century. The

regional investigation by Harker (1904) and Richey (1932) was followed up by Tilley (1951) and

Hoersch (1981) who investigated the contact of the granite. The structure of the granite was

investigated by King (1960), Whitten (1961) and Hoersch (1979) (Emelus and Gyopari, 1992).

Despite the intense research, a definitive answer is yet to be found in order to explain the origin of

the Beinn an Dubhaich granite and its contact metamorphism with the surrounding Durness

dolomitised limestone.

1.2 Aims and Objectives

The aim of this research project is to determine the field relationship of the Beinn an Dubhaich

granite and its contact metamorphism of the surrounding Durness dolomitised limestone. The

Durness dolomitised limestone sits on top of the Torridonian sandstone via the Kishorn Thrust fault.

The Durness dolomitised limestone, Beinn an Dubhaich granite, basalt and Torridonian sandstone

will be discussed in detail as a means of a description and an interpretation. However, the

documentation of the remaining lithologies will only be brief because they are of little presence and

significance on the geological map.

The primary data that was collected during the mapping on the Isle of Skye was used to support the

description and interpretation of individual lithologies. Hand specimens were carefully selected to

be made into thin sections. The thin sections were used for the analysis and interpretation of each

selected lithology’s mineralogy which was then used to aid in the determination of the scope of

metamorphism. The lithologies will be depicted from both the field notebook and geological map.

1.2.1 Aims

To determine the effect of contact metamorphism on the Durness dolomitised limestone.

To determine the origin of the Beinn an Dubhaich granite and its effects on the surrounding

lithologies.

To produce a 6km² geological map of the study area and a field notebook.

To produce a rose diagram and cross section for the study area.

1

1.2.2 Objectives To identify any new mineral compositions and textures within the Durness dolomitised

limestone.

To analyse and explain the contact metamorphism that has taken place between the Beinn an

Dubhaich granite and the Durness dolomitised limestone.

To identify the origin of the Beinn an Dubhaich granite and the Durness dolomitised

limestone that is found within.

To explain the irregular boundary between the Beinn an Dubhaich granite and the Durness

dolomitised limestone.

1.3 Area of StudyFigure 1.1 shows the location of the Beinn an Dubhaich granite on the Isle of Skye. The area of

study in question is located in the south east of Isle of Skye along the Beinn an Dubhaich granite

(figure 1.2). The Isle of Skye is located along the west coast of Scotland which places it within the

Scottish highlands. Skye is the largest island in the Inner Hebrides (Isleofskye.com, 2015).

2

Figure 1.1 - A map showing the location of the

Isle of Skye within Scotland and the location of

the Beinn an Dubhaich granite (Wikipedia,

2015).

Figure 1.2 - A schematic showing the location

and size of the area of study on the Isle of Skye

(Digimap, 2015).

The area of study is a 6km² section along the Beinn an Dubhaich granite. Specifically, the section in

question is approximately 3 miles west from Broadford and 2.8 miles east from Torrin (Google

Maps, 2015). The Beinn an Dubhaich granite starts at Loch Slapin and runs up until it reaches

Kilchrist (figure 1.3). The granite is part of the Tertiary igneous complex which is situated in the

Eastern Red Hills centre with Cambrian-Ordovician aged lithologies surrounding it (Goodenough,

2006).

3

Figure 1.3 - A general map used to show the location of the Beinn an Dubhaich granite intrusion

in relation to the surrounding Durness dolomitised limestone (Emelus and Gyopari, 1992).

2 Literature ReviewThe Torridonian Sequence can be split into three groups; the Stoer, Sleat and Torridon (Batchelor et

al, n.d.). The sequence unconformably overlies both the Lewisian gneiss complex and the Stoer

Group, and is in turn unconformably overlain by the Lower Cambrian sequence (Geological Society

of London, 2002). According to the British Geological Survey Map Viewer (2015) the Torridonian

sandstone in the area of study is found within the Torridon Group which sits between 1000-970Ma

in the stratigraphic chart (Barber et al, 2008). The Torridon group is approximately 7km thick and is

part of the upper Torridonian in the Neoproterozoic age (Earthwise, 2015b). Specifically, the

sandstone which sits in the area of study is part of the Applecross formation (British Geological

Survey Map Viewer, 2015).

Cambrian-Ordovician rocks unconformably overlie the Torridonian sandstone of the Torridon

Group (Geological Society of London, 2002). At the start of the Cambrian period the Iapetus Ocean

had opened to a width of 2000km and Scotland was situated far south of the equator (figure 2.1).

The quartzite that was formed in the Early Cambrian was due to the Caledonian orogeny that

produced the Moine thrust zone which eventually lead to the formation of the Kishorn thrust fault

(The Geological Society, 2012). This subjected the quartz-rich sediment to high pressures and

temperatures leading to production of the quartzite.

4

Three main horizons of the limestone exist in the Durness group and are exposed in the contact

aureole of the Beinn an Dubhaich granite (Hoersch, 1981). They include the Ben Suardal Limestone

(a dark limestone containing chert layers and nodules) and the Beinn an Dubhaich dolomitised

limestone. The final horizon is the Strath Suardal dolomitised limestone, a dark grey dolomitised

limestone containing chert nodules, pipes, and bands (Hoersch, 1981). However, according to the

British Geological Survey Map Viewer (2015) there are four horizons in the Durness dolomitised

limestone group. These include the Strath Suardal Formation, Ben Suardal member, Kilchrist

member and the Lonachan member. The Skye marble formation is an example of where Durness

dolomitised limestone has been metamorphosed as a result of direct heat and heated groundwater.

Within the area of study the Cambrian sequence comprises of quartzite at the base and the

dolomitised limestone at the top. “The basal unconformity of the sequence is planar showing that

the tilted Torridonian rocks had been eroded to a near-level peneplain” (Barber et al, 2008).

The Torridon group consists of red, fluviatile sandstones that have been thrust over the Durness

dolomitised limestone on the northern slopes of Ben Suardal and in Glen Suardal. Additionally, the

Torridon Group sandstones are overlain by much younger sedimentary rocks in a synclinal fold

(Goodenough, 2006). However, according to Brown (1969) “west and below the thrust-plane, the

Durness sediments are particularly well exposed in a large anticlinal structure”. Emelus and

Gyopari (1992) also suggested the contacts between the Durness dolomitised limestone and the

Beinn an Dubhaich granite are part of the Broadford anticline with the granite intrusion coinciding

with the anticlinal axis.

