2. 2
Study on the Morphology and Formation mechanisms of
Metabasite outcrops on the Ross of Mull.
Lucas Jacobs
Department of Geology, Meston Building, Aberdeen University, Scotland.
Received 29/01/16
Abstract: The lithological sub groups in this report have been described through outcrop observation
and hand lens analysis in the field. The Moine stratigraphy has all been metamorphosed to amphibolite
facies and there have been two deformation events, δ1
produced the main folds shown in cross section
and parasitic folds of all sizes. This folding regime’s axial planes are orientated NE-SW. The second
event δ2
has created large kinks, their axial planes are perpendicular to δ1
making the fold hinges plunge
NE or SW alternatively as you move inland. The geological history for this report has been interpreted
from field work. The metabasite intrusions are of interest because they have previously had very little
morphological investigation. Garnets found within the metabasites become larger towards the edge of
each intrusion, this could be due to initial fractional crystallisation. Other features further investigated
within this report are the mineral lineation orientations and fracture analysis data. Previous literature
about the Ross of Mull has never focused on the metabasite lithology as this report does. Absolute
timing of metabasite emplacement is not fully constrained however intrusion of these features happened
early on in the history of the Ross of Mull Moine.
1. Introduction
The Ross of Mull located on the southern arm
of the island of Mull, the section we are
concerned with for this report is within the red
box shown on Map 1. This stretch of coastline
between Scoor and the headland of Rubh’
Ardalanish has had a complex geological
history, combining igneous and metamorphic
processes to become as it can be seen in
outcrop today. This report focuses first on the
individual lithologies in detail and then gives
an appreciation for the wider structural setting
and geological history. The focus there after
is on investigating the formation mechanisms
of metabasite outcrops and the relative timing
of their initial intrusion. Using analysis of
mineral lineation orientation data, garnet size
distribution, and fracture analysis data to gain
a further understanding of these characteristic
features which are an intrinsic yet relatively
unexplored part of the Ross of Mull Moine.
Map 1: Diagrammatic map showing Scotland and
Mull with the area that was mapped for this report
in the red box. (after reaserchgate)
3. 3
2. Rock Units / Lithostratigraphy
Moine Metasediments
For the purpose of map piece, the Moine
metasediments including Pelites, Psammites
and Calc-silicates have all been mapped as
one unit because they are in succession,
closely interbedded in constantly varying
ratios and thicknesses.
1. Pelites / Schists:
Lithology: These rocks are greyish and
sparkling in colour. The rock has
recrystallised entirely suggesting that it is of
medium to high grade regional metamorphic
facies. The individual mica flakes are clearly
visible without a hand lens, suggesting that
these pelites are of schistose grade. The micas
present are both muscovite and biotite. It is
these micas that create the schistosity of the
rock and give the pelite its name, Muscovite-
Biotite Schist. Across the vast majority of the
area that was mapped from Scoor House west
to Aird Dubh the pelitic layers have very
similar crenulation cleavage formed within
each layer as shown in Figure 1.
To the west into Ardalanish bay, within the
area effected by contact metamorphism from
the Ross of Mull Granite the pelites have
taken on a slightly different characteristic.
Location 29.0, NM(3772,1876), while still
full of platy muscovite grains there is an
added component of white translucent
crystals. These appear to be quartz produced
in situ over a long period of heating,
distributed unevenly throughout the pelites as
small lenses and not laterally continuous. At
Location 29.3, NM(3754,1885), pelites make
up 90% of the outcrop. This location is also of
interest because it shows kyanite and
andalusite crystals up to 4cm long in hand
sample, and even sillimanite in thin section.
This and a number of other outcrops in a
linear pattern towards the NE all display
kyanite crystals.
Outcrop example: Location 38.1, NM(3938,
1903) is a great example of a pelite dominated
fold structure with a thin band of psammite
surrounded by pelite showing an axial planar
foliation. There is evidence at location 35.0,
NM(4158,1903) of organic rich material still
preserved in the pelite. It is in the form of
dark, elongate nodules of around 6cm long
with microscopic grain size.
Contacts: The only boundary that the pelite
has with another rock type in this area is with
psammite these are relatively sharp contacts,
easily distinguishable by the naked eye below
the high tide mark where the sea has
weathered the rock. However, above the high
tide mark distinguishing the two becomes
increasingly difficult.
