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UniversityofAberdeen

GeologicalMapProject2015-2016.GL4023

StudyontheMorphologyandFormationmechanismsofMetabasiteoutcropsontheRossofMull.

LucasJacobs

GeologyandPetroleumGeology(Fourthyear)

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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)

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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 Bedding Crenulation

Pattern, muscovite grains

Crenulation Cleavage in Muscovite rich pelites

7mm

Figure 1: Crenulation cleavage in muscovite rich pelites

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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 SE21/204

20/205205/90

030/81 SE

25/025Psammite

Top of exposure

NW SE

Figure 2: M-fold of calc-silicate band. Orientated facing NE in outcrop view.

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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

7mmFigure: 3.1Short axis

Figure: 3.2Long axis

Figure: 3.3Long axis

Figure 3.1, 3.2 and 3.3: Orthoclase porphyroblasts, three examples of different orientations.

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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 BasalticDike

Granite

3metersMetasediment(highly fractured)

Metasediment xenolithsFractures

SE NW

Figure 4: Basalt dike cross cutting granite and metasediment xenoliths. Plan view.

Metasediment

Basalt

fractures

Cont

act2m

eter

s

Cooled Margin

N S

Figure 5: Large horizontal basalt dike cross cutting metasediments in outcrop. Vertical section orientated facing E.

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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.

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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

WFigure 7: Diagram used to illustrate the effect of folding caused by deformation event δ1(NW-SE) and δ2(NE-SW).

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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.

PsammiteMetabasite

Contact surface is pitted by pressure

during metamorphism

Contact

W

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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 SEMetabasite

3.73m

1.2m

Thinning ofBedding

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

VerticalPsammite 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.

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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 SEMetasediment 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

MetabasiteContact 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.

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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.

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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.

Gar

net

long

axi

s (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 10

1

2

34

5

67

89

10

11

1213

14

1516

1718

1920

21

22

232425

26

2829

30

31

32

33

3435

36

3738

39

40

41

42

4344

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 DistributionKey: =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)

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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

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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.

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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/227840731_fig5_Figure-1-Outline-geological-map-of-the-island-of-Mull-Argyllshire-western-Scotland [accessed Jan 26, 2016]

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