STRUCTURAL AND METAMORPHIC GEOLOGY OF THE SADDLE AREA, WESTER ROSS AZ INVEREESS
by
Philip S. Simony B.Sc., M.Sc. (McNaster)
A thesis submitted for the Degree Ph.D.
Imperial College University of London 1963 The Saddle seen from the north. April 17, 1962. ABSTRACT
.An area of about 25 square miles,. southwest of Glen Shiel in
Wester Ross and including the "Saddle" (3317 feet) was mapped in
detail. The main rock types are micaceous and quartzo-feldspathic
schists of the Moine Series. Sheets and lenses of rocks which
resemble the Glenelg Lewisian and which occur at three main
structural levels are demonstrated to be Lewisian.
The rocks of the area were folded in three distinct phases which
are correlated with plylses 1, 2 and 3 of the Loch Hourn Region
(Ramsay 1960). During phase I the Moine and Lewisian became
interlayered by isoclinal folding and by sliding. It is tentatively
suggested that the Lewisian sheets have their roots to the northwest
of the area.
During phase II the rocks were refolded into nearly isoclinal
folds about southeast plunging axes concomitant with amphibolite
facies metamorphism and the development of nigmatites in three zones.
The main zone is a concordant sheet more than one mile thick. Modes
of the granitic phase of the migmatite fall along the line separating
the quartz and feldspar fields in the granite-water system, suggesting
that the granite formed by partial fusion of the host-rocks.
In phase III the structural succession of phase II was refolded
into large box folds coaxial with the phase II folds and which have
nearly orthorhombic symmetry. The orientation of the axes of
maximum stress during phase III was calculated as having been nearly
horizontal with a N.E.-S.W. orientation. Metamorphic grade was dropping during phase III such that it has an early plastic stage and a later brittle stage.
Movement in the Strathconon fault zone occurred after phase III
and after the intrusion of the Ratagain igneous complex and associated felsite and lamprophyre dykes. The total horizontal movement was
Sirnstral and of the order of 2 or 3 miles. The vertical component of the movement was 1 or 2 miles with the northwest side having moved upw-ards.
A correlation of the Moine schist of the Saddle Area with the succession in Knoydprt (Ramsay and Spring 1962) is suggested. CONTENTS
Abstract Inge
I. INTRODUCTION A. Location B.Topography C.Previous Work 4 D.Purpose of the Investigation 5 E.Present Work 6 F.Acknowledgements 8 G.Summary of the General Geology 10
II. THE MOINE SCEISTS 13 A. Semipelite and Psammite 17 B.Pelite 25 C.Calc-silicate Bands 30 D.Garnet Amphibolite Bodies 33
III, THE LEWISIAN AND LEWISIANOID ROCKS 35 A.Leudsian 35 B.The Lewisianoid Rocks 36 C.Age of the Lewisianoid Rocks 54- D. Metamorphic Grade of the Rocks 60
IV. MIGMATITE 67
A.Description of the Granite 79 B. Origin of the Migmatite Complex 92 iv
C.Possible Temperature and Pressure During Migmatization 98 D.Pegmatite 102
V. POST —1,ETMORPHIC DYKES 106
VI. STRUCTURE 111 A.Faults 111 B.Folding 114 a)Folds of Phase III 114 b)Folds of Phase II 152 c)Folds of Phase I 176
VII. STRATIGRAPHY 184- VIII. SUMMARY OF CONCLUSIONS 188 V
FIGURES 1. The Location of the Saddle page 2 2. General Geology of the Saddle Area 9 3. Current Bedding at Achnangart 15 4, Crinkled Pelite, Glen More 31 5, Textures in Lewisianoid Rocks 42 6. The Loch Shiel Lewisianoid Body 53 7. Migmatization Map 68 8. Relation of Granite to Phase II Folds 75 9. Relation of Granite Abundance in Semipelitel Pelite and Psammite 78 10. Composition of Granite in the Migmatites 84 11. Plot of Q-Igb-Or Values of Moine Schists 86 12. Textures in Granite 87 13. Composition of Plagioclase from Migmatitic Granite 89
14. Distribution of Potential Trondhjemite in Pelitic Schists 99 15. Stability of Muscovite and the Ternary Minimum 100
.16. Hinge of IITTI in Choire nam Meirleach 117 17. ITPoles of Foliation and Schistosity Below Sgurr na Creige Pelite 119 18. Intersection of IIIB and IIIG Axial Planes With Sgurr na Creige Pelite 120
19. Phase III Minor Folds 123 20. Axes and Axial Planes of IIIE and IIIG Minor Folds 125 21. The Axial Plane of IIIL 128 vi
22. Calculation of Axis and Axial Plane of III' 130
23. Calculation of IIIL Axis 131 214.. Axis of IIIN, M aand A from II' Plot of Foliation and Schistosity 136
25. Calculation of IIIN, M and A Axial Planes 137 26. Axes and Axial Planes of minor Folds on the Limbs of IIIN, IIIM and IIIA 141
27. Thickness of Bands Folded by Phase III Minor Folds 143
28. Calculation of the Major Stress Axes 146 29. Displacement of Phase III Axial Planes by Faults 151 30. Axes of Phase II Minor Folds on the Limbs of the Choire Chaoil Folds and in Ground to the southeast 158 31. Phase II Minor Fold Axes on Either Side of the Slide : A'Mhuing 16o 32. Phase II Axial Lineations in Regions Below Slide 163 33. Phase II Minor Fold Axes in Glen Aoidhdailean 166 34. Phase II Axial Lineations Between Beinn Aoidhdailean and Strathconon Faults 168 35. Phase I Minor Folds 177 36. Section Parallel to Phase II Axial Trend. 179 37. Possible Time Relations Between Movement and Metamorphism 190 38. Profile Looking S.E. Down Plunge of Phase II and Phase III Folds 194 vii PLATES
The Saddle seen from the north. frontispiece
1. Current beading in Moine psannite. page 16
2. Lewisianoid migmatite at Loch Shiel. 48 3. Deformed Lewisianoid migmatite. Early stage. 49 4. Deformed Levvisianoid migmatite. More advanced stage. 49 5. Small augen of trondhjemite in polite. 71 6. Large augen of trondhjemite in pelite. 71 7. Granite lenses in semipelite. 74 8. Granite bands in psammite. 74 9. Row of small pegmatite pods with phase II fold. 104- 10. Pegmatite dyke parallel in attitude to IIIG axial plane. 104-
11. Phase II fold, Sgurr a Gharg Gharaidh. 169 12. Phase II fold. Druim na Firean. 169 13. Phase II folds. Glen Aciada=ilean. 170 14, Quartz veins folded by phase II folds. 170 15. Phase II granite rods parallel to axes of IIIG folds. 164 16. Granite layers in "similar" folds. 175 17. Granite layer in partly "concentric" fold. 175 I. II iRODUCTION
A Location The Saddle Area is located in the Northwest Highlands of Scotland
and straddles the county boundary between Wester Ross and Inverness. It is limited in the northwest by Glen Shiel, in the north by the Ratagan Forest, and in the southwest by Glen Aoidhdailean (Figs. 1 & 2). The south boundary joins the head of Glen Aoidhdailean and Glen More to
the Saciale, and then follows the county boundary along the watershed to Sgurr Beag. For convenience, the area is named after its highest and
best known mountain - the Saddle (3317 feet). That portion of the area lying in Ross-shire is readily accessible from the road A87(T) which follows Glen Shiel. The other portion lying to the west of the
watershed, in Inverness-shire, is more conveniently reached from the
road which, branching off the Glenelg-Mam Ratagan road, follows Glen More to Moyle. Geologically the area lies within the region of outcrop of the
Moine Series. Its north margin is some eight miles southeast of the outcrop of the Moine thrust, and the area as a whole lies just east of the western edge of the zone of "regional injection". The northwest edge of the area is crossed by the Strathconon fault zone, thus most of the area lies in the block south of the fault.
B._ Topography
The area has a total topographic relief of slightly more than 3000 feet. The main portions of the larger valleys are generally only a few hundred feet above sea-level and the crests of the main 2 3 ridges vary in altitude between 2000 and 3000 feet. In brief, the area consists of a series of such ridges left between glaciated valleys trending north-south, and which have at their heads, north facing corries. prom east to west these are; Glen Shiel, Choire
Chaoil,Choir' Uaine, Glen More and Glen Aoidhdilean. The largest and best developed of the corries is Choir' Uaine. From its floor at
2000 feet, the walls rise steeply to an arcuate ridge at about 3000 feet. The Saddle is the highest point on that ridge.
With the exception of the Strathconon fault zone which is topographically expressed as a marked lineament, the topography does not markedly reflect the geologic structure. The ridges connecting the peak of Faochag to Sgurr na Sgine and Druim a Choire Reidh to
Sgurr Beag, are strike ridges with steep, northward facing scarp slopes and more gentle dip slopes falling away to the south.