“Between 50 and 60 million years ago, the Atlantic Ocean began to form, separating Scotland and

North America. The event caused large scale volcanic activity in the UK” (Geolsoc, 2012) which

created the Tertiary igneous complex. The Tertiary igneous complex includes the intrusive gabbro,

granites and dikes, as well as extrusive basalt flows (Hoersch, 1981). The Beinn an Dubhaich

granite is approximately 55 ± 4 million years according to Rb-Sr age determination (Bell and

Moorbath, 1965). “The Beinn an Dubhaich intrudes and deforms Durness limestones and Tertiary

dykes in west-central Strath on Skye” (Coward and Longman, 1979). Additionally, according to

Coward and Longman (1979), radial compression from the centre to the west end of the granite

5

Figure 2.1. A schematic showing the location of Scotland during the Late Cambrian.

“During this time Scotland continued to move northwards from about 60 degrees below

the equator in the early Cambrian to nearer the equator in the late Cambrian” (Age of

outcrop is the reason for the limestones being deformed. This is due to the forceful intrusion of an

early phase of the granite.

Due to the volcanic origin of the Isle of Skye, the Torridonian Sequence has gone through contact

metamorphism either at a baked or chilled margin (Chris, 2011). As a result the mineralogy and

texture of the initial rock has changed overtime through solid state diffusion. As well as this, the

volcanic origin has led to the deposition of volcanic tephra in the form of ash, lapilli and bombs

which had been erupted from past volcanoes (Batchelor, 2005).

The western end of the Beinn an Dubhaich granite is in steep contact with the Durness dolomitised

limestone near Camas Malag. On the other hand, near Kilchrist in the east, the relation between the

dolomitised limestone and the granite is irregular (Emelus and Gyopari, 1992). This has led to wide

variety of interpretations for the presence of the Beinn an Dubhaich granite. King (1960) and

Whitten (1961) suggested that the granite has a sheet-like form and that the irregular boundary was

caused by erosion through the granite sheet into underlying sediments. On the other hand, Hoersch

(1979) suggests the granite cooled in the fractures present in the dolomitised limestone to form

dyke-like lobes creating an irregular and curved boundary. They also implied the Kishorn Thrust

Plane had emplaced the granite. According to Raybould (1973) “sub-surface borehole evidence

(<50m) suggested the granite is part of a shallow funnel-shaped intrusion” (Castro et al, 1999)

(figure 2.2). On the other hand, Goulty et al (1996) suggested that the granite is a steep-sided stock

extending down to about 1km according to local geophysical investigations (Castro et al, 1999).

6

Figure 2.2 – (A) a plan

view, (B) a cross section

of a shallow funnel-

shaped intrusion which

is a suggested form of

the Beinn an Dubhaich

granite (Ocean Drilling

Program, 2007).

3 MethodologyThis research project essentially uses primary data yet secondary data was still of some importance

for background research and comparing results from the field. The primary data collected from the

field was used in conjunction with secondary data. Journals, academic papers, library textbooks and

previous geological maps on the study area formed the basis of secondary data used in the project.

Online academic papers published by the Scottish Journal of Geology were used because they are

informative and engage a range of scientists’ theories.

3.1 FieldworkWhilst undertaking field work, primary data was collected and recorded in a field notebook or on a

geological map. Hydrochloric acid was used to test for the presence of calcium carbonate whereas

the grain size card was mainly used on sedimentary rocks to analyse various properties. Other

equipment used included a pen knife to test a lithology in terms of hardness. Also a hand lens (10x

magnification) was used to identify the mineralogy and texture of each lithology in the field.

Plunge and trend measurements and strike and dip measurements were collected via the use of a

compass-clinometer. Data was recorded in the notebook and then transferred onto the geological

map using grid references obtained using the GB Grid Converter application. Additionally,

geological boundaries were followed by using vegetation which was then recorded on the field map

using grid references. Hand specimens were taken, using a geological hammer, depending upon

their importance to the project. The rock specimens collected had to be approximately 12cm x 12cm

or bigger. The locations of the samples were noted for future reference.

A systematic approach was taken when covering the map geologically in the field. The action taken

was to start in the north of the map and work southwards. At each recorded location a detailed

description along with any important features in the lithology were noted and photographic

evidence was taken for future reference. Photographic evidence was taken alongside various types

of equipment (e.g. pencil, grain size card, hammer and notebook) to illustrate the true scale. The

7

interpretation used information from the description to determine the lithology and give reasons for

the presence of any important features. The exact location was identified on the field map and then

the relevant colour was used to represent the lithology by differential shading. At the end of each

day in the field a summary was produced to show what was done during the day, lithologies found

and structures found.

3.2 Laboratory WorkBasalt, Kilchrist hybrid, Beinn an Dubhaich granite, Durness dolomitised limestone (2 samples at

different localities) and Torridonian sandstone were all the samples collected from the field. These

samples were transferred over to the lab technician (Matthew Hunt) to be made into thin sections.

However, the limit of six samples for thin section production prevented more dolomitised limestone

samples from being collected. This would have been beneficial for the analysis into the effects of

metamorphism on the dolomitised limestone. The thin sections were examined under a microscope

in the student laboratory to determine the mineralogy of each lithology by observing the

characteristics of the minerals under both plane and cross polarised light. Furthermore, the grain

size, contacts between grains and any new textures in the lithology were noted.

8

4 Lithologies

4.1 Beinn an Dubhaich Granite

4.1.1 Hand Specimen and Field DescriptionThe Beinn an Dubhaich granite (NG 61450 19382) has a light brown appearance as well as being a

relatively dense rock which doesn’t scratch with a penknife and is very hard to break with a

geological hammer. In the field, a hand lens was used to identify the mineralogy as quartz,

plagioclase feldspar and biotite. Hydrochloric acid was applied which produced no reaction

indicating a lack of carbonate minerals such as dolomite or calcite. Figure 4.1 at NG 61276 19968

shows part of the Beinn an Dubhaich granite which is smooth and rounded in appearance and

contains both jointing and quartz veins.

At the western part of the Beinn an Dubhaich granite, 950m defines its width, however at its most

eastern point its width stretches to 750m suggesting it slightly thins out from west to east.

Additionally, the granites contact with the Durness dolomitised limestone is shown as curved and

irregular on the geological map (appendix A). The topography of the Beinn an Dubhaich granite is

fairly flat with only a rise of 71m from its most northern point to its most southern point. The

granite is seen as being in steep contact with the dolomitised limestone.

9

Figure 4.1 – a photo from the

field (NG 61276 19968)

showing the Beinn an

Dubhaich granite where

jointing and quartz veins can

be clearly seen (scale:

notebook is 21cm long).