2. Psammites:
Lithology: These layers in the Moine
sequence are of predominantly sandy texture
due to the high proportions of quartz. Biotite
and muscovite are still present however in
such small quantities that they do not produce
a fabric. In the west of Ardalanish Bay there
are layers of granoblastic quartzite but these
are on such a small scale that they where not
mapped. The quartz grains which make up the
psammite are between fine and medium grain
size, they have been recrystallised through
dissolution during metamorphosis to create a
045/55 NW
045/68 SE
NWSE
Short axis Angle of
BeddingCrenulation
Pattern, muscovite
grains
Crenulation Cleavage in Muscovite rich pelites
7mm
Figure 1: Crenulation cleavage in muscovite rich
pelites
4. 4
tightly interlocking grain structure. There is
also evidence of fracturing within the
psammite beds where quartz has filled these
spaces to make veins.
Outcrop example: Location 2.5
NM(4164,1844), Psammite layer 2.4 meters
thick, this is by no means the biggest layer in
the mapped area, but it is a good example of a
well defined psammite bed between pelitic
layers.
Contacts: Psammite is the main lithology in
the contact with the Ross of Mull granite
(which will be described shortly). This
contact is sharp in places, yet can be gradual
and unclear in others, where the granite
melted the metasediments in situ. The
psammites are also interbedded with pelitic
layers and commonly have calc-silicate bands
within.
Thickness: Highly variable from tens of
meters across to a few centimetres.
3. Calc-silicates:
Lithology: Outcrops of this lithology were
always found as a concentrated band within a
psammite bed. After closer inspection of a
hand sample it is possible to see hornblende,
quartz, grossular garnets and biotite. The calc-
silicate bands where discovered to come in
many different morphologies from large pods,
to entire bands, some showing folds, or small
elongate lenticles and wisps. Calc-silicates
such as these could form due to areas of high
calcite concentration within the sands during
diagenesis.
Outcrop Example: Calc-silicate m-fold at
location 30.0, NM(3910, 1882), in the centre
of Uisken Bay. See Figure 2, it would
originally have been enclosed in psammite
but weathering has left it exposed.
Contacts: Contact is only ever with
psammite, the calc-silicate can show either a
positive or a negative relief within the
psammite bed, it appears to vary from one
location to another.
Thickness: The maximum thickness of a calc-
silicate band was 8cm, average thickness was
5cm.
Metabasite Intrusions
Lithology: This rock contains noticeably red
garnets these are aluminium rich almandine
and pyrope garnets. The main body of the
outcrop is dark in colour caused by the
mineral hornblende. The hornblende minerals
are not aligned as the micas in the pelite beds
are, but do still produce a definite foliation.
Small white, narrow ellipsoid shaped
lineations show the preferred orientation of
elongate minerals, and are present in varying
quantities. These are likely to be plagioclase
feldspar lineations due to the basaltic protolith
of the intrusions. The total recrystallisation of
these igneous intrusions suggests medium to
high grade regional metamorphism, most
likely to garnet amphibolite facies. Using the
previous observations, I will classify these
distinctive outcrops as Garnet-hornblende
schist (amphibolite facies). However, for the
purpose of this study I will continue to refer
to them as metabasites.
Outcrop example: A typical example of what
was found throughout the area is Location
4.4, NM(4045, 1886), all the features that
have been analysed in each metabasite
Psammite
Garnetiferous
Calcsilicate
Beach Sand
32cm
026/84 SE
21/204
20/205
205/90
030/81 SE
25/025
Psammite
Top of exposure
NW SE
Figure 2: M-fold of calc-silicate band. Orientated
facing NE in outcrop view.
5. 5
intrusion are clearly visible here. These will
be elaborated on later in the report.
Contacts: The metabasite shows sharp
intrusive contact mainly with the psammite
layers of the metasediments. On an exposed
surface the psammite often shows a pitted
surface where the garnets of the metabasite
once pressed against the adjacent layers of
rock.
Thickness: The average metabasite intrusion
thickness is about 4m.
Ross of Mull Granite Complex
1. Red Facies:
Lithology: This is the darkest sub facies of the
granitic complex manifesting itself in a deep
red colour. The orthoclase crystals are
abundant and so dark that it can be classified
as Mesocratic and phaneritic. The grains are
course and equigranular, and show no distinct
alignment of minerals. The minerals present
are; orthoclase feldspar (70%), quartz (26%),
and biotite (4%). The largest crystals present
are orthoclase of between 0.3 and 1cm across.