The tops and steep upper slopes of the ridges as well as the floors of some of the corries are generally well exposed. The more gentle slopes, particularly those that are nearly parallel to the dip of the foliation, such as the southeast slope of Sgurr Mhic Bharraich, are generally covered with grass and yield only small sporadic exposures. The floors and lower slopes of the larger valleys are, over much of their lengths, covered with drift, and with more recent scree and boulder fans. Terminal moraines can be recognised in the hummocky deposits of Choire ChaoilIabove the junction of Alt a'
Choire Chaoiland Alt a' Choir' Uaine; and in Glen More above the confluence of the Alt a' Ghleannain with the Glen More River. Up to about the 1000 foot contour the only exposures found in Glen More are those laid bare by the river and its tributaries.
On the ridge tops and on the corrie walls, the rocks are frost shattered and present very irregular, angular surfaces. The only places where rounded ice-polished and striated surfaces are common,
are the floors of some of the corries and locally on the floors of the
main valleys,
C. Previous Work
With the exception of the western margin of the area which is included in sheet 71 of the Geological Survey of Scotland, no work has been published concerning the geology of the Saddle Area. However, when completing sheet 71, C.T. Clough continued mapping beyond the sheet boundary, eastward to the county boundary. The writer was given the opportunity briefly to examine Clough's field sheets at the
Edinburgh office of the Geological Survey.
More recently, the ground to the west of the Saddle Area has been remapped in detail by Ramsay (1958, 1960), Sutton and Watson (1959), and Spring (1960), (Ramsay and Spring 1963). This work has proven
Clough's earlier contention that the Lewisian-like rocks at Glenelg are indeed a part of the pre-Moine basement. The recent work has demonstrated that the rocks have undergone three distinct phases of folding and metamorphism, all later than the deposition of the Moine
Series, and earlier than the movements on the Strathconon fault. By allowing for these superposed movements, Ramsay and Spring have erected a stratigraphic succession which is found applicable throughout the coastal area from Morar to Loch Duich. It is based 5 on the recognition that the strips of Lewisian rocks around Glonelg
are in the cores of first phase anticlines and that these basement cores are enveloped at least in part by a basal Moine politic schist.
In a number of localitio the order of succession can be verified from
the attitude of current bedding structures.
Dhonau (1900) mapped the Vivo Sisters of Kintail immefti the northeast of the Saddle Area, Ho accepted TnN Clifford's (1957) reconnaissance tracing of Ladhar Bhein polite which runs from
Knoydart across tc, head of Glen Shiel, and correlated the succession he established in. the Five Sisters with the upper three units of the
Knoyde.:(..-t; succession Dhonau tentatively correlated the main episode of folding and matamophism in the Five Sisters with the second phase of folding recognised. by Ramsay (1960) at lion21 hourn.
The Gleneig-Ratagain (Ratagan) Igneous Complex lying immediately to the north of the Saddle Ax-ea was studied in detail by Nicholls (;1951),
Be made some observations on the adjacent country rocks, noting in particular cummingtenite schists near Suardalain and small amphibolite bodies south of the Strathconon fault on Sgurr. Mhic Bharraich,
D, Paroose of the Investigation
Thu main objectives of this study are the follawing:-
(1)the determination, as accurately as possible, of the geometry of
the structures constituting the ground: C0707?Cd. by the map area
(2)elucidation of the tectonic history which includes at least three
phases of deformation, and to correlate these with the phases
established to the west of the Saddle Area 6
(3) determination of the time relations of the metamorphic and
tectonic phases
()+) determination of the shape, time of emplacement and petrology
of a migmatite complex which occupies the northern portion of
the area
(5)demonstration of the stratigraphic relationship between small bodies of rocks, resembling the Lowisian of Glenelg, and the more
dominant quartzo-feldspathic schists generally ascribed to the
Moine Series
(6)to gain some information concerning the mechanism of deformation
involved in the different phases of folding from the geometry of
the major and minor folds associated with them.
E. Present Work
A total of twelve months were spent mapping in the field; from
May to September 1961 and from April to October 1962. As no geological map was available for the Saddle Area on any scale, a reconnaissance map was prepared, using as base a one inch to one mile Ordnance Survey map photographically enlarged to 4 cm. = i mile (approximately q inches , 1 mile). This preliminary phase of the work was completed in three weeks and proved very useful in planning the more detailed research.
The detailed geological mapping was carried out using as a base,
Ordnance Survey six inch sheets in conjunction with R.A.F. low-angle oblique aerial photographs of approximately the same scale. Three methods of mapping were tried in the field: 7
(1)mapping ajrectly on the 0.S. six inch sheets, locating each
outcrop by resection with the prismatic compass and using the
photographs only as a guide in drawing the shape of the outcrops,
and as an aid in the search for outcrops in poorly exposed ground
(2)mapping ajrectly on the O.S. sheets on to which the outlines of
the outcrops had been plotted from aerial photographs with a
sketchmaster before going into the field
(3)mapping on a transparent overlay on the air photograph and
locating on the six inch sheets sufficient prominent features
with the prismatic compass, to permit accurate transfer of the
geologic anta from the transparent overlays on to the base map.
Method (1) was found to be accurate providing many points were located and that the location of the more important points was checked carefully. But the method is time consuming and particularly inefficient in areas of many small outcrops through which many geological boundaries have to be traced. Because of the 3000 foot relief and the variable, but not indicated, obliqueness of the photographs, the errors in locating outcrops with the sketchmaster were in many parts of the area significantly greater than the errors involved when outcrops were located by resection. Method (3) was found to be the most efficient; it was particularly well suited for tracing thin bands, where accurate relative location of points close to one another was essential. This method was used to map most of the ground.
The greater portion of the area was mapped at a rate of about
1V6 square mile per day. Portions of particular interest such as the north and west slopes of Sgurr Mhic Bharraich were mapped even more slowly and a small area northeast of Loch Shiel was mapped on a scale of one inch to 200 feet using chain, hand-level and prismatic compass.
The strip of ground between Alt an Choire Toitel and Alt an Fraoch
Choire was mapped at a rate of approximately* square mile per day because of its rather monotonous lithology and apparent structural simplicity.
While mapping in detail, the orientation of bedding, schistosity,
mineral lineation and of the axes and axial planes of minor folds. were
measured wherever this could be done accurately. In additions the relative proportion of discrete granitic material to host-rock was
measured on the exposures wherever this was possible.
The laboratory work consisted largely of the examination of rocks in thin section, with the following main objectives in view:-
1)identification and estimation of the composition of the constituent
minerals
2)determination of the modal composition
3)study of the textures, mainly as a means of discovering time
relations between mineral growth and deformation.
Slides prepared for modal analysis were etched with EF fumes and stained with a concentrated solution of sodium cobaltinitrite to facilitate rapid identification of untw±nned potash feldspar and plagioclase.
Acialowledzements
The writer wishes to express his gratitude to Dr. Gilbert
Wilson for supervising and encouraging his research and for critically
LOCH DUICH Fig. 2. General Geology • , • \ • •• . . trr an t • of the Saddle Area. •• arroich • • \•• • • N ( N Intrusive rocks Beinn a' • N N Chaoinich N Psammite
N. • • N. • Semipelite Pelite Lewisian and Lewisianoid
• • • • • Range of Calc-silicate bands • Aut a' h n n • ..", • • • • • Axial plane traces of PhaselI ...--7- / / and Phase III major folds • •• 411% Dip of axial plane •• ALc.\ T: r\ ai: 4F:ht./d t h: Ii. C. r.n . h- ...., - • 4‘i3 E IN / • r 49 / ‘10,. . 0. 1.g. /J - • -V- , ' 4 us 1.1.t. ----," t,, , . Faults CHA1 stitt.l. -_. 4:( Lt., 1.114. ---t • . ---, • . • • ,:e'r / I - • . e ../ 1 . County boundary • , .... .• . ___,_ • \C ; • .. mr•Tor, recv--,L:--___, / -.<. 717*-,--,-- O Libh: / .., , One mile -/ 7
,- 7.— .1.•••*" Meallan Odhar / u.r r Leac • •• tio ..•••••1•••. N Sgus nn v7h("nr)(0.7. • add I A • • • • • • • S un Ohorrauill M,' (4; aire Br a nit Bhric T4 .'\ "Mi. 7- • Druirr\ na Fireon • • Faochas` r AN • 'inn • I r :CA ORA H • /147, a'Choire • Reidh k Fr-noc 0 Sgurr‘n a Sgine -•- •• i Cl V ••• OC) Sgurr Cil la
Chaolais•, " 10
reading the manuscript. Much helpful advice was also received from Prof. H.H. Read, Prof. J. Sutton, Dr. J.G. Ramsay and Dr. I.
Carmichael. The many informal discussions that the writer had with his fellow students were a great source of help and encouragement.
The warm hospitality and helpfulness of the people of Letterfearn
and Glen Shiel, are here gratefully acknowledged. A. particular debt of gratitude is owed by the writer to his wife, Vera, for typing the thesis and for much help and encouragement.
In 1960-61 and in 1961-62 the research was supported by a scholarship from the Royal Commission for the Exhibition of 1851, and in 1962-63 it was completed under a NATO Science Scholarship granted by the National Research Council of Canada. The great financial help, without which this work could not have been done, is deeply appreciated.