Jointing

Quartz veins

4.1.2 Thin Section Description

In thin section, the Beinn an Dubhaich granite is crystalline and inequi-granular. Overall, the granite

is coarse grained (>5mm) which gives it a phaneritic texture (Jackson et al, 2011). Under the

microscope, the grain contacts are curved and irregular. Specifically, the majority of the contacts

are long whilst a minority are tangential, showing along with incomplete alterations, that the granite

is caught in a state of disequilibrium.

Under the microscope, quartz minerals constitute as the main mineral forming 40% of the lithology.

Quartz minerals were identifiable by their irregular shape and colourless transition under plane

polarised light. The quartz minerals obtained first order interference colours, along with extinction

which occurred parallel to the crystal faces (Deer et al, 2013).

Hornblende is present (5% coverage) however it was in the process of altering to biotite (figure 4.3

and 4.5). This was identified because the brown colour of biotite becoming more noticeable in the

hornblende mineral that is green in colour. Hornblende is a mafic mineral that is part of the

amphibole group. It contained perfect cleavage at approximately 120 and second order interference

colours.

Biotite isn’t as abundant as quartz in thin section with it covering approximately 5%. Biotite is a

mafic mineral that was identified by its brown colour in plane polarised light, perfect cleavage and

the presence of pleochroism. Biotite alteration to chlorite is clearly seen under the microscope

(figure 4.7 and 4.9) because green colour of chlorite is becoming more prominent in the biotite

mineral that is of a brown colour. Chlorite is a phyllosilicate mineral (sheet silicate) which is

categorised as a mafic mineral. It was distinguished by its low relief and birefringence,

blue/brown/black interference colours and pleochroism which range from pale yellow to green.

Chlorite accounts for approximately 5% of the thin section.

Carlsbad twinning and very low relief are both distinguishing features that were used to identify

orthoclase. Orthoclase is classified as a felsic mineral which accounts for 15% of the sample. Also it

is a type of alkali feldspar which exhibits a perthitic texture. A perthitic texture is an indication that

exsolution has occurred at some point in the granite. According to the Jackson et al (2011), a

perthitic texture is defined as “an igneous texture consisting of parallel or subparallel intergrowths

of sodic plagioclase (typically albite) occurring as small stringers or irregular veinlets in potassium-

rich feldspar”. Therefore, the granite must have cooled slowly and albite must be present in the

perthitic texture caused by exsolution (figure 4.2).

10

Plagioclase feldspar widely exists in the granite covering 30% of the thin section. Plagioclase is

classified as a felsic mineral which has multiple twinning and is colourless under plane polarised

light. The majority of the plagioclase minerals are in the process of altering to the secondary mica,

sericite (figure 4.3 and 4.5).

The Beinn an Dubhaich granite started as a plutonic rock in a magma chamber below the earth’s

surface. Due to this, magma has cooled at a very slow rate allowing the felsic minerals to grow

overtime and become >5mm suggesting its coarse grained. However, the dolomitised limestone is

found both above and below the Beinn an Dubhaich granite indicating the granite has been intruded

through the surrounding country rock. Also, its sheet-like form in the study area suggests it was

intruded through the country rock from a shallow depth indicated by the long and tangential grain

contacts present in the sample. Upon intrusion of the granite through the country rock the minerals

cooled rapidly halting their growth.

11

Figure 4.2 – a thin section image at 10x magnification showing a perthitic texture in the

orthoclase minerals which indicates exsolution has occurred. Field of view is 2mm.

Perthitic

texture and

albite present

within

orthoclase.

Quartz

12

Figure 4.3 - a thin section viewed at 2.5x magnification in plane polarised light showing

plagioclase feldspar altering to a secondary mica known as sericite. It also shows hornblende

altering to biotite. The field of view round circle is 4.6mm.

Figure 4.5 – a thin section viewed in cross polarised light at 2.5x magnification showing the

mineralogy and textures observed in the Beinn an Dubhaich granite. Quartz, hornblende, biotite,

plagioclase and sericite are all present in the thin section. It also shows plagioclase being altering

to sericite and hornblende altering to biotite. Field of view round circle is 4.6mm.

Figure 4.4 – a sketch of figure 4.3 in plane polarised light at 2.5x magnification. Field of view is

4.6mm.

13

Figure 4.7 – a thin section under plane polarised light at 2.5x magnification showing the

alteration of biotite to chlorite. Field of view round circle is 4.6mm.

Figure 4.6 – a sketch of figure 4.5 in cross polarised light at 2.5x magnification. Field of view is 4.6mm.

14

Figure 4.9 – a thin section under cross polarised light at 2.5x magnification showing the

alteration of biotite to chlorite. Field of view round circle is 4.6mm.

Figure 4.8 – a sketch of figure 4.7 in plane polarised

light at 2.5x magnification. Field of view is 4.6mm.

4.2 Durness Dolomitised Limestone

4.2.1 Hand Specimen and Field DescriptionThe Durness dolomitised limestone is a light grey rock which reacts with hydrochloric acid but not

as vigorous as with limestone. Fractures are occasionally found in the dolomitised limestone which

is less dense than the Beinn an Dubhaich granite and is easily scratched with a penknife or

fingernail. Within the hand specimens chert nodules and quartz veins were more frequent at locality

NG 61425 19153 compared to the locality NG 61127 18486. In the field, under a hand lens

minerals such as dolomite, calcite and plagioclase feldspar were identified. Boudins were

occasionally found within the schistosity and chert nodules on the dolomitised limestone in various

locations.

The geological map (appendix A) shows that there are eight separate units of Durness dolomitised

limestone that vary in size from 20m to 370m which have been integrated into the granite. Their

boundaries are curved and irregular due to the effect of the granite. These integrated units must

have been subjected to high grades of metamorphism, therefore new minerals and textures would

have been formed. The geological map also shows that dolomitised limestone can be found both

above and below the Beinn an Dubhaich granite. The topography increased quite rapidly from

15

Figure 4.10 – a sketch of figure 4.9 in cross polarised light at 2.5x magnification.

Field of view is 4.6mm.

approximately 140m to 190m over a distance of 500m. Solution pits were frequently present in

areas where dolomitised limestone was found.

4.2.2 Thin Section DescriptionTwo samples were collected at contrasting distances to the Beinn an Dubhaich granite. The sample

at NG 61127 18486 is approximately 250 metres from the granite, whereas the sample at NG 61425

19153 is 50 metres from the granite. The two thin sections were applied with alizarin red carbonate

staining which showed up as red indicating the presence of iron free calcite. In both samples the

dolomite and calcite minerals were well rounded with a high sphericity. The minerals obtain long

contacts and are mostly fresh with talc altering to dolomite. They were also well sorted and fine-

grained with a low porosity (1% coverage), partly due to being metamorphosed by the granite. Both

are mainly grain supported with areas being supported by a matrix. Samples contained micro-

crystalline calcite known as micrite which is found between the grains. Micrite covers

approximately 10% of the each sample and was identified by it being very fine-grained and its dark

colours.