There are no small scale (breccia like)
fractures thus producing rounded outcrops,
however cooling fractures are present, these
have been preferentially eroded in places
where they are highly concentrated, forming
small hills and valleys.
2. White Facies
Lithology: This facies is in stark contrast to
the red facies, it is leucocratic and phaneritic
however the grains are smaller, of medium
grain size and angular. This facies contains
19mm individual plagioclase crystals so it is
inequigranular and porphyritic. There is no
alignment of the minerals, these include;
plagioclase feldspar (50%) which could
possibly be microcline, quartz (48%), biotite
mica (1%), and is some locations muscovite
is also present.
3. Pink Facies
Lithology: Outcrops of this type are found
close to the contact zone. The pink granitic
facies is leucocratic and phaneritic, the
mineral crystals are medium grained and
visible. The minerals present are quartz,
plagioclase feldspar, biotite mica, and a small
proportion of orthoclase feldspar, there is no
alignment of the minerals.
4. Granite as a whole
Contact: It has been observed that the red
granite is not found along the contact zone
only towards the centre of the pluton. The
pink facies is the primary lithology along the
contact zone with areas of the white granite
facies distributed almost randomly
throughout. The contact area is also full of
metasediments xenoliths which may have
melted in situ to form the pink eutectic point
granite facies.
Orthoclase Porphyry
Lithology: This rock is mesocratic and
phaneritic it is also inequigranular and
porphyritic. The matrix is medium to fine
grained and the minerals present are; quartz,
2cm
7mm
Figure: 3.1
Short axis
Figure: 3.2
Long axis
Figure: 3.3
Long axis
Figure 3.1, 3.2 and 3.3: Orthoclase porphyroblasts,
three examples of different orientations.
6. 6
biotite and hornblende, however the
porphyroblasts are orthoclase feldspar. There
is no clear alignment of the minerals and there
are about 5 tiny vesicles per cm2
. The
porphyroblasts are between 22mm and 2mm
along their longest axis, all of them appear to
adhere to a similar structural shape. The short
axis is very square (Figure 3.1) with a darker
pinky orange outer edge and a lighter centre.
The longer axis is a bit more varied, some
shaped like Figure 3.2 and others are more
rectangular as in figure 3.3.
Outcrop example: Location 15.1,
NM(3673,1753) is where this rock type was
first discovered and described.
Contacts: This lithology has a sharp, perhaps
intrusive, contact with both the granite and the
metasediments.
Thickness: It was not possible to determine
the thickness of this lithology from the
outcrop that was available.
Basalt Dike Swarm Formation
Lithology: The dikes are melanocratic, dark
grey almost black on a fresh surface, yet
oxidised brown on much of the exposed
surface due to iron content. Some of these
dikes have laterally continuous vesicles along
their centre, others have no vesicles at all. The
grain size is very fine or aphanitic, any
fractures present are perpendicular to the
length of the dike (span its width) like cooling
columns. All the dikes cut indiscreetly across
granite or metasediment xenoliths and
country rock alike.
Outcrop example: Figure 4 depicts a basalt
dyke cutting across the granite in location:
15.2, NM(3669, 1775). This one is exposed
for about 13meters and stays between 32-
28cm in thickness throughout its length.
Contact: Sharp, intrusive.
Thickness: Average is 30cm wide, however
very laterally persistent.
Large Horizontal Basalt Dikes
Lithology: These basaltic formations are the
result of a single event due to their cooling
columns running without interruption from
top to bottom. Their total thickness is 3.8m on
average and there is no evidence of
phenocrysts, the whole feature is
homogeneous. The colour of a fresh surface is
light brown and the weathered surfaces are
covered in patchy white lichen making this
lithology identifiable from some distance. A
moderate number of fractures are present,
either following the columns or perpendicular
to this. The horizontal basalt dike as a whole
cuts perpendicular to the metasediments and
shows cooled margins where the columns
have not formed on the edges of the feature.
The sediments where most likely tilted
upright prior to the intrusion of this basalt.
30cm
Basaltic
Dike
Granite
3meters
Metasediment
(highly fractured)
Metasediment
xenolithsFractures
SE NW
Figure 4: Basalt dike cross cutting granite and
metasediment xenoliths. Plan view.
Metasediment
Basalt
fractures
Contact
2meters
Cooled Margin
N S
Figure 5: Large horizontal basalt dike cross cutting
metasediments in outcrop. Vertical section
orientated facing E.