G._ Summar of the General Geol„.9417- To place the more detailed discussion of the various aspects of the structural and metamorphic geology into a general framework, a
brief outline of the geology of the Saddle Area is given below and the main facts are summarised on the accompanying map (Fig. 2).
The dominant rock types are micaceous, qunrtzo-feldspathic schists (semipelite), quartzo-feldspathic schists (psammite) and garnetiferous
mica schists (pelite), generally referred to the Moine Series. Thin
sheet-like bodies composed of a suite of diverse amphibole and pyroxene- bearing schists and gneisses, not unlike those forming the Lewisian inliers of Glenelg, are found in three main zones; (i) on the north
and west slopes of Sgurr Mhic Bharraich, (ii) a belt passing from 11
Shiel Bridge, through the north slopes of Sgurr na Creige, Sgurr Lcac
nan Each and Druim na Firean, and (iii) a thin sheet extending from
Am Fraoch Choire, across Glen Shiel through the Glen Shiel Forest.
Because of their lithological similarity to the Lewisian rocks of
Glenelg they will be called Lewisianoid.
Both the Moine and Lewisianoid rocks are host-rocks to a migmatite
complex, and are also cut by concordant and discordant pegmatite
bodies. The main zone of migmatite occurs in an arcuate belt from 1 to
3 miles wide, extending from the head of Loch Duich to the northeast
slopes of Boinn Aoidhdnilean. This migmatite body has the shape of a
folded sheet. Another, much smaller sheet-like body can be traced from
Bad an Fhithich Mhoir to Glen More. Migmatite is also weakly developed
in the pelite that forms Sgurr Beag and Druim a Choire Reidh.
At least three phases of fol(9ing can be recognised in the Saddle
Area, all later than the deposition of the Moine rocks and predating
the movements on the Strathconon fault.
Large open folds were developed during the third phase of folding;
these can readily be recognised on the map (Fig. 2) and profile (Fig. ).
The largest structure is a box fold with two axial planes which dip
towards each other; one has a trace passing eastward from Sgurr Mhic
Bharraich to Glen Shiel, and dips south, the other approximately follows
Glen More and dips to the east. These open folds refold the axial
planes of earlier, nearly isoclinal folds, whose axial traces can
only locally be identified with confidence.
The more obvious of these early folds are: (i) the synform whose axial trace extends from Choire nan Laogh on the north slopes 12 of Sgurr Lhic Bharraich, over Sgurr a Gharg Gharaidh to Glen More;
(ii) the antiform, recognised by Dhonau (1960), which runs northwest through Sgurr an t-SF_tarraich, and the pair of folds whose axial traces extend from Glen Shiel, over Biot an Fhithich to the Saddle. These folds belong to the second movement phase with which the emplacement of the migmatite was also associated. The pegmatite veins however post-date the migmatite. It can also be shown that these (second) folds are in turn folding even earlier structures, but the geometry of these early isoclines and/or slides could only be elucidated in part. The folds developed in the second and third episode are approximately coaxial over most of the Saddle Area, and their axes plunge from southeast to south. The meagre data available concerning the folds which pre-date the second phase suggests that their axes generally have gentle plunges in directions markedly differing from those of the later structures. 13
II. THE MO= SCIUSTS
The most widespread rocks in the area are gunrtzo-feldspathic and
micaceous schists and gneisses similar to, and in part continuous with, those generally referred to the Moine Series in nearby areas. For
mapping purposes they are subdivided into three types in the field:-
Psammite:- mmrtzo-feldspathic granulite or schist with less than
106 mica
Telite:- mica schist and quartzo-feldspathic mica schist,
generally garnetiferous, with over 30% mica
Semipelite:- micaceous quartzo-feldspathic schists and granulites
intermediate in composition between psammite and pelite.
This classification does not take into account variations in the feldspar:uartz, or the K-feldspar:plagioclase ratio, as these ratios are
exceedingly difficult to estimate in the field. It is, however, roughly
the same classification as used by most other workers in the Moines.
Recent extensive modal work by Dhonau (personal communication) has shown
that this classification has been fairly consistently applied by different workers. The three rock types grade into each other and are interlayered
on all scales. This makes accurate presentation of their distribution
difficult even on a six inch map, because a certain amount of grouping
is involved even in plotting in the field.
The Moine schists of the Saddle Area only rarely present a uniform
aspect. Commonly banding (layering) is seen on all scales. This
banding may be obvious, such as the interlayering on foot to inch scale
of psammitic layers with semipelite on Sgurr na Ls-ire Brice, or it may
be very subtle, such as the fine lamination that results from the 14 interlayering, on millimeter to centimeter scale, of slightly more and less micaceous psammite which is characteristic of much of the psammite south of Me..4aan Odhar. Current bedding is locally outlined by the fine laminations in the psammite beds in this great psammitic and semipelitic group. These structures have been recognised in ten localities. The current bedded units are generally one half to two feet thick and the individual cross-laminae are about one quarter to one inch thick. The best examples were observed in a road cut on the A87T, 300 yards north of the Achnangart bridge. The field sketch shown in Pig. 3 was drawn in September 1961. During the following winter this road cut was blasted again to widen the road. Very good examples of current bedding can now be seen in three dimensions on large blocks; a photograph of one is shown in :Plate 1.
The psammitic unit immediately north of the pelite which caps
Sgurr Beag is well banded; the bands vary in thickness from one to six inches and in two localities about mile west of the Shiel River, concentrations of magnetite, epidote and sphene were found lying on the contacts between bards. These are probably sedimentary heavy mineral bands.
It would appear, from the observations above, that the widespread compositional banding (layering) in the Moine rocks of the Saddle Area is indeed sedimentary bedding. Such features as graded bedding, conglomeratic units, relic elastic textures and undoubted sedimentary slump structures have, however, not been observed.
In the succeeding section the individual Moine rock ty;,Jes and their distribution in the Saddle Area will be described. 15
CURRENT BEDDING AT ACHNANGART 16
Elate 1: Current bedaing in Moine .Dsammite.
Loose block in road cut at Achnangart. 17
A. Semi elite and Psammite
The most abundant Moine rock type of the Saddle Area is semipelite.
Next in abundance and commonly associated with it is psammite.
Microscopic examination of more than 50 specimens indicates that the only difference between semipelite and psammite is the abundnnee of micas and that there is a complete transition from the one rock type to the other. They will, therefore, be described together.
Both rock types occur either as thick mappable units or as thin interbedded strata within units of another lithology. The main bodies of semipelite are:
(1)the band comprising A1 Chrannag and the north slopes of Bad an
Fhithich Mhoir. It will be called the AlChrannag semipelite.
(2)the great body of semipelite underlying much of Sgurr Mhic
Bharraich, the Sgurr a Gharg Gharaidh ridge and both sides of
Glen More. It will be referred to as the Gharg Gharaidh semipelite.
(3)the band that extends from Beinn Aoidhdailean, over Sgurr na
Laire Brice, Sgurr Leac nan Each and Sgurr na Creige to AtMhuing.
It is well developed on Sgurr na Laire Brice and will be named
after that mountajn.
(4)the tongue of semipelite out of which the head of ChoirlUaine is
carved. It is well exposed on Spidean Dhomhuill Bhric and will
be called the Dnomhuill Bhric semipelite.
(5)the thin band covering the dip slopes of Faochag and thickening
westward to comprise much of Sgurr na Sgine. It will be termed the Paochag semipelite. 18
(6) the band extending northeastward from Creag nan Damh.
Semipelite, in the form of beds a few inches to several feet thick, is very common throughout the great thickness of psammite between Biot an
Fhithich and Creag nan Damh.
The units which are dominantly made of psammite are:
(1)the group of interbedded psammite and semipelite outcropping
between the Glenelg-Ratagain igneous complex and the Strathconon fault
(2)the dominantly psammitic unit extending from Glen Aoidhaniloan,
north of the Lewisian, across Choire SgamacWil to Glen More. It
will be called the Sgamadail psammite.
(3)the unit northwest of the Beinn Aoidhdailean fault on the southeast
slopes of Bad an Fhithich Mhoir
(4)the band of psammite that surrounds the pelite on the northwest
face of Sgurr Mhic Bharraich. It is well exposed on the west
side of Choire nan Iaogh and is referred to under the name of that corrie.
(5)the band of psammite that frames Sgurr Mhic Bharraich, extending
from the Strathconon fault southeastward to Alt Undalainj then
following the stream for 2 miles and finally turning westward up
the east slope of Sgurr a'Gharg Ghnraidh. It will be called the
Undalain psammite.
(6)the great psammitic group, with semipelitic interbeds,
constituting all the ground between AlMhuing and Am Fraoch Choire.,
It is the southwest continuation of the Main Psammite group
in the Five Sisters (Dhonau 1960). 19
(7) the band of psammitc running through the upper part of Am Fraoch
Choire and along the north slope of Druim a Choir° Reidh. It
will be called the Reidh psammite.
Of these psammitic units, only (4), (5) and (7) are essentially free of interbedded semipelitic material. Psammitic beds a few inches to several feet thick are not uncommon in the semipelitic as well as the pelitic units.