The fine nature of the grain size suggests that deposition occurred in an environment with a low

level of energy. High sphericity and well roundness of grains show that the dolomitised limestone is

mature which in turn shows it has undergone prolonged transportation and reworking. Long grain

contacts also show the depth of burial was shallow. Dolomitisation was an early process that

occurred before the Caledonian orogeny, possibly during the opening of the Iapetus Ocean in the

Cambrian period (Merritt and Stephenson, 2006). The presence of micrite along with dolomitisation

shows that the lithology has undergone marine diagenesis.

Marble appears at localities near to the granite intrusion but further away it is Durness dolomitised

limestone. Marble was identified in the field by its white and crystalline nature as well as being easy

to break with a hammer.

4.1.2.1 Sample NG 61127 18486 Olivine is scarce in the sample at NG 61127 18486 covering approximately 1% of the thin section

(figure 4.11 and 4.12). Specifically, the minerals are known as forsterite which is part of the olivine

group and is categorised as an ultramafic mineral. The grains were identified by their second order

interference colours and under plane polarised light, it was colourless as well as obtaining high

levels of relief.

16

Calcite is a prominent felsic mineral within the thin section of the Durness dolomitised limestone

covering approximately 38% of the sample. Calcite was identified by its 120 cleavage as well as

being colourless under plane polarised light.

Like calcite, dolomite minerals which cover 41% of the sample, were identified by it being

colourless under plane polarised light. However, dolomite minerals were distinguished from calcite

by a lack of cleavage. Dolomite is in the process of altering to talc which subsequently covers 10%

of the sample. Talc was identified by its fourth order interference colours along with being

colourless under plane polarised light. The alteration was identified because fourth order

interference colours were becoming present within dolomite minerals.

17

Figure 4.11 – a thin section photo taken under both plane and cross polarised light at 10x

magnification. It shows the mineralogy and textures in the dolomitised limestone at NG

61127 18486. Field of view is 2mm.

Quartz

Vein

Talc

replacing

dolomite

Dolomite

4.1.2.2 Sample NG 61425 19153The presence and size of olivine minerals had significantly increased in the sample from NG 61425

19153 compared to the sample from NG 61127 18486 (figure 4.13 and 4.14). Forsterite covers 50%

of the sample and was identified by its second order interference colours.

Talc has a small presence in thin section (1% coverage) which is in the process of alteration with

talc replacing dolomite. It was identified by its fourth order interference colours and parallel

extinction. The alteration of dolomite to talc was, like the sample at NG 61127 18486, identified by

fourth order interference colours becoming more present in the dolomite mineral.

Calcite and dolomite were both identified by their lack of colour under plane polarised light.

However, calcite was distinguished from dolomite by its 120 cleavage. Calcite covers 20% of the

sample whereas dolomite covered 24%.

18

Figure 4.12 – a sketched version of the sample at NG 61127 18486 at 10x magnification

which shows a quartz vein running through the centre. Field of view is 2mm.

Dolomite

Talc

replacing

dolomite

Quartz

Vein

19

Figure 4.13 - a thin section photo taken under both plane and cross polarised light at

10x magnification. It shows the mineralogy and textures in the dolomitised limestone at

NG 61425 19153. Field of view is 2mm.

Dolomite

Forsterite

Calcite

Calcite

Forsterite

Dolomite

4.3 Torridonian Sandstone

4.3.1 Hand Specimen and Field DescriptionThe Torridonian sandstone from NG 61700 18307 has a red/brown appearance and like the Durness

dolomitised limestone, it does scratch with a penknife. By using a hand lens in the field the

mineralogy was seen as quartz, feldspar and micas (biotite and muscovite). Upon application of the

hydrochloric acid the lithology didn’t react suggesting a lack of carbonate minerals. Figure 4.15

shows the sandstone containing quartz veins as well as being granular, moderately well sorted and

fine-grained. Bedding planes were very hard to distinguish but they were present along with

jointing.

20

Figure 4.14 - a sketched version of the sample at NG 61425 19153 at 10x

magnification which shows a high forsterite content. Field of view is 2mm.

Quartz

Vein

4.3.2 Thin Section DescriptionIn thin section, the Torridonian sandstone is fine-grained, moderately well sorted and grain

supported with long and sutured grain contacts. Overall, the rock has scarcely populated pore space

along with grains which are sub-rounded and have a low sphericity.

Quartz is the most abundant mineral covering 38% of the thin section. It was identified by the lack

of colour under plane polarised light and first order birefringence colours. The grains are fresh and

experience undulose extinction.

Quartz cement is present in the form of syntaxial overgrowths. The cements occur between the

grains and covers approximately 12% of the sample.

Muscovite was primarily identified by its high third order interfernce colours under cross polarised

light. It has a relatively small quantity with 8% coverage. Muscovite is a felsic mineral that is

classified under the mica group. Under plane polarised light muscovite was colourless and had low

levels of relief.

The Torridonian sandstones’ mineralogy also consists of microcline which was first identified by its

cross-hatched twinning. Microcline, which was colourless under plane polarised light with no

pleochroism, covers 17% of the overall slide.

Plagioclase feldspar minerals cover 25% of the thin section, which is distinguished by its multiple

twinning and first order birefringence colours. The lack of colour in plane polarised light as well as

an inclined extinction are also identifiable features of plagioclase.

Using the Dott sandstone classification, the sandstone is interpreted as an arkose due to the fact that

it has a feldspar content that is >25% and the cement covers less than 30%. The sandstone hasn’t

undergone prolonged transportation and reworking because the grains are sub-rounded and have a

low sphericity. This also shows that the sandstone is texturally immature. Its fine-grained nature as

well as being moderately well sorted shows deposition occurred where there was low environmental

energy. Long and sutured grain contacts show that degree of burial was fairly deep. Undulose

extinction within quartz minerals shows that the Torridonian sandstone has experienced at least one

deformation event.

21

Figure 4.15 – a field photo (NG 61163 18279) showing the Torridonian

sandstone which contains quartz veins (scale: pencil is 15cm long).

22

Figure 4.16 – a thin section photo taken under both plane and cross polarised light at 10x

magnification with a 2mm field of view. It shows the mineralogy and textures in the

Torridonian sandstone.

Figure 4.17 – a sketched version of the Torridonian sandstone sample in

figure 4.16 at 10x magnification. Field of view is 2mm.