7. 7
Outcrop example: Location 3.1, NM(4140,
18260), is a good example of this lithology
showing it’s full thickness.
Contacts: This geological formation shows
very sharp, intrusive contacts with the Moine
metasediments, Figure 5 illustrates this
clearly.
Thickness: The horizontal basalt dike is
3.8meters thick on average.
3. Structural Setting
The area that is included in this report is the
only location on the Ross of Mull, or the rest
of the island, where the Moine Metasediments
are exposed as outcrop. They have been
through a complex structural geological
history. The metasediments are found across
the vast majority of the area mapped for this
study, they contain folds of many variations
and every scale. There are pelite rich isoclinal
folds, psammite rich concentric folds, and
small psammite bands folded within foliated
pelite rich layers. The metasediments are
closely linked with the metabasite intrusion
features and are cross cut by small basaltic
dyke swarms plus larger horizontal basalt
dykes. Moving further west there is a kyanite
baring pelite band that shows the regional
trend in the pressure/temperature conditions
reached by the Moine. Continuing west
intrusions of orthoclase porphyry are found
and finally there is the emplaced granite
pluton which covers the entire western section
of the map, its contact with the metasediments
running N to S.
Refer to figure 6 it shows the plunge and trend
of all the fold hinges for (which this data was
collected) across the entire metasedimentary
area. What it reveals is that all the fold hinges
plot on a parallel axial plane to one another,
along the bearing 020° to 210°. This is
indicative of a major structural folding event
(δ1
) with a shortening direction from the NW-
SE (120°-300°), perpendicular to the axial
plane of the folds. However, it is also
important to note that although the axial plane
of these folds is similar, the actual plunge and
trend is split into two sets, those dipping to the
NE and those dipping to the SW. To account
for this there must have been a second major
deformation event (δ2
) with its shortening
direction perpendicular to δ1
. Therefore the δ1
event created folds out of the previously
undeformed sedimentary layers forming the
mesoscale folds that have been mapped and
measured on the outcrops. Subsequently the
already corrugated layers where then put
under stress by the δ2
event. This created
macro scale folds (like kinks) that are to large
to notice at outcrop level. Figure 7 shows a
diagram of the interpreted, over all effect of
these two folding events through the
formation of a corrugated roof pattern.
Throughout mapping this area, the thickness
of the beds where never large enough to be
drawn accurately from reality onto a cross
section, so the layers drawn where
extrapolated from measurements taken across
Figure 6: Stereonet showing the plunge of fold
hinges across entire metasedimentary area.
8. 8
the section in the field. The area from Scoor
to Uisken Bay is characterised by near
isoclinal folding in the metasediments,
displaying many S, Z and M-folds in outcrop.
Between Uisken Bay and the granite contact
there are no more M-folds because the whole
area is located on the rising limb of an
antiform to the NW, however the S and Z
folds are found on an increasingly larger
scale.
The Ross of Mull granite has a very
complicated contact zone on its boundary
with the Moine metasediments but I have
mapped it’s furthest extent. The contact zone
is complicated simply because it contains
many xenoliths and rafts of metasediments,
some of which display melting of the pelites
to form eutectic point granite. Cordierite can
be found in the contact metamorphic zone,
and pegmatite dykes also present, these are
the remnants of the final stages of
magmatism. As mentioned before there is
evidence at location 29.2, (NM3754,1885) of
kyanite, andalusite and sillimanite being
present within the rock outcrop
simultaneously. The conditions necessary to
allow all three minerals to be stable were
therefore at a triple junction eutectic point on
the aluminosilicate phase equilibrium
diagram. This corresponds to a pressure of
0.45Gpa and temperature of 550°C. This band
of pelite continues to the NE, where it bares
kyanite alone.
4. Geological History
Sediments where deposited in a dynamic
shoreface environment this formed varying
thicknesses, and repeating units, of sand and
mud layers.
After burial and diagenesis, regional
metamorphism begins and through orogeny
the lithified sediments start to buckle and fold.
Now basalt intrusions form at the weak points
around the folds and through bedding planes
(the timing of this section is the subject of
further investigation, see part 5 of paper).
Metamorphism continues to intensify in
accordance to δ1
(shortening direction 120°-
300°) this is when the aforementioned
mesoscale folds are produced, their axial
planes orientated along 020°-210°. At the
same time the basalts recrystallise to become
metabasite.