Ptammite and semipelite consist essentially of quartz, plagioclase, potash feldspar, biotite and muscovite. Epidote, sphene and apatite are almost invariably present but garnet and opaque minerals are rare.
Modes of a group of representative samples are given in Table I.
Plagioclase is largely untwinned and, in most specimens, is partly altered to sericite. Its anorthite molecule content appears to be limited between An and An with the plagioclase of most specimens 10 30 having compositions falling between An15 and An25. No systematic variation in the composition of plagioclase was found across the areaj the variations observed apparently resulting, at least in part, from fluctuations in the bulk composition of the rocks.
Potash feldspar is generally less abundant than plagioclase, particularly in the sernipelites, and in some specimens was found to be absent. It is generally unaltered, The grains tend to have irregular boundaries and they appear to occupy the interstices between quartz, oligoclase and mica. Most grains are not twinned on a scale discernible with the microscope, but in about half the slides examined a few grains, at least in part grill—twinned, were seen. Of the grains determined, all had 2V values higher than 70° and it would appear probable that 20
TABLE
Modes of Psammites and Semioelites
Rock No. Grid Ref. (az T47f1s. Biot. Must. Gnt. RII13e 9271 75 22.5 52.0 21.5 1.5 2.5 0.0 0.0 0.0 R1126a 9201 79 75.4 7.0 6.2 2.3 5.1 0.0 3.0 0.6 R111 9a 9211 78 48.6 31.7 6,1 9.0 3.8 0,0 0.8 0.0 RII 7a 9181 73 38.9 38.2 8.2 12.5 0.0 0.7 0.8 0.8 RIV 2 91 8152 44.6 37.2 2,9 12.4 0.0 0.0 2.7 0.2 RII13d 926174 47.3 31.1 4.6 11.0 5.0 1.0 0.0 0.0 RII 20 9251 82 79.2 1.8 1.6 5.9 8.3 0.0 2.9 0.2 RII13b 9281 75 39.4 38.5 4.4 15.3 0.3 0.2 0.0 1.9 RIV13 920146 41.9 32.9 5.2 11.9 6.5 0.0 1.5 0.3 R1125b 920183 70.0 5.4 0.8 11.1 8.2 0.0 3.6 0.9 R1122b 922183 73.6 1.3 1.2 9.4 9.4 0.0 4.7 0.5 RIV10a 928150 71.6 1.3 0.0 10.9 12.2 0.0 3.9 0.1 RII13c 9281 75 22.9 42.2 7.8 22.4 4.0 0.3 0.0 0.4 RII 5b 930175 19.8 47.5 5.0 15.8 6.4 0.0 5.0 1.5 B11 27 91 71 77 49.7 10.8 9.0 8.6 10.6 0.0 8.8 2.5 RIV 9a 930167 26.4 24.2 18.9 23.4 2.9 0.0 3.3 0.9 YI 3a 9141 76 37.2 20.1 1.9 17.3 15.4 0.0 6.9 1.2 RV 5b 928132 38.0 31.7 12.2 16.1 0.0 0.0 1.8 0.2 RVI 2 936131 48.5 21.3 18.2 5.1 3.7 0.0 1 .6 1.6 RVII1 96214.8 35.3 39.2 13.6 8.7 2.5 0.0 0.7 0.0
The first 17 modes were determined by T. Dhonau . 21
the untwinned potash feldspar is untwinned microcline rather than
orthoclase. Similar observations were made by Lambert (1959) at Morar.
In view of the complex variations in triclinicity and twinning that may
be present even within single grains (MacKenzie and Smith 1961), it is
best, with the meagre data available, to call this mineral by the
noncommittal term K.-feldspar.
MIrmekite was observed along contacts between K-feldspar and
'plagioclase in all rocks that have these two minerals, both from within,
as well as from without the zones of migmatization. It would appear,
therefore, that the development of myrmekite is not directly related
to the development of migmatite. Myrmekite was not observed on all
plagioclase - Kfeldspar contacts, but no specimen containing these two
minerals was found to lack myrmekite. Moreover, myrmekite was not
found in samples lacking K-feldspar. The plagioclase in -which the quartz
"worms" are present does not differ sufficiently in composition from
the rest of the grain for any difference to be optically detectable.
It would seem clear that myrmekite is in some way developed as a
result of reaction between adjacent plagioclase and K-feldspar grainsy
but no unambiguous textures were found in psammite or semipelite to
indicate .reulacement of plagioclase by K-feldspar or vice versa.
Myrmekite occurs either in cauliflower shapes apparently sprouting from
the plagioclase into the K-feldspar, or as a narrow strip, in the
plagioclase grain, parallel to the boundary with the adjacent K.-feldspar
grain.
Biotite is present in all the psammites and semipelites. With
the exception of certain garnetiferous, biotitic semipelites closely 22
associated with polite, all psammite and semipelite samples examined
have biotite pleochroic in dark brown and pale yellowish or greenish
brown. Fleochroic haloes have been observed, but they are rare. A
little chlorite along cleavage planes is ubiquitous and in some rocks
many of the "biotite" flakes are made up almost completely of pale green
chlorite with low, to anomalous blue birefringence colours.
Muscovite is less abundant than biotite in all the samples examined,
and in some psammites it is almost lacking. It occurs in two distinct
manners:
(1)closely associated or intergrown with biotite flakes, and having
the same orientation and size
(2)as poikiloblastic porphyroblast in the form of books 0.25 - 0.5 cm.
thick and 1-2 cm. wide, apparently with no preferred orientation.
These porphyroblasts are restricted to semipelite and micaceous
psammite south of the main migmatite complex.
Epidote is almost invariably present in the semipelites and
psammites of the Saddle Area. It is particularly abundant and well
crystallized in the rocks north of the Dhomhuill Bhric semipelite.
4idote is more abundant in the mica-rich layers and is commonly seen in
contact with, or included by, biotite. As is common with epidote
(Winchell 1951) birefringence varies even within a single grain. Some
grains are well zoned and have cores of altered material with low
birefringence possibly orthite, surrounded by zones having progressively
higher birefringence outward. Reversals in this sequence of the zones
have been observed. A few grains, twinned on (100) were seen. The
value of X'Ac varies between 2° and 2+0 and the maximum value of On 23 measured in each slide varies between .015 and .025 suggesting that the epidote is pistacite with 12 - 18 mol.% (Pe,Mn) end member (Deer, Howie and Zussman 1962). This compares well with observations made by
Lambert (1959) on similar Moine rocks in Morar except that in Morar some of the epidote is much richer. in iron than any in the Saddle Area.
hene and ,ppatitc are ubiquitous accessories in the psammites and semipelites of the Saddle Area, but garnet and opaque minerals such as magnetite and pyrite are rare. However, magnetite, epidoto and sphene are found concentrated together in the heavy mineral bands of the Reidh psammite at the southeast end of the area.
Two main types of psammite and semipelite occur in the Saddle Area: those rocks involved in migmatization and those that are outside the zones of migmatization. The unmigmatized rocks are typically fine to medium grained and even grained and have normally a granular texture.
The micas lie mainly in the schistosity which is parallel to the axial planes of phase II folds. Some mica, however, lies on the bedding planesi even where these diverge from the axial plane orientation. This can be clearly seen on the hinges of some minor phase II folds. A mineral lineation, resulting from quartz and feldspar being arranged in trains, and the parallel orientation of prismatic minerals such as epidote is relatively weakly developed. Quartz and plagioclase occur as approximately equant grains with simple boundaries. In quartz, undulous extinction is not uncommon, but only rarely is it strongly developed.
In hand-specimen the rocks vary in colour from glassy pale grey for quartz-rich psammites, to lustrous dark grey for biotitic semipelites.
Bands of micaceous semipelite or biotite schist are black. These bands 24 are not uncommon in the semipelite of Spidean Dhomhuill Bhric. In the
field these schists were mapped as "black pelite", but as their mineral
assemblage is more akin to biotitic semipelite than to that typical of
the pelites they are described here.
The rocks involved in the migmatite complexes differ from those
not so involved on the following main points:
(-0 Even those rocks in which no granitic material is present are much coarser grained than the equivalent rocks unaffected by
migmatization. This coarsening affects all the minerals, but is
particularly noticeable in the micas and it can be recognized in
the field.
(2)The orientation of the micas parallel to the phase II axial plane
schistosity is better developed than in the unmigmatized rocks.
(3)The quartz-feldspar lineation parallel to the phase II fold axes, described above, is better developed. Eloidote grains commonly
occurring as well-formed prisms lie with their long axes
(crystallographic b) sub parallel to this lineation.
(4)The grain boundaries of quartz and feldspar are somewhat more involved, and the grains themselves carry inclusions; in
particular, plagioclase has inclusions of small rounded quartz
grains. In many specimens, quartz was seen to occur in composite
grains flattened in the plane of foliation.
Many of the rocks, inside as well as outside the migmatitic zones, are weakly to strongly altered. Biotite is chloritized, quartz and. feldspar granulated and feldspar sericitized as well as being reddened with haematite. Joint planes, intersecting rocks so altered, are locally coated with haematite.