Microcline

Quartz

Quartz cement

Microcline

Quartz cement

Quartz

4.4 Basalt

4.4.1 Hand Specimen and Field DescriptionA lithology described as being black/dark grey in its appearance and doesn’t react when

hydrochloric acid is applied. The hand specimen has a dark colour due to the high percentage of

dark minerals, which shows the colour index is melanocratic (66 - 100%). It is also crystalline,

inequi-granular and scratches with a penknife. The basalts’ mineralogy using a hand lens consists of

plagioclase and chlorite. Figure 4.18 also shows that phenocrysts of 2-3mm are present. Both the

dykes and sills trend in a north-west to south-east direction (appendix B).

4.4.2 Thin Section Description

Under the microscope, the basaltic dyke at NG 61509 20712 has a porphyritic texture which

contains phenocrysts of plagioclase feldspar and quartz. The basaltic dykes and sills have a very

fine-grained texture which shows it is aphanitic because they were exposed near or at the surface.

Therefore, the lithology cooled rapidly preventing the growth of any coarse crystals.

Vesicles which are identifiable by areas that are black and do not change in colour upon rotation of

the stage cover approximately 9% of the sample.

Plagioclase feldspar minerals are a minority in the thin section sample with approximately 5%

coverage, it was established by its multiple twinning and lack of colour under plane polarised light.

Magnetite is an ultramafic mineral that covers approximately 20% of the slide. Magnetite is a type

of naturally-occurring oxide of iron which is identifiable by its opaque colour in both plane and

cross polarised light. Magnetite differentiates from a vesicle because vesicles are typically small

(roughly 1mm in width) and rounded.

23

Figure 4.18 – a photo

from NG 61589

20816 showing part

of a basaltic dyke

which contains

phenocrysts of 2-3mm

(scale: grain size

card is 80mm long).

Chlorite which is identifiable by its low relief and birefringence, first order interference colours and

pleochroism that ranges from pale yellow to green, covers the majority of the thin section (40%

coverage). The process of alteration has occurred with chlorite replacing pyroxene. This was

identified because under plane polarised light the green colour of chlorite is becoming more

prominent in a pyroxene mineral.

24

Figure 4.19 – a thin section photo in XPL at 4x magnification showing the phenocrysts of

plagioclase feldspar and quartz along with chlorite and magnetite. Field of view is 4mm.

Figure 4.20 - a thin section photo in PPL at 4x magnification showing the phenocrysts of

plagioclase feldspar and quartz along with chlorite and magnetite. Field of view is 4mm.

Magnetite

Plagioclase

QuartzChlorite

Magnetite

Chlorite

Quartz

4.5 Kilchrist HybridsA grey lithology located at NG 61122 20184 that doesn’t react with hydrochloric acid and doesn’t

scratch with a penknife. It is crystalline and has a colour index of 33-66% which suggests it is

mesocratic. Figure 4.21 shows that jointing is present in the lithology. The minerals are slightly

bigger when compared to the Beinn an Dubhaich granite because the lithology was situated below

the earth’s surface for more time than the granite. The mineralogy was identified as quartz, biotite,

chlorite, plagioclase and alkali feldspar (figure 4.22 and 4.23). The presence of minerals such as

chlorite and phenocrysts like quartz and alkali feldspar in a granitic matrix proves that the lithology

is encompassed by basaltic material.

25

Figure 4.21 – a field photo

at NG 60137 20417

showing the jointing

present in the Kilchrist

hybrids (scale: pencil is

12cm long).

Figure 4.22 – a thin section photo in cross polarised light at 4X magnification, showing the

mineralogy and texture of the Kilchrist hybrid. The centre is occupied by chlorite growing

round the biotite. Field of view is 4mm.

Jointing

Chlorite

Biotite

Quartz

4.6 Pyroclastic BrecciaThe pyroclastic breccia obtained a fine-grained matrix that was black in colour as well as being

crystalline and breaking easily with a hammer. The matrix supports angular rock fragments that

range in size from 0.5cm to 12cm. Due to the matrix being black and fine-grained it can be

interpreted as volcanic ash. The pyroclastic breccia is part of the Skye Eastern Red Hills Centre

(Digimap, 2015). It is deduced that this lithology formed during a volcanic eruption that spewed out

volcanic ash along with lapilli and angular bombs. The volcanic ash (<2mm) acts as the matrix

which supports the lapilli (2-64mm) and angular bombs (>64mm).

4.7 Interbedded Sandstone and ConglomerateThe interbedded sandstone and conglomerate is part of the Stornoway formation (Digimap, 2015),

which has a fine-grained matrix containing rounded clasts and a small quantity of chert nodules.

The matrix is granular, well sorted and red/brown in colour. Overall, its mineralogy consists of

quartz and feldspar along with clasts that range in size from 2mm to 15cm which are in contact

therefore the rock is clast supported (figure 4.24). Additionally, some of the clasts reacted with

hydrochloric acid which shows there are clasts of limestone. The matrix is interpreted as

Torridonian sandstone because of its colour and mineralogy. The depositional environment must

have had low levels of energy, for grains of various sizes to settle. The boundary between the

26

Figure 4.23 – a plane polarised light photo of the Kilchrist hybrid as seen in figure 4.21

at 4x magnification. Field of view is 4mm.

Quartz

Biotite

Chlorite

lithology and the Torridonian sandstone is curved, however when in contact with the quartzite it

becomes straight.

4.8 QuartziteQuartzite is a metamorphic rock which shows the protolith was quartz-rich sandstone. As a result,

the quartzite contains 100% quartz along with the presence of jointing (figure 4.25) and a white,

crystalline appearance which scratches with a penknife. The quartzite’s crystalline appearance

suggests it was deeply buried subjecting it to high temperatures and pressures. Normal faulting has

affected the quartzite at two different periods. Quartzite has a small presence on the south western

part of the map at around NG 60452 18256.

27

Figure 4.24 – a

field photo from

NG 60585 18027

showing the clasts’

range of size in a

fine-grained matrix

in the Interbedded

sandstone and

conglomerate

(scale: hammer is

32cm long).

Jointing

4.9 LimestoneThe limestone has a light grey appearance and contains quartz veins; however no schistosity or

chert nodules were present. Using a hand lens, its mineralogy consists of quartz, feldspar and calcite

which were identifiable by a more vigorous reaction to hydrochloric acid compared to the Durness

dolomitised limestone. Consequently, the lithology hasn’t undergone dolomitisation unlike the

Durness dolomitised limestone. This particular limestone unit is part of the Broadford Beds

Formation (Digimap, 2015) which is located in the south eastern part of the study area.