Metamorphism now reaches amphibolite
facies and switches orientation to δ2
forming
the macro scale kinks that control the fold
hinge plunge angle and trend. It is possible
that this may also be when the kyanite band
formed due to the need for high pressures to
create the mineral. After this regional
metamorphism ends.
Magmatism begins under the metasediments
to the west, initially forming the orthoclase
porphyry facies. This magmatism continues,
and the granite pluton rises up through the
metasediments until it is no longer buoyant
bringing up rafts of Moine and orthoclase
porphyry with it from below.
The heat from such a granite body dissipates
over a long period of time. This allows it to
melt the pelites along it’s margins to produce
NW
SE
NE
W
Figure 7: Diagram used to illustrate the effect of
folding caused by deformation event δ1
(NW-SE)
and δ2
(NE-SW).
9. 9
eutectic point granite melt, which is white
with microcline of plagioclase feldspar. The
heat also spreads into the metasediments
causing contact metamorphism and the
production of the ternary eutectic point
containing andalusite, sillimanite and kyanite
at Location 29.3, NM(3754,1885).
During the last stages of magmatism wet
pegmatite builds up enough pressure to
fracture through some of the metasediment
rafts, forming very distinctive pegmatite dikes
almost a meter wide in places. Quartz veins
are also produced in large quantities.
After the granite has cooled and crystallised,
there is a prolonged period of uplift and
erosion, basaltic intrusions in the form of dike
swarms are emplaced. Finally, the area is
intruded one last time by horizontal basalt
dikes of a larger scale. Further erosion and
weathering concludes the geological history
of this area and brings us to the present.
5. Metabasite Study
Introduction
The focus of this investigation into the
metabasic (garnet-hornblende schist)
outcrops of the Moine is to determine the
relative timing of their emplacement along the
geological sequence. This was achieved by
observation of outcrop features and collecting
data from each location on the orientation of
mineral lineations, garnet size distribution,
and facture analysis.
Overview of Outcrop Locations
Location 4.4, NM(4045,1886): 3.8m wide,
linear in shape, shows bedding parallel
foliation and lineation. Figure 8 shows a
representative example of what the garnets
look like at the centre, intermediate zone and
at the edge of the outcrop. Figure 9 shows
Edge Garnets
Centre Garnets
N
Figure 8: Photos in plan view, mechanical pencil for
scale, top photo shows the garnets within 10cm of the
outcrop edge. Bottom photo shows the smaller garnets
in the centre.
Figure 9: Exposed, pitted contact surface between
psammite and metabasite.
Psammite
Metabasite
Contact surface is pitted by pressure
during metamorphism
Contact
W
10. 10
where the garnets have been pressed into the
psammite contact layer.
Location 5.0, NM(4028,1864): About 50m
wide, is blocky in shape, lineations not as
parallel, fracture data collected here. The
bedding of the metasediments has been bent
around this large intrusion during
metamorphism, like the foliation bending
around a garnet but on a larger scale.
Location 33.0, NM(4028,1878): 3.73m wide,
linear intrusion, shows evidence of minimal
melting possibly cross cut calc-silicate band
no evidence of psammite melting. Figure 10
shows brittle fracturing (either normal or
strike slip faulting), across the whole
intrusion with the surrounding sediments
bending around the offset.
Location 21.2, NM(4004,1868): In
juxtaposition to the brittle deformation shown
at the previous outcrop, this location (see
Figure 11) shows ductile deformation.
Alternatively, this could be due to intrusion
into previously heavily folded layers.
Location 34.0, NM(4017,1892): 3.98m wide,
linear intrusion that runs parallel to the strike
direction of surrounding bedding. There is
often quartz associated with the contact
possibly due to melting of pelite. This outcrop
also includes a boudinage train, it’s total
extension is 152.52% it’s original length.
Location 7.1, NM(3960,1872): 15.82m wide,
lineations and garnets are present, this
outcrop becomes wider towards the sea and
cross cuts some psammite beds. The
psammite shows baked contacts through a
change in colour and texture.
Location 29.0, NM(3752, 1871): 4.36m wide,
the metasediments show a clear deformation
around this outcrop, a psammite inclusion is
situated within the metabasite. The contact
around this inclusion is very gradual, this may
be because it is the only part that survived the
initial melting.
Location 41.0, NM3803,1856): 3.92m wide
this intrusion is linear however it bifurcates
and pinches out at the south west end.
Lineations and garnets are present.