B. Polite
Of the three types of Moine schists in the Saddle Area, pelite is clearly the least abundnnt. It tends to be restricted to certain lithologic units and is therefore a more "useful" marker rock type than the psammites and semipelites.
The main units made up largely of polite, or those to which pelite is restricted, are:
(1)the pelite forming the top of Bad an Fhithich Mhoir which will be
called the Fhithich Mhoir polite.
(2)the band of semipelite with pelitic beds that forms the lower,
west-facing slopes of Beinn Aoidhdailean and Sgurr na Laire Brice.
As it lies in Glen Aoidhdailean it will be referred to as the
Glen Aoidhdailean semipelite pelite.
(3)the irregular lens of pelite on the north slopes of Sgurr Mhic
Bharraich; this will be called the Mhic Bharraich pelite.
(4.) the pelite covering the southeast, dip slope of Sgurr Mhic
Bharraich. This is a thin lens less than 100 feet thick and is
surrounded by semipelite.
(5)the belt of polite extending from Glen Shiel over Sgurr na Creige
and Sgurr Leac nan Each to the head of Glen More and the east
slope of Beinn Aoidhdailean. It will be called the Sgurr na Creige polite.
(6)the thick unit dominantly composed of politic rocks that forms
the ridge joining Sgurr Beag and Druim a Choire Reidh. Only 26
the lower 500 feet of this unit are included in the map area. It will
be refelu.,.id to as the Sgurr Beag pelite. It is a direct continuation
of Dhonau's (1960) easternmost politic unit and is thought to be the
continuation of the main pelitic horizon in Central Ross (Sutton & Watson 1962).
Two bands of polite, separated by somipelite enter the area
immediately to the south of the Strathconon fault. They pinch out
shortly after crossing the Aoidhdaiiean River, and the intervening
semipelite thickens to make up the whole band. On sheet 71 of the
Geological Survey of Scotland the entire band is mapped as polite but,
using the present classification, it is better described as a biotitic semipelite.
Bands of fine grained) but massive polite only a few feet thick,
are also found in the Main Psammite group in a few localities, such as
the ridge joining Sgurr na Creige to the Saddle or on Blot an Fhithich.
None of the six polite units listed above are made entirely of
polite. Interbeds of semipolite and psammite a few feet to several
tens of feet thick are common. The units are classed as politic
because abundant bands of garnetiferous pelite are restricted to them.
In hand-specimen polite varies in appearance from dull, dark grey
and massive, fine grained sandy loolito, through shiny dark grey,
coarse biotite, muscovite polite, to shiny, silvery grey muscovite rich polite. The pelites and certain clearly related micaceous semipelites consist essentially of quartz, plagioclase, biotite,
muscovite and garnet. Accessory apatite, spheno, magnetite and pyrite are ubiquitous. Epidote and potash feldspar are rare. Modes of 27 representative specimens from migmatized and unmigmatized polite are given in Table II.
Plagioclase anda=tz occur in approximately equal proportions and constitute about half the rock. The plagioclase is seen beneath the microscope as roughly equant grains, some grains being quite fresh while others are clouded with fine sericite flakes. About one third of the grains in each slide are twinned. The plagioclase in all the specimens examined is oligoclase which commonly has a composition falling between Ana) and Any).
Potash feldspar is characteristically absent or rare in the pelitic rocks and associated micaceous semipelites. A few grains, however, were observed in a number of sanples, particularly in more massive, sandy pelites. In about half the slides in which it was observed, at least some grains show grid twinning and the 2V ij commonly large 700-80°. Like the potash feldspar of the psammitic rocks, it is probably microcline, but will be referred to simply as K.-feldspar.
Biotite in all the polite samples examined is pleochroic in pale brown and dark reddish brown. Pleochroic haloes around inclusions of zircon, orthite and epidote are abundant. Biotite and muscovite are closely associated or intergrown and constitute between one third and two thirds of the rock.
Garnet varies greatly in abundance and size in the different varieties of pelite, but it is invariably present. It has the typical claret colour of almandine and inclusions are common, particularly in the larger grains. Inclusions of all the other minerals in the rock 28 TABLE II
Modes of Pelites and Associated Semi- elites
Rock No. Grid Ref. Qtz . Fla . K-fels. Blot, Musc Grit . Acc. YII 8e 907177 22,9 27.3 0.0 27.9 16.1 4..3 0.0 1.5 YII 8c 907177 22.3 31.0 0.0 24.4 20.1 1.6 0.0 0.6 YI 24c 910166 21.9 19.8 0.0 18.6 28.6 10.7 0.0 0.4 YI 2 914176 25.3 30.0 0.7 27.9 10.0 5.0 0.0 1.1 YI 27 914165 30.3 20.3 0.0 30.1 16.0 2.3 0.0 1.0 Y1 20a 909180 3665 18.9 0.1 24.1 17.9 3.3 0.0 1.1 YI 19a 91 01 82 4.041 16,5 0.0 21.7 18.2 2.3 040 0.9 RI 14.a 9441 80 25.5 28,1 0.2 15.1 27.7 1 .6 040 1.5 YII11 c 9051 77 33.7 20.2 1.3 25.7 16.6 1 .2 0.0 0.7 YVII2 901126 30.6 28.1 0.0 21.0 17.7 1.9 0.0 0.7 YV11 4 916137 34.9 36.2 0.4 13.4 11.2 3.2 0.0 1.3 YVIII2 91 21 29 15.8 11.7 0.0 14.8 44.8 12.6 0.1 0.2 YVI13 916138 29.1 27.9 0.4 14.9 16.8 10.2 0.0 0.7 YV 6 889143 31.5 40.2 0.2 14.7 11.5 1.6 0.0 0.2 111111 901125 32.4 25.4. 12.1 22.7 7.1 0.0 0.0 0.0 RIII18 943165 27.3 38.2 0.2 20.7 11.5 1.6 0.0 0.3 RIV1 9 921138 28.1 27.2 0.3 15.0 21.0 7.2 0.0 0.9 29
are found, but by far the most common are small, round inclusions of
quartz. The density of inclusions has in some grains a zonal
arrangement in which the core is crowded with inclusions, and is
surrounded by a zone essentially free of inclusions. Some crystals show
a marginal zone with inclusions and deep embayments of the adjacent
minerals. In such examples, the marginal inclusions are coarser than
the interior ones. Trails of inclusions are rare, but where they are
observed, they are parallel to the schistosity outside the grain, or to
the margins of the grain. Cracking and alteration of garnet was only
rarely observed.
Apatite, sphene, magnetite and pyrite are ubiquitous as accessories
in the pelitic rocks. Apatite and sphene occur as small, well-shaped
crystals; magnetite and pyrite are closely associated or intergrown
with biotite and garnet. Small grains of epidote were found in four
specimens, but in general epidote is not present.
All the migmatized pelites are coarse-grained with a marked
schistosity parallel to the axial planes of phase II folds, and the
micas tend to be concentrated into thin, discontinuous foliae which
define the schistosity. Quartz and feldspar are segregated between
these foliae. Garnet grains, 1 mm. to 5 mm. in diameter, are randomly
scattered throughout the rock and the micaceous foliae either abut
against the garnet grains or sweep around them. Quartz, which is
unstrainea or weakly strained occurs either as individual grains or in
aggregates flattened in the schistosity plane. The very micaceous pelite on the south side of the Sgurr na Creige pelite, lying at the
outer edge of the main migmatite complex, has a similar texture. 30
The more sandy and massive garnetiferous pelites at the head of Glen
More and in the Main Psammite are finer grained and less schistose because the mica flakes, though equally abundant, are disseminated and are not segregated into foliae.
The schistosity in the pelites, lying parallel to the axial planes of phase II folds, is locally thrown into small crinkles the axial planes of which are 3-10 'am. apart and sub parallel to the axial planes of phase III folds. The crinkles are commonly defined by a pattern of untwisted interlocking flakes of mica, but bent micas and micas lying in the axial plane of the crinkles are rare. This type of crinkling is analogous to that described by Johnson (1960) from the Dalradian and figured by Sander (1950, page 279) as illustrating paracrystalline folding. In a few localities, biotite and garnet in the axial plane zone of the crinkles are chloritized. Where crinkle axial planes come against garnet grains, the axial plane is seen to be deflected or the fold stopped, the fold pattern on the other side of the garnet being completely different, as is shown in Fig. 4. Quartz grains which occur at the crest of crinkles and are bent, now consist of a number of unstrained individuals.
C. Cale-silicate Bands
In a zone about one mile wide northwest of the Iewisianoid band of Am Fraoch Choire, there occur small bodies similar to rocks termed
"talc-silicate" elsewhere in the Moines, (Kennedy 1946), (Ramsay and
Spring 1962). These talc-silicate bodies occur in the form of lenses and bands one inch to one foot thick. Some of the bands can be traced 31
Figure 4
CIRIN 1: L D P 17, LITE, GLEN .,.ORE
Folded quartz lens
Luscovite and a little biotite
Garnet with quartz inclusions at centre
5 Lira. 32
for tens of feet along strike but, more commonly, they pinch out after
two or three feet. Lenses and balls are most common. In several
exposures it was observed that bodies with circular or elliptical shape
are, in fact, rods that extend several feet down their plunge, parallel
to the phase II fold axes.