4.10 Limestone with Interbedded SandstoneIn the field, the grey lithology scratched with a penknife and reacted more vigorously to

hydrochloric acid than the Durness dolomitised limestone. This suggest the lithology hasn’t

undergone dolomitisation, unlike the Durness dolomitised limestone. There were patches of

red/brown sandstone but no specific beds were found (figure 4.26). The sandstone was well sorted,

granular, fine-grained and its red/brown colour meant it was interpreted as Torridonian sandstone.

Solution pits were found in close proximity to the unit.

28

Figure 4.25 – a field image from NG 60452 18256 showing that jointing is

Figure 4.26 – a field photo at NG

61686 18085 showing the limestone

with interbedded sandstone (scale:

pencil is 15cm long).

Limestone

Sandstone

4.11 MudstoneA dark brown/black lithology that is granular and scratches with a penknife (figure 4.27). It is very

fine-grained and its mineralogy under a hand lens consists of quartz, pyroxene and clay minerals. It

didn’t react with hydrochloric acid suggesting a lack of carbonate minerals. Its depositional

environment must have acquired very low energy to allow the very fine sediment to settle out of

suspension.

4.12 RhyoliteA light grey lithology that is crystalline and doesn’t scratch with a penknife or react with

hydrochloric acid. Figure 4.28

shows jointing within the

lithology which, by using a hand

lens, has a mineralogy consisting

of quartz, plagioclase and

potassium feldspar. It is fine-

grained therefore it is a volcanic

rock that has been exposed at or

near the surface suggesting cooled

rapidly preventing the growth of

coarse minerals.

29

Figure 4.27 – a field

photo at NG 61720

18020 showing the

mudstone (scale: pencil

is 15cm long).

Figure 4.28 – a field

photo at NG 61661

18157 showing the

jointing in the

rhyolite (scale:

5 Geological Structures

5.1 FoldsWithin the Durness dolomitised limestone s-folds and m-folds were found at NG 61451 20295 and

NG 61231 19069 respectively in the schistosity suggesting the presence of a fold (figure 5.1 and

5.2). Additionally, boudins of chert nodules and schistosity, suggest the dolomitised limestone has

been subjected to compressional stress. Plunge and trend measurements in the Durness dolomitised

limestone above the Beinn an Dubhaich granite show the unit is dipping in a north-west direction.

The average plunge and trend for the northern dolomitised limestone was 246 and 33 respectively,

but plunge and trend measurements from below the Beinn an Dubhaich granite show the

dolomitised limestone is dipping in approximately a south-east direction (appendix A). The average

plunge and trend for the southern dolomitised limestone was 175 and 42 respectively. Therefore,

an anticlinal fold exists with the Beinn an Dubhaich granite sitting on its axial plane which trends in

30

Figure 4.28 – a field

photo at NG 61661

18157 showing the

jointing in the

rhyolite (scale:

Jointing

approximately a north-east-east direction. Undulose extinction in quartz minerals were found in the

Torridonian sandstone suggesting it was, along with the Durness dolomitised limestone, subjected

to the compressive stresses which produced the anticline.

31

Figure 5.1 – a photo from NG 61451 20295 showing an s-fold from the schistosity

in the dolomitised limestone (scale: pencil is 15cm long).

Figure 5.2 – a photo from NG 61231 19069 showing an m-fold from the schistosity in

the dolomitised limestone (scale: pencil is 15cm long).

5.2 FaultsTwo separate normal faults were identified in the southern sector of the geological map (appendix

A). At NG 60503 18047 the normal fault was identified due to the displacement that was caused to

the interbedded sandstone and sandstone. A displacement of 130m is seen to affect the interbedded

sandstone and conglomerate on the geological map. The hanging wall consists of the units to the

east of the fault line including quartzite and interbedded sandstone and conglomerate. On the other

hand, the foot wall consists of the units to the west of the fault line which include the limestone,

Durness dolomitised limestone and interbedded sandstone and conglomerate.

The second normal fault located at NG 60652 18156 was identified due to the planar contact

between the Torridonian sandstone and quartzite. The hanging wall is the Torridonian sandstone

whereas the quartzite is the footwall.

The Kishorn Thrust fault is located in the southern part of the map (appendix A) and is part of the

Moine thrust zone which was formed during the Caledonian orogeny. It was first distinguished by

the unconformable contact between the Torridonian sandstone and the Durness dolomitised

limestone. Additionally, a rapid rise in elevation defines the line of contact with the sandstone

sitting higher than the dolomitised limestone (figure 5.3). Therefore, the older Torridonian

sandstone has been thrusted up onto the younger Durness dolomitised limestone. Cheeney and

Matthews (1965) also suggest that a foreland is present around the Torridonian and Cambrian

lithologies (figure 5.4). The foreland was formed by both normal and thrust faults, when the

Caledonian orogeny took place.

32

Figure 5.3 – A digital elevation model showing the areas

of elevation along the terrain in the study area. The

Kishorn thrust fault (shown in the red box) in the southern

part of the map is depicted from the map (Digimap, 2015).

33

Key

(m):

6 DiscussionThe intrusion of the Beinn an Dubhaich granite has led to the contact metamorphism of the

surrounding country rock known as Durness dolomitised limestone. Contact metamorphism

occurred when heat provided by the granite came into contact with the dolomitised limestone. High

34

Figure 5.4 – a simplified map showing the location of the thrust faults, nappes and

Torridonian and Cambrian foreland as shown by the red box (Cheeney and Matthews,

temperatures along with low pressures characterize contact metamorphism and cause contrasting

chemical reactions which alter the mineralogy and texture of the surrounding country rock.

In the localities closest to the granite it is evident that forsterite marble is present. The forsterite

marble formed when the surrounding country rock was subjected to high grades of metamorphism

from the granite intrusion. The amount of forsterite minerals in the Durness dolomitised limestone

significantly increases to the point of contact with the granite. This is indicated by the sample

collected from NG 61425 19153 containing bigger and more forsterite minerals in comparison to

the sample from NG 61127 18486. At NG 61425 19153 the sample contained 1% talc and 50%

forsterite along with dolomite, calcite and micrite. On the other hand, at NG 61127 18486 the

sample was made up of 10% talc and 1% forsterite along with dolomite, calcite and micrite.

Forsterite was of a larger volume in the sample at NG 61425 19153 indicating an increase in the

temperature between the two localities. Evidently, there is a 49% increase in forsterite content and

9% decrease in talc content as the intrusion is reached.