Location 41.1, NM(3814,1834): Very small
metabasite intrusion just 1.35m across, it is
NW SE
Metabasite
3.73m
1.2m
Thinning of
Bedding
Left lateral normal fault if
bedding was horizontal, but
alternatively strike slip fault.
Psammite +
pelites
Figure 10: Metabasite outcrop that shows brittle
deformation with ductile deformation of
metasedimentary allowing the displacement.
Figure 11: Metabasite intrusion ductile
deformation or intrusion into pre folded
psammite layers.
W E
Folded
metabasite
intrusion
Concentration of
quartz
Psammite
Quartz
1.7m
Pelite
NW
SE
1.35m
Vertical
Psammite beds
Horizontal metabasite
outcrop, however it
cross cuts the
overlaying beds
towards the NE- The psammites drop down on either side of the
metabasite outcrop.
Figure 12: Field sketch of metabasite outcrop cross
cutting the bedding of psammite.
11. 11
linear and parallel with surrounding bedding
however the outcrop clearly cross cuts the
metasediments above it, as shown in
Figure12. The variation in the size of the
garnets across this outcrop is more gradual
than the abrupt change seen in some locations.
Location 41.2, NM 3809,1859: 4.02m wide,
linear outcrop containing foliation lineations
and garnets, marks the end of Ardalanish
beach.
Location 41.3, NM(3816,1831): This is a very
complex metabasite outcrop with many
branches and offshoots creating included
fragments of metasediment within the
intrusion. The long axis of the intrusion is
parallel to the strike of the bedding but the
beds are cross cut in many other orientations.
Figure 13 shows the cross cutting
relationships, it is possible to trace and
connect the fold on either side of the outcrop.
Location 26.0, NM(3886,1835): 5.8m wide,
shows all the basic foliation, lineation and
garnets. An interesting feature present here is
the preservation of an entire psammite layer
within the intrusion. It is now at a discordant
angle to the surrounding metasedimentary
folds. Figure 14 illustrates how the metabasite
surrounding the psammite band has allowed it
to react differently to the δ1
orientated stresses
experienced during the garnet amphibolite
facies metamorphism.
Orientation of Mineral Lineations
All feldspar lineation orientations where
measured on a planar surface so as to avoid
errors. Refer to Figure 15, these rose diagrams
show a strong correlation in the orientation of
the feldspar lineations in the metabasite
outcrops. The outcrops of widths below 4m
have such a high correlation that almost all of
SENW
029/72 NW
027/44 SE
Metasediment folds match up on
either side of the metabasite
Metabasite
3.5m
Figure 13: Field sketch of metabasite outcrop cross
cutting a synformal fold hinge, exploiting the
weaknesses in the metasediments.
SE NW
48cm
Thickening of
bed where it
foldssmaller garnets
larger garnets
psammite
Metabasite
Contact with main
psammite layers
Figure 14: Psammite bed included and preserved
within metabasite intrusion
Figure 15: Rose diagrams depicting the orientation
of feldspar mineral lineations. Comparing the
correlation of lineations in outcrops of different
sizes; outcrops of under 4m wide and over 4m wide.
12. 12
the feldspar lineations are within 20° of each
other. Amongst the outcrops wider than 4m
the lineations orientate around the same 20°-
40° NE as those in the outcrops below 4m but
have a wider variation of about 60°. As an
example from in the field the metabasite
outcrop at location 34.0 is 3.98m wide and all
the lineations are aligned to within 16° of one
another. On the other hand the data from
location 7.1 (a 15.82m wide outcrop) reveals
less alignment of to within only 30°. With
this information I suggest that during
metamorphoses and the recrystallisation of
these basic intrusions the dominant
compressive force, (δ1
) was from 120° and
300° (NW – SE). This compressive force had
a more intense effect on the elongate feldspar
grains in the narrower outcrops, where as
those in the centre of larger outcrops where
protected from the pressure. This orientation
is also consistent with the production of
mesoscale folds in the metasediments.
Garnet Size Distribution
For the purpose of this investigation into
garnet size and distribution I have created a
specialised graph, see figure 16 page 13. The
results show that garnets situated on the edge
of the outcrop (within 10cm of the contact
with the metasediments) are larger than those
in the centre. Not a single central garnet is
larger than 7mm (longest axis) and the
majority are around 1-2mm. The edge garnets
however have a minimum size of 3mm and a
maximum 44mm, the vast majority are
around 10-11mm. There are two outcrop
locations (6 and 1 on the graph) where I
managed to differentiate an intermediate zone
and in both of these cases the results show a
transition from small to larger garnets, from
the centre to the edge.