The calc-silicate bodies consist of quartz, plagioclase, actinolite,
clinozoisite, garnet and calcite with accessory sphene, apatite and
pyrite. The quartz has simple boundaries, is untrained and elongated
parallel to the length of the rods. The plagioclase is strongly
altered andesine. The clinozoisite is colourless, has anomalous blue
birefringence colours with On - .010 and XtAc = -2° suggesting about
10 mel. % PeNn end member. Elongated grains have their crystallographic b-axes crudely parallel to the length of the rod. Actinolite occurs as long laths lying parallel to the length of the rods and filled with inclusions of small round quartz grains. The actinolite is pale green with pleochroic formula: X-very pale green, Y--greenish yellow and o Z-pale green. ZAc = 18° and 2V '' -70 . It is in part altered to pale green chlorite. Orange-red garnet occurs as large spongy grains, partly replaced by calcite.
Two other rock types which might loosely be grouped with
"calc-silicate" rocks occur in the Saddle Area. The one variety is an epidote, quartz, plagioclase rock with accessory apatite and sphene occurring in bands 2 to 2 inches thick, widely scattered in the psammite and semipelite of AtMhuing and Biot an Fhithich. The other type was found in a one foot bed on the ridge joining Sgurr na Creige to the
Saddle. The rock is a semipelite differing from that adjacent to it 33 in containing, besides several percent epidote, porphyroblasts of dark actinolite lying on the schistosity planes and elongate parallel to the phase II lineation.
D. GarnetAmehiboliteBodies
A few spherical and elliptical bodies of garnet amphibolite occur singly, and in groups, in the Sgurr na Creige pelite. The bodies vary in median diameter from 1 to 6 feet and, except for the spheroidal ones, they are elongate parallel to the axes of phase II folds. The schistosity of the polite sweeps around them, and they themselves show a marginal schistosity between 2 and 5 inches thick. In this schistose outer zone, large flakes of brown biotite and small grains of claret coloured garnet become increasingly abundant outwards. The interior of the bodies is coarse grained and massive, consisting of hornblende, quartz, plagioclase, garnet, and a little brown biotite with accessory spheno, apatite, rutile, magnetite and minute grains of zircon(?) in biotite. About half the rock is made of a greenish hornblende which has a pleochroic formula: X.-Tale yellowish green, Y-pale brownish green,
Z-dark grey-green. ZAc = 17° and 2V - 75°. It contains small inclusions of quartz, particularly in the outer schistose zone, and sphene is concentrated in clusters within it. Quartz is coarsely crystallic and interstitial with weakly undulose extinction and is more abundant than plagiocalse. Plagioclase is in part clear as well as altered and is complexly zoned, with a mean composition of about And.
Red garnet makes up 15 to 240 of the rock. It is riddled with quartz 34 inclusions which vary greatly in size. The garnet grains have a diameter of 2 - 15 mm. and locally form nearly pure garnet clusters 3 - 6 inches in diameter.
It is possible that the garnet amphibolite bodies represent fragments of basic intrusives that have broken, deformed and metamorphosed. Similar bodies have been reported from other parts of the Moine outcrop and locally their intrusive relation can be demonstrated (P. Clifford 1956). 35
III. THE LE\VISIAN MD LEWISIAROID ROCKS
A. Lewisian
The eastern end of one of the Glenelg Lewisian "inliers" (Geol.
Surv. Scot. Map 71) enters the western edge of the area in the lower portion of Glen Loidhdailcan. It is not well exposed and only its outer limits can be mapped with any confidence. The structural relations of the different Lewisian rock types within the "inlier" could not be deduced.
The north margin of the body is an apparently normal, premetamorphic contact and Ramsay and Spring (1962) interpret it as being the unconformity between the Lewisian basement and the Moine cover, All the other contacts are faults of Strathconon fault age.
The main rock typos in this portion of the Glenelg Lewisian are; hornblende schist, hornblende-biotite schist, banded hornblende gneiss and massive amphibolite. The rocks along the northern boundary of the outcrop are banded and flaggy hornblende schists. Along the south boundary the rocks are mainly banded amphibolitic migmatite sheared to various extents by the faulting. A line of outcrops which passes just north of Creagan LUbh, and possibly all belongs to ono continuous band, consists of diopsidic forsterite marble, diopside amphibolite and calcite-actinolite schist; and a one inch seam of graphite schist crops out immediately northwest of this lime-silicate band, near the river.
It consists of small flakes of graphite disseminated in a quartzo- feldspathic schist. Near the graphite schist is an outcrop of massive garnet-pyroxene amphibolite possibly derived from eclogite. 36
Northwest of Creagan aahh and north of the diopsidic bandi is a lens of kyanite bearing, garnet-rich and rusty-weathering mica schist containing bands of fine grained garnetiferous amphibolite about one inch thick, and occasional knots of quartz tourmaline and garnet.
Unfortunately, the kyanite cannot be unambiguously dated. With the exception of phase III minor folds and crinkles which clearly post-date the biotitegarnet-kyanite assemblage, there are no folds in the polite which can be dated and to which the kyanite can be related.
The kyanite grains are broken and in part altered to "schimmer aggregate"; biotite and garnet are replaced by large flakes of chlorite.
This retrogression might be the result of low to medium grade metamorphism imposed during Caledonian folding on the earlier Lewisian kyanite grade metamorphism, or it might be the result of phase III low grade metamorphism imposed either on a Lewisian or an earlier Caledonian assemblage. Moine polite and semipelite near the Lewisian are fine grained and lack kyanite; they appear not to have reached kyanite grade.
B. The Lewisianoid Rocks
Bodies of rock which lithologically resemble the Lewisian inliers of Glenelg occur in the Saddle Area. Their true stratigraphic position is not directly apparent and they will be referred to as "Lewisianoid" solely because of their similarity to the Lewisian.
The Lewisianoid rocks crop out in three main zones:-
(1) an irregular lens-shapedzone mainly within the Mhic Bharraich
polite, on the north and northwest slopes of Sgurr Mhic Bharraich.
It will be referred to as the Sgurr Mhic Bharraich zone. As will 37
be discussed in a later section, the small body on Sgurr a Gharg
Gharaidh may perhaps a3so belong to the Sgurr Mhic Bharraich zone.
(2)a great arcuate zone stretching across the Saddle Area from the
head of Loch Duich, through Loch Shiel, Sgurr na Creige, across
the head of Glen More to the head of Choire Sgamadail. As the
bodies of this zone are restricted to the Sgurr na Creige polite,
the zone will be named after that mountain.
(3)a thin, poorly exposed, but apparently continuous band extending
from Am Fraoch Choire across the Shiel River and the .A87T road
up Glen Shiel, into the Glen Shiel Forest. It will be called the
Am Fraoch Choire zone.
In addition, small bodies are found;
(4)in the Strathconon fault zone
(5)between the fault zone and the- Glenelg—Ratagain igneous complex
(6)on the east side of Glen Aoidhdailean, below Beinn Aoidhdailean.
The main zones are made up of individual masses which are flat lenses and sheets varying greatly in size. The largest is a sheet that can be traced continuously from Alt a Choire Chaoil, for four miles along the strike to the head of Choire Sgamadail, where it is truncated by a fault. At the head of Glen More the sheet reaches its maximum thickness of nearly 500 feet but in ChoirtUaine it is locally only a few feet thick. It is quite possible that the outcrops to the east of Alt a
Choire Chathiconnect this sheet to the one that can be traced continuously from Shiel Bridge to the ridge of A'Mhuing. If this connection is correct, the body is a curved sheet with a N.E.—S.W. extention of more than 5 miles (Fig. profile at back). A lamprophyre dyke, striking 38
N.N.W. and having a nearly vertical dip, at the head of the west branch of Alt a Choire Chaoil carries inclusions of hornblende gneiss, and if these are brought up from the down dip continuation of the Lewisianoia, then the sheet has a N.W.-S.E. extension of at least 3 miles and a surface area of at least 15 sq. miles. Some of the smaller bodies, such as those near the mouth of Alt Undn1ain, are only a few feet thick and cannot be traced along strike for more than a few yards.
Perhaps the most outstanding feature of the Lewianoid bodies, which sets them in sharp contrast to the normal Moine-6type units, is the great diversity of the rocks that constitute them. Each of the bodies consist of more than one rock type, but not all types occur together in each body.
The main lithologies may be grouped together into the following classes;
(1)marble, lime silicate rocks and pelite metasediments? -
(2)massive, melanocratic amphibolites - basic igneous rocks? -
(3)migmatite and hornblende gneiss
(4)flaggy, banded hornblende schist and hornblende-biotite schist.