The Durness dolomitised limestone has been subjected to progressively lower temperatures from

NG 61425 19153 to NG 61127 18486, as a result the metamorphic grade has decreased. Due to the

granite being 1km in diameter, figure 6.1 is used to illustrate a decrease in temperature in

correlation to an increase in distance from the igneous contact. Specifically, it shows that at the

point of contact with the host rock, the temperature was approximately 630C. After that at 0.5km

from the contact the temperature was 450C and at 1km it was 325C. This has led to the growth of

contrasting minerals at different distances to the granite, which is shown by the presence of more

forsterite at NG 61425 19153 and more talc at NG 61127 18486. Mason, R. (1990) found that the

chert nodules in the Durness dolomitised limestone brought SiO₂ (quartz) into the surrounding

country rock. New minerals were found in a sequence in the reaction rims with increasing grades of

metamorphism; talc, tremolite, diopside, forsterite, periclase and wollastonite (Gillen 1982).

However, the high forsterite content in the sample collected at NG 61425 19153 in the field proves

that periclase and wollastonite are not

part of the sequence.

35

According to Yardley (1989) the contact metamorphism caused a reaction between dolomite and

quartz which led to the early formation of talc in impure dolomite. This reaction occurred in the

sample from NG 61127 18486 because it shows the formation of talc which is caught in the process

of altering from dolomite. The reaction is shown by the chemical equation: dolomite + quartz +

water = talc + calcite + 3CO₂. After an increase in the metamorphic grade, tremolite is formed and

is expressed by the chemical equation: dolomite + quartz + water = tremolite + calcite + 7CO₂. Further increases in the metamorphic grade form both diopside and forsterite respectively. The

former is expressed by the equation: dolomite + quartz = diopside + 2CO₂ and the latter is

expressed by: diopside + dolomite = forsterite + calcite + CO₂ (Mason, 1990). The latter reaction

has occurred in the sample from NG 61425 19153 because it possesses a 50% forsterite content.

According to Hoersch (1981) four zones of increasing temperatures are recognized with talc-

bearing zones forming at 350-425C, tremolite-bearing zones forming at 425-440C, diopside-

bearing zones forming at 440-520C and forsterite-bearing forming at 520-600C. It can be derived

from the reactions which form the sequence of new metamorphic minerals that with increasing

metamorphic grade, progressive decarbonation and hydration occurs (Gillen, 1982). This shows the

sequence of reactions experience an influx of water and a loss of carbon dioxide. The reactions also

show that dolomite is being replaced by calcite which is caused by its reaction with quartz

suggesting the process of dedolomitisation has occurred. Gillen (1982) states that low grade

minerals such as talc and tremolite are hydrous and higher grade minerals such as diopside and

forsterite are anhydrous. Figure 6.2 shows the triangular diagrams which illustrate progressive

decarbonation in the sequence of minerals towards the granite. As a result, metamorphic aureoles

36

Figure 6.1 – Temperature against distance from igneous contacts

(Gillen, 1982).

are formed which are separated by isograds that indicate a change in the index mineral in the

Durness dolomitised limestone. This coincides with the British Geological Survey Map Viewer

(2015) which shows that the sample at NG 61425 19153 is part of the Kilchrist member group and

the sample from NG 61127 18486 is part of the Lonachan member group.

Alteration has occurred in the Beinn an Dubhaich granite sample with plagioclase altering to

sericite, biotite altering to chlorite and hornblende altering to biotite. Additionally, chlorite is

altering from pyroxene in the basaltic material and talc is altering to dolomite in both the

dolomitised limestone samples. The process of alteration is catalysed by the presence of

hydrothermal fluids which clearly exist in the form of quartz veins and chert nodules. Evidently,

chemical alteration of the mineralogy in the granite, basalt and dolomitised limestone implies

37

Figure 6.2 – Triangular diagrams which show the progressive

decarbonation throughout the sequence of new minerals towards the

Beinn an Dubhaich granite contact (Mason, 1990).

metasomatism has taken place and according to Emelus and Gyopari (1992) skarn minerals have

been produced. The skarn zones are grouped (table 6.1) with group 1 not containing any boron-

fluorine minerals and group 2 containing boron-fluorine minerals (Tilley, 1951). These skarn zones

are approximately 3 metres wide from the granite (Emelus and Gyopari, 1992). Curved and

irregular grain contacts exist along with incomplete alterations in the Beinn an Dubhaich granite,

indicating the rock has been caught in a state of disequilibrium. This implies that the rock did not

have enough time to reach equilibrium, before a change in temperature which occurred when the

granite reached the earth’s surface, causing it to cool quickly.

The granitic magma which eventually formed the Beinn an Dubhaich granite was produced by the

partial melting of the Torridonian sandstone below the earth’s surface (Merritt and Stephenson,

2006). This is evident because feldspar minerals such as plagioclase and k-feldspar are found in

both the granite and the sandstone. Analysis of the Beinn an Dubhaich granite sample depicts both

long and tangential grain contacts which evidently suggests its depth of burial was fairly shallow,

coinciding with the geophysical investigations by Goulty et al (1996). Specifically, the

investigations found the granite to extend to 1km in depth, as a result the origin of the granite must

38

Table 6.1 – Minerals that are present in the two groups of skarn

zones (Tilley, 1951).

be a sheet intrusion. Contact between the granite and the dolomitised limestone is steep, showing

the granite is a steep sided discordant stock intrusion (appendix C). Additionally, geophysical

investigations by Goulty et al (1996) interpreted the Beinn an Dubhaich granite as a steep-sided

stock. This falls in conjunction with the findings by Hoersch (1979), in which a Bouger anomaly of

+500 gamma was found suggesting a steep contact between the two lithologies. Consequently, the

inclusion of Durness dolomitised limestone within the granite intrusion suggests they are roof

pendants which have been forced up along with the granite stock intrusion. Evidently, the

dolomitised limestone has been subjected to high grades of contact metamorphism due to the

closeness of the lithology to the granite. Therefore, analysis of the dolomitised limestone sample

from NG 61425 19153 suggests the dolomitised limestone roof pendants contain a high forsterite

content and are a Kilchrist member.

The boundary between the Beinn an Dubhaich granite and Durness dolomitised limestone is shown

as curved and irregular on the geological map (appendix A). Upon the sheet intrusion of the granitic

stock, magma passed through the fractures present within the dolomitised limestone creating an

irregular boundary between the two lithologies. This fits into conjunction with Hoersch (1979)

theory which states that the irregular boundary “consists of many dike-like lobes which may have

intruded the limestone along pre-existing fractures”.

7 Geological HistoryThe low sphericity and sub-rounded nature of the grains shows the Torridonian sandstone hasn’t

undergone prolonged transportation and reworking. Also the long and sutured grain contacts prove

the depth of burial was fairly deep. Therefore, during the Neoproterozoic age, sediment was

deposited in braided river systems as planar-laminated and cross-bedded coarse sands, which as a

39

result of overlying pressure, compacted the sediments forming the Torridonian sandstone within the

Torridon Group (1000-970Ma). Specifically, it is part of the Applecross formation (Owen, 2006).