Therefore, a relationship does exist between
the position on the outcrop and the size of the
garnets produced. This relationship may be
related to each intrusion fractionally
crystallising inwards from the contacts, as it
occurred and cooled. Potentially allowing the
essential minerals for later garnet production,
such as aluminium, to be concentrated along
the edge. I had a hypothesis that the larger
metabasite outcrops would contain larger
garnets. However, in figure 16 the outcrops
are displayed in order from the narrowest to
the widest allowing the graph to disprove this
hypothesis due to garnet sizes staying very
constant throughout the Centre section of the
graph. Surprisingly across the Edge part of the
graph there are larger garnets found in the
medium sized outcrops.
Fracture Analysis
This fracture analysis was completed to try
and gain an understanding of whether the
Figure 17: Rose diagrams illustrating fracture
analysis data comparison between metabasite and
psammite outcrops.
13. 13
Figure 16: Graph of garnet size and distribution data. Outcrops studied for these results are explained in the
key above. All garnets where measured at their longest axis with a ruler in the field at the outcrop.
Garnetlongaxis(m)
Outcrop location (1-14) and location on outcrop
Centre Intermediate Edge
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 146 1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
27
7
17
11
7
8
19
5
6 4
15
10
20
10
10
5
5
13
15
4 4
11
15
9 24
7
5 9
6
8
15 12
9
7
4
4
4
12
5
8
9
4
4
4
4
5
7
5
6
4 5
7
4
4
4
4
5
4
4
4
5
4 7
8 4 5
5
5
8
6
6
6
Insufficient
data
Insufficient
data
Metabasite Garnet Size and Distribution
Key: =one garnet recording
Outcrop nembers are in order of size from smallest to largest
1 = Location 41.1, 2 + 4 = N.A, 3 = 41.3, 5 = 33.0, 6 =4.4, 7 =41.0, 8 =34.0, 9 =41.2, 10 =29.0, 11 = 26.0,
12 = 42.0, 13 = 7.1, 14 = 5.0
m)
14. 14
metabasite has experienced the entire, or just
a part of the folding and fracturing history
experienced by the metasediments. Refer to
Figure 17 these rose diagrams show the
fracture pattern of the metabasite outcrop at
location 5.0 and the psammite layer adjacent
and in contact with it. The results show that
the two lithologies have the same overall
fracture orientations, but the psammite shows
the majority of
fractures in this layer are perpendicular to the
main regional compressive force (δ1
). The
bedded psammite and pelite layers
surrounding the metabasite have taken the
strain through flexural slip, flexural flow, and
fracturing in the orientation mentioned above.
On the other hand, the unbedded metabasite
has been put under shear stress this is
supported by the X shaped fracture pattern
shown in the rose diagram. The two
lithologies may have reacted differently to the
compressional regime however it could be
that the metabasite displays less fractures in
the orientation of 130°-330° due to having
intruded part way through the δ1
kinematic
deformation event.
Summary and Conclusion
Evidence from the metabasite outcrops
overview supports the conclusion that their
protolith was certainly of basic igneous
composition. This intruded in the form of
sheets, sills and dikes that preferentially
entered the metasediments at weak locations
such as into fold complexes, and along
bedding planes. The fracture analysis results
suggest that this happened after diagenesis in
the early to middle stages of metamorphism,
when folding had already begun. The basalts
where subjected to continued metamorphism,
reaching garnet-amphibolite facies. This
caused recrystallisation, forming hornblende
and feldspar lineations parallel to bedding,
the wider the intrusion the less aligned that the
lineations become due to being subjected to
reduced compressional force from δ1
.
Almandine and pyrope garnets where
produced during this recrystallisation, the
position and size of which was controlled by
the fractional crystallisation of the initial
basalt features.
5. Discussion
The work in this report up until this point has
been entirely my own analysis of the
geological area. Now I will investigate and
compare my own interpretations to those of
others in previously published reports.
Compared to Holdsworth (1987) who mapped
the whole Ross of Mull Moine inlier the
reduced area that was mapped for this report
is an issue because it includes only the
Ardalanish Stripped Formation (A.S.F.).