Marbles and lime silicate rocks are not quantitatively very important in the Lewisianoid masses, but they do occur in several bodies of the three main zones. Marble, which is rare in the Moine outcrop, is of particular interest. Three bands of it have been found in the
Sgurr na Creige zone:
(1)on the east slope of A'Mhuing and extending into the small forest
south of Loch Shiel
(2)a band one to two feet thick in the corrie at the head of Glen More
(3)a band. possibly as much as 20 feet thick in the corrie carved
out of the north slope of Sgurr na Laire Brice. 39
After the field work was completed, a local resident of Glen Shiel,
Mr. E. MacRae, kindly informed the writer that it is said that marble
had been found in the N.E. extension on the Am Fraoch Choire band,
above the Glen Shiel forest and was used as a source of lime more than
a century ago.
South of Loch Shiel near the road A87T) the marble is white,
coarse grained and massive. Large calcite grains have simple, smooth
boundaries with each other and are little twinned. They are in part
repinoed by a finer, granular calcite. Tremolite, phlogopite, diopside
and quartz constitute together about 10% to 15% of the rock and epidote,
andesine and apatite are common accessories. Further up the slope of
A'Mhuing along the strike, the marble is made of coarse, little twinned,
calcite and rounded grains of diopside and olivine, the last two
minerals making up about 20% of the rock. The olivine has a large
(85°-90°) negative optic axial angle which suggests that it is almost
pure forsterite. Both the diopside and the forsterite are partly
replaced along cleavage planes, cracks and grain boundaries by
serpentine and a little magnetite. In the hand—specimen, the marble
is white with greenish brown to black clots which represent the
weathered serpentinized ferromagnesian minerals and form knots on the
surface.
The marbles of Glen More have an identical appearance to those of
Loch Shiel in the field. In thin section it is seen, however, that
here serpentinization has progressed virtually to completion. Other
minerals present include phlogopite, apatite and scapolite.
Lime silicate rocks are closely associated with the marbles but 11_0 they also occur without marble in the Lewisianoid of Sgurr na Creige and in particular in the Sgurr Mhic Bharraich zone. Only a little actinolite schist was recorded in the An Fraoch Choire band south of the Shiel River; however, Dhonau (personal communication) has found abundant actinolite and diopside schist north of the river. The common mineral assemblages in the lime silicate rocks are given below. Actinolite Actinolitoplagioclase Actinolite-plagioclase-quartz -calcite Actinolito- diopside -calcite Actinolite- diopside -plagioclase-quartz Diopside Diopside - plagioclase Diopside •-• plagioclase - epidote Diopside - plagioclase - epidote - calcite Diopside - plagioclase - epidote microcline - quartz The actinolitic assemblages are the most widespread. Bands and lenses of actinolite schist occur in some bodies of all the Lewisianoid zones. The actinolite is acicular to fibrous, apple green to dark green in mass and with ploochroic formula; X - colourless, Y - very pale green and Z - pale bluish green. The optic angle is negative and large
(700-80°) and ZAc - 20°. In many localities it has a marked preferred orientation of its long axes (crystallographic c) parallel to the phase II lineation. Elsewhere this orientation is only weakly developed or lacking, the crystals forming a felted mass. Where present, rp_la-rd2 is strongly scricitized oligoclase (An25 .-an ). Apatite 30 and sphene are common accessories.
Diopsidic assemblages are far less common than actinolitic ones and wore only found in the Sgurr na Creige and Sgurr Mhic Bharraich zones. The aside is apple green in mass, and colourless in thin section
with 2V- 4.60°-65°, ZAc = 39-42 and on - .030. Where it has a prismatic habit, it is oriented with its long axis (crystallographic c)
parallel to the phase II lineation. The asSociatedplase varies
in composition from An23 in a diopside-oligoclase granulite on Sgurr
Mhic Bharraich to An in a diopside-andesine-epidote granulite on the 35 south side of Glen Undnlain. k149-t€2, where present in diopsidic rocks,
is in large equant grains locally sieved with quartz and plagioclase.
Twinning on100 is not uncommon and zoning was observed in almost every
specimen. The zoning is diffuse or sharp and in some specimens it is
oscillatory. Commonly an internal zone or group of zones with low on
(low FeO), is surrounded by zones which are progressively richer in
Fe0 outwards, the outer zones having On = .020 to .028.
In four specimens a diopside-andesine-epidote granulite and
actinolite-andosine-quartz-calcite schist were found closely interbanded.
At the contacts of the two assemblages, the following textures (Pig. 5) suggest that the actinolitic band was widened at the expense of the
diopsidic one.
(1)Actinolite porphyroblasts are developed in the diopsidic rock
near the contact.
(2)Small "islands" of the diopsidic rock are seen surrounded by
actinolite schist.
It is possible that after the climax of metamorphism had passed_ and the grade began to drop, the association of the two assemblages became unstable and the actinolitic (more hydrous)one started to replace the diopsidic one before all reaction ceased. 112
Figure 5
TEXTURES IN LETTISIANOID ROCKS
Netagabbros Head of Loch Duich
Hornblende aggregate with magnetite and rutile Clinczoisite
Groumdmaas of granular plagioclase with sericite alteration 2 ram.
Diopside Granulite with Actinolite South of Alt Undalain
Island of diopside-plagioclase granulite
Diopside granulite with actinolite needle
Acid Portion of Loch Shiel Migmatite
Clinozoisite
Granular plagioclase partly altered Apatite 43
Pelitic rocks aiffering in appearance from the typical Moine pelites described earlier, are found within Lcwisianoid bodies at three localities:
(1 ) in the small bodies north of Torr Beag, associated with hornblende gneiss and cummingtonite schist (Nicholls 1950)
(2)on the east slope of A'Mhuing
(3)on the north shoulder of Sgurr na Laire Brice. At the last two localities the polite occurs as beds a few feet thick, interlayerod with marble and actinolito schist. The pelites are highly micaceous and all have rusty weathering. North of Torr Beag, thin bands of hornblende schist are interlayered with the polite. About 6C of the rock is made of golden brown biotite and muscovite. Garnet, as large porphyroblasts or as small grains, amounts to aYloroximately 10A, of the rock. The rest is made of quartz and oligocalse. These rocks can be distinguished from the normal Moine-type polite of the Saddle Area by their high mica content, rusty weathering, golden brown rather than red brown biotite, and the way in which they are interbedded with hornblende schist. In all these aspects, these Lewisianoid pelites are similar to some of the pelites described by Peach & others (1910) from the Glenelg Lewisian and seen by the writer.
Relatively massive amphibolite occurs in several bodies of the three main zones. It is best developed in the Sgurr na Creige zone around Shiel Bridge in two localities:
(1)immediately south of the head of Loch Duich
(2)beside Loch Shiel.
At the first locality there are two masses surrounded by flaggy, 44 banded hornblende schist. The rock in the interior of the bodies is dark, coarse grained, massive and heavy amphibolite which has the appearance of a metamorphosed gabbro or "metagabbro" (Hewitt 1957), and consists of aggregates of black hornblende, magnetite and rutile set in a "matrix" of granoblastic, altered plagioclase (ka38) in which are scattered little rods of clinozoisite. The hornblende aggregates are elongate parallel to the phase II lineation (Fig. 5).
At Loch Shiel the rock is a medium grained, dark and massive amphibolite, locally garnetiferous. Here again, the massive and homogeneous character of this rock is in sharp contrast to that of the banded and schistose amphibolite separating it from the normal Moine- type schists. It has the appearance of a metamorphosed dolerite and consists of hornblende, andesine (An42), and little rods of clinozoisite.
The common assemblages in the more massive amphibolites like the ones described above are:
hornblende - epidote - plagioclase ± quartz garnet+magnetite (clinozoisite hornblende - epidote. - plagioclase - diopside (relic?) hornblende - epidote - plagioclase - scapolite hornblende - PIS ION ilm• - plagioclase - quartz - biotite (late) hornblende - IOW.. .11 - plagioclase
Sphene, apatite and pyrite are common accessories.
Hornblende forms 50% or more of the rock and is similar in its optical properties in all the specimens examined. It has a pleochroic formula: X - pale yellow, Y brownish grey-green, Z smoky green with a distinct bluish tint. The absorption varies somewhat from specimen o o. to specimen. 2V , -(65 -75 ) and ZAc varies between 17° and 22°. This data does not permit a precise estimation of the composition 45
(Deer, Howie and Zussman 1963) but does suggest a composition intermediate between hastingsite, ferrohastingsite and actinolite.
Five to thirty percent of the rock is made of oluA.2plase. It is
generally sericitized and varies in composition between An15 and An42.
Two varieties of mL12:te mineral occur; either clinozoisite with on = .010, or pistacite with On = .020 - .027. The latter is
generally zoned with an inner iron poor zone and an outer zone richer
in iron.
The accessory occurs either as small grains scattered
through the rock or, more commonly, as lens-like aggregates of small
grains within or near hornblende suggesting that the space now occupied by the hornblende was once occupied by a more titaniferous mineral.
Large, disoriented porphyroblasts of bronze biotite are not uncommon in the massive amphibolite. They carry inclusions of P11 the other minerals, and are clearly later than the hornblende plagioclase assemblage.