After this, during the Comley Epoch (528-508Ma) in the Early Cambrian, quartz-rich sediments

were deposited on a shallow, tide-washed marine shelf (University of Leeds, n.d.). The Caledonian

orogeny occurred between the late Cambrian (490Ma) and mid-Devonian (390Ma) forcing the

quartz-rich sediment to be subjected to regional metamorphism which is characterised by high

pressures and temperatures (The Geological Society, 2012). The result is the formation of

crystalline quartzite that is approximately 160m thick and is part of the Eriboll formation (British

Geological Survey Map Viewer, 2015). A time gap of approximately 442Ma shows there is an

unconformity between the quartzite and Torridonian sandstone.

The long grain contacts present in both samples of Durness dolomitised limestone indicate the depth

of burial was shallow. Additionally, a fine-grained nature proves the depositional environment had

low levels of energy. As a result, calcium carbonate minerals were deposited in the Arenig Epoch

(479-467Ma), along the passively-subsiding, continental margin of the Laurentian craton (Raine,

2009). Then at a later period, dolomitisation occurred before the Caledonian orogeny, replacing

calcite minerals with dolomite. The Durness dolomitised limestone is split into several different

groups depending upon their mineralogy. A time gap of approximately 29Ma shows there is an

unconformity between the quartzite and dolomitised limestone. After that, the Caledonian orogeny

formed the Moine thrust zone which in turn produced the Kishorn thrust fault. Therefore, after the

deposition of the Durness dolomitised limestone, the Kishorn thrust fault forced the Torridonian

sandstone up to sit above the dolomitised limestone. Later, folding occurred, producing an anticline

which is shown by the plunge and trend measurements in the dolomitised limestone.

The deposition of fine-grained sediment along with coarse pebbles formed the interbedded

sandstone and conglomerate. According to Trewin (2002) the sediment was deposited along the

faulted western margin of the minch basin. This specific interbedded sandstone and conglomerate is

part of the Stornoway Formation which formed in the Late Permian – Early Triassic (271-246Ma).

Due to a time gap of 196Ma between the dolomitised limestone and interbedded sandstone and

conglomerate, there is an unconformity. Normal faulting also occurred producing the fault located

at NG 60651 18154.

In the Hettangian – Sinemurian (200-190Ma) the limestone, limestone with interbedded sandstone

and mudstone were formed. The Breakish formation limestone with interbedded sandstone is

40

overlain by Ardnish formation mudstone (Earthwise, 2015a). The limestone that is part of the

Broadford Beds Formation (Digimap, 2015) then overlies the mudstone. The deposition of

sediments (mud and sand) along with the presence of calcium carbonate occurred in a shallow tide-

washed marine shelf. Ultimately the pressure from overlying units caused compression and

compaction leading the lithification of the units. After this normal faulting occurred again at the

grid reference NG 60451 18110 causing a displacement of 130m.

Volcanic activity in the Palaeogene (66.4-23.7Ma) caused an eruption of volcanic ash along with

angular fragments of lapilli and bombs, which vary in size. As a result, the Kilchrist pyroclastic

breccia was formed and in turn was followed by the mixed-magma intrusions termed the Kilchrist

hybrids which intruded through the Kilchrist pyroclastic breccia. After that, the Beinn an Dubhaich

granite intruded through the country rock in the form of a stock which brought up roof pendants of

dolomitised limestone. The granite formed from the slow crystallization of magma below the

earth’s surface and then intruded through the Durness dolomitised limestone along with the

intrusion of basaltic dykes and sills at a later period. These intruded through the Beinn an Dubhaich

granite and the dolomitised limestone. As well as this, rhyolite intruded through the Torridonian

sandstone, interbedded sandstone and conglomerate, limestone, mudstone and limestone with

sandstone.

8 ConclusionThe intrusion of the Beinn an Dubhaich granite caused the Durness dolomitised limestone to be

subjected to contact metamorphism, forming new minerals and textures. Forsterite marble was

41

identified in localities closest to the intrusion suggesting the Durness dolomitised limestone is

subjected to high grades of metamorphism which led to a high forsterite content. At NG 61425

19153, the sample contained 50% forsterite and 1% talc compared to the sample at NG 61127

18486 which contained 1% forsterite and 10% talc. Evidently, the temperature increases towards

the granite intrusion along with the metamorphic grade. Therefore, the result of contact

metamorphism is a sequence of new minerals which form with an increase in metamorphic grade;

talc, tremolite, diopside and forsterite. Dedolomitisation occurred throughout the sequence because

dolomite reacted with quartz to alter back to calcite. Gillen (1982) states that low grade minerals

such as talc and tremolite are hydrous and higher grade minerals such as diopside and forsterite are

anhydrous. Progressive decarbonation and hydration reactions took place with increasing

metamorphic grade due to the loss of carbon dioxide and introduction of water. This then produced

metamorphic aureoles which were separated by isograds. The metamorphic aureoles indicate a

change in the index mineral within the dolomitised limestone which is shown by different members

of the Durness group. The sample from NG 61425 19153 is a Kilchrist member whereas the sample

from NG 61127 18486 is a Lonachan member (British Geological Survey Map Viewer, 2015).

Minerals in the Beinn an Dubhaich granite, Durness dolomitised limestone and basalt have been

caught in the process of alteration suggesting along with curved and irregular grain contacts, they

are in disequilibrium. Alteration occurred because of the presence of hydrothermal fluids which

exist in the form of quartz veins and chert nodules. This essentially concludes that metasomatism

has materialised, producing two contrasting groups of skarn zones which measure up to 3 metres

wide (Emelus and Gyopari, 1992). Group 1 does not contain boron-fluorine minerals and group 2

consists of boron-fluorine minerals (Tilley, 1951).

Geophysical investigations by Hoersch (1979) show a + 500 gamma Bouger anomaly which implies

the granite is in steep contact with the Durness dolomitised limestone. Therefore, the Beinn an

Dubhaich granite has been intruded through the surrounding country rock suggesting the existence

of a steep-sided discordant stock intrusion. Long and tangential grain contacts along with

geophysical investigations also show the granitic stock extended to a depth of 1km. An irregular

boundary appears between the granite and dolomitised limestone which was produced by granitic

magma cooling within the fractures found in the dolomitised limestone. Roof pendants consisting of

Durness dolomitised limestone country rock are found within the Beinn an Dubhaich granite. The

roof pendants were subjected to high grades of metamorphism, therefore analysis of the sample at

NG 61425 19153 suggests they consist of a high forsterite content and is a Kilchrist member.

42

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10 Appendices

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