According to Holdsworth (1978) the A.S.F is
only one of 3 formations that make up the
Assapol Group. The adjacent Shiaba Group is
said to show the younging direction of this
metasedimentary sequence, a thing that I tried
and failed to find within the A.S.F. There are
some points on which this report is in
agreement with Holdsworth (1987) such as
the the presence of variably banded and
stripped pelite, and micaceous psammite, plus
isolated laterally persistent bands of kyanite
carrying pelite. We also agree on the presence
of calc-silicate lenticles within psammite
layers and that the garnetiferous amphibolites,
the equivalent of the Metabasites I refer to, are
normally concordant with metasedimentary
layering but locally cross cut the bedding.
Where Holdsworth (1987) and my reports
disagree is that the metabasites originated
only as intrusive mafic sheets, this may be
true for most, however some of them are far
15. 15
more batholith or dike related, such as
location 5.0 which is not concordant with
bedding. According to my field work the
metabasites did not experience all of the
structural history of the adjacent country
rocks, Holdsworth (1987) states that they
have, and that they are repeated around
mesoscale δ1
orientated fold closures. In
hindsight I must agree that there is evidence
around Aird Dubh to suggest the metabasite
outcrops repeat around fold closures.
However I am convinced that they where not
subject to all the kinematic events.
On the whole structurally there are two
kinematic events which Holdsworth (1987)
describes as happening before the δ1
event
that I suggested. The evidence for both of
these events appears to also have been
concentrated in the section off the east end of
my maps thus it was missed. The first event is
suggested to have created what I referred to in
my notebook as white translucent crystals but
which are actually quartz-oligoclase
segregations where the pelites have become
gneissose (Holdsworth 1987). To elaborate on
this, according to Barr (1985) these where
formed without partial melting which in its
self would require mid-amphibolite facies
metamorphic conditions. The second event is
said to be characterised by near isoclinal
folding, axial planer to δ1
however with
steeply dipping fold hinges. I did actually find
on outcrop, Location 35.1 (NM4126,1877)
that fits this description but I was unsure what
to make of it at the time. Therefore what I
described as δ1
kinematic event is equivalent
to Holdsworth’s third deformation event.
Subsequently what I suggested for δ2
kinks,
Dubey and Cobbold (1977) dismiss as simple
whaleback folds caused during the same
deformation event as the other folds.
All the other literature that I have read is only
concerned with either the emplacement of the
Ross of Mull granite or the contact
metamorphic aureoles associated with it,
displayed by the outcrops baring kyanite,
andalusite and sillimanite. There is no
literature available which analyses the
metabasite outcrops of the Ross of Mull
Moine in the same detail as this report. They
are only ever mentioned briefly as an
accessory lithology to the Ardalanish
Stripped formation.
6. Summary and Conclusion
There are a number of things that could be
improved upon, if the study area were to be
expanded for example it could pick up on
earlier kinematic folding events. The use of
thin section data would have been incredibly
advantageous and in hindsight a larger
number of fracture analysis location should
have been undertaken. On the other hand,
compared with existing literature this is
currently a relatively detailed investigation
into the secrets of these slightly unusual
metamorphosed basaltic intrusions. With
regards to the metabasite study the literature
suggests that they intruded as basic sheets and
have experienced the entire kinematic history
of the metasediments. My field results have
some evidence to support a slightly later
emplacement, resulting in less fracturing
however this difference in the over all
sequence, is of little consequence and can be
dismissed. In conclusion to this report there is
plenty more detailed work that could be
followed up on the metabasite outcrops.
Finally, this area of the Ross of Mull was a
challenging yet intriguing place to map and
write up on.
16. 16
7. Bibliography
Barr, D. 1985. Migmatites in the Moines. In:
Ashworth, J.R. (Ed.), Migmatites. Blacker
and Son, Glasgow. 225-264.
Dubey, A.K. and Cobbold, P.R. 1977. Non-
cylindrical flexural slip folds in nature and
experiment. Tectonophysics, 38, 223-239.
Holdsworth, R. E., Harris, A. L. and Roberts,
A. M. (1987) ‘The stratigraphy, structure and
regional significance of the Moine rocks of
mull, Argyllshire, W. Scotland’, Geological
Journal, 22(2), pp. 83–107.
Researchgate: Outline geological map of the
island of Mull, Argyllshire, Scientific Figure
on ResearchGate. Available from:
https://www.researchgate.net/figure/2278407
31_fig5_Figure-1-Outline-geological-map-
of-the-island-of-Mull-Argyllshire-western-
Scotland [accessed Jan 26, 2016]