Garnet-rich anlphibolites occur as small lenses and bands within the Sgurr na Creige Lewisianoid zone, north of Loch Shiel and on the east slope of Sgurr na Creige. The common mineral assemblage in these rocks is;
garnet - hornblende - biotite - (oligoclase) - quartz - magnetite, ( An18-30 ) with accessory sphene, epidote, apatite and rutile. Diopsidic pyroxene is also present along with the other minerals in a band a few inches thick north of Loch Shiel, but it is clearly in part replaced by hornblende. 46
The garnet is red to brownish red and occurs in small round grains
0.2 mm. in diameter. The hornblende has the same optics as in the massive amphibolites and is characteristically dark. The origin of these rocks is uncertain, but as they were never found associated with the lime silicate or pelitic rock, but rather with the amphibolites, and as they have mineral composition similar to the amphibolites except for an excess of garnet, they may well be related to them.
Hornblende migmatite and hornblende_zneisses which differ in several aspects from the rocks developed in the Caledonian migmatite complexes of the Saddle Area, occur locally in the three main Lewisianoid zones.
Hornblende gneiss also occurs in the small Lewisianoid bodies in Glen
Aoidhdailean and north of Torr Beag.
Migmatite is best developed in the Sgurr na Creige zone near Loch
Shiel. Typically this rock is patchy rather than banded and consists of irregular bodies of hornblende-rich rock set in a matrix of plagioclase and clinozoisite with only little quartz (Fig. 5). These bodies or patches tend to be flat in the plane of schistosity and elongated parallel to the phase II lineation; they vary in length between 1 inch and 2 feet.
They consist of large hornblende crystals with which are intergrown smaller, and perhaps slightly later, hornblende crystals of the same composition; both are identical to that described from the massive amphibolites. Sphene and magnetite are minor constituents associated with the hornblende. The feldspathic matrix consists of granoblastic, considerably sericitized, andesine (An ) with small prisms of 42 clinozoisite scattered throughout. Biotite porphyroblasts which have no preferred orientation are widely distributed through the rock. 47
A. photograph of this migmatite is shown in Plate 2. To the right of
the hammer large blocks of amphibolite are separated by veins of
feldspathic material. To the left of the hammer can be seen the more
usual feldspathic rock with small patches of amphibolite. The hornblendic
masses yield readily to deformation, and in many outcrops near Loch Shiel
they can be seen drawn out into thin plates. Lenses of rock similar to
the more deformed migmatite of Loch Shiel occur along the trend of the
Lewisianoid body as far as Sgurr na Creige. Photographs showing two stages of deformed migmatite are presented in Plate 3 and Plate Li. In many of its features, as seen on the outcrop and under the microscope,
the Loch Shiel type of migmatite is identical to migmatized amphibolite
from the Lewisian of Glenelg which the writer has examined.
In the Am Fraoch Choire zone a very dark amphibolite is locally
streaked by bands of feldspathic material and on the northeast side of
Am Fraoch Choire this rock also passes into well banded migmatite.
Characteristically the granitoid phase of the migmatite is rich in
plagioclase and has a granulitic texture.
Hornblende gneisses are best developed on the north slopes of
Sgurr Mhic Bharraich, but are rare in the Sgurr na Creige zone.
Typically, they are quartzo-feldspathic rocks with thin bands and lenses
of amphibolite, the acid phase of which contains 50 to 1270 hornblende
porphyroblasts and small flakes of brown biotite. Granular epidote with on . .020- - .025 is locally present. The hornblende is similar to that
described from the other basic rocks, and has a pleochroic formula:
X pale yellowish green, Y - grass green and Z - dark bluish green.
-2V - 70° - 80° and ZAc - 20°. The plagioclase is usually fresh and I8
Plate 2: Lewisianoid migmatite at Loch Shiel. 49
Plate 3: Deformed Lewisianoid migmatite. Early stage. South of Loch Shiel.
Plate 4-: Deformed Lewisianoid migmatite. More advanced stage. South of Loch Shiel. :>c varies in composition between Ani5 and. An20. Accessory sphene and apatite are locally abundant. Hornblende and spindle-like aggregates of quartz are oriented with their long axes parallel to the phase II lineation,
Bands poor in hornblende and rich in biotite in the Lewisianoid hornblende gneiss look very similar to Moine-type semipelite and are difficult to distinguish from it in the field, and in samples of hornblende gneiss taken from exposures close to highly migmatized
Moine-type semipelite or psammite, a little 1K-feldspar and myrmekite way observed. Biotite was seen to he closely associated or intergrovrn with hornblende, such as to suggest it had grown at the expense of the hornblende,
banded hornblende schist is by far the most common rock type of the Leuisianoid and it occurs in all the bodies, Its banded appearance results from the interlayering on inch to foot scale of mafic rich and mafic poor laminae. In the more mafic laminae, the common mineral assemblages are:
hornblende - plagioclase - quartz - epidote + biotite garnet hornblende - plagioclase - quartz - - biotite hornblende * plagioclase 7 - epidote (minor hornblende) - plagioclase - quartz - epidote - biotite
Apatite, sphene, garnet, magnetite and pyrite are common accessories.
Porphyroblnts of bronze biotite with no preferred orientation are scattered through the rock.
The hornblende is very dark green or black in the hand-specimen and has similar optical properties in all the slices examined, with pleochroic formula: X - pale yellowish green, Y - brownish grey-green$ Z - dark bluish green. The optic angle is negative and large (650-800),
ZAc = 170 - 21°. This hornblende, then, is very similar to that from
the massive amphibolites and the migmatites described earlier. The plagioclase is partly altered oligoclase with composition between
An and An but most commonly fallinF between An and Epidote 17 35 20 An30' is twinned and zoned and has on . .015 - .026 indicating it contains
10 to 15 mol. (Fe,Mn) end member (Deer, Howie and Zussman 1962).
The more felsic bands are made of the same mineral assemblages,
but contain more quartz and plagioclase (An2o 30).
In both the acid and basic schists, aciculate: minerals such as
hornblende and epidote have their long axes preferentially oriented
parallel to the phase II lineation, Biotite and flattened quartz grains
lie in the schistosity plane which is parallel to the axial planes of
phase II folds. In those rocks in which the schistosity surfaces are
defined by micaceous foliae, some post crystallization granulation has
been observed. The granulation trains never cut across the mica foliae,
but follow them and the edges of the biotite flakes only are locally
affected,
Acid bands with little or no hornblende are not unlike Moine-type
semipelite and differentiation between them is exceedingly difficult.
The occurance of thin lenses, rich in hornblende, the presence of a few hornblende crystals and of much more epidote than is common in the
Moine-type semipelites, locally, but certainly not invariably, help to identify the acid Lewisianoid schists. For example, in a large new road cut, just south of Loch Shiel, the transition from banded hornblende schist to a rock indentical to biotitic Moine semipelite can be well 52 observed. It is impossible, at that locality, to place the boundaries of a band of hornblende schist to within six feet. In another locality one of the small bodies mapped as Lewisianoid, near the bottom of the north slope of Sgurr Mhic Bharraich, in fact consists of a number of small lenses of actinolite and hornblend schist a few feet thick, as well as two wider bands of hornblende gneiss, set in a quartzo-feldspathic schist which is identical to Moine psammite. The boundary drawn on the
map is simply a line which encloses all the local amphibolitic occurances.
The effect of Caledonian migmatization on the hornblende schists will be described in more detail later, but it may be said here that the hornblende schists are locally affected. Where the hornblende schists are adjacent to highly migmatized Moine-type semipelite or psammite, biotite and K-feldspar are developed along the contact zone, suggesting an influx of K' from the adjacent migmatite.
Many of the thinner Lewisianoid bodies consist largely of banded hornblende schist with only a few lenses of other rock types such as lime silicate, massive amphibolite or migmatite, isolated in the schist.
In the larger bodies, a greater variety of rock types is found, and the laminated schist is largely, though not entirely, confined to the outer margins of the Lewisianoid body. Here again it contains lenses of other rock types, particularly of those that predominate in the interior of the body.
These relations are illustrated by the detailed map of a portion of the Loch Shiel Lewisianoid body given in Fig. 6. In spite of the masked ground separating exposures, it can be seen that the contacts between massive amphibolite and migmatized amphibolite are truncated
53
Figure 6
THE LOCH SHIE.i„ DODY
N \\sio
N •
•
6‘° '‘‘c \•,. w. 4 \ y,,,,<:, \ \ \:‘,..... \`,.`\ "•••t,. \ `so 4 '-::::' 42*,0,e, \ \\‘•\•0' \ — \ ., ...... --...-,-- --...- 1A,„,,, *\'''•--\.\11„„ .\\ :„ .. `----- , ---, .v le. '''? „,::• • I \ \- ' , 1.- \ ...... ____ ...... --.-. (ty \ • S 0 • N,,, • V.V...., o ' ... •''. 'I!' ' .1 A. "--...... ' . \ ; I /
\ 'r \iLoch Spiel --.
O
Legend
Per,netite Hornblende Schist Ps.tchy Minatite iltu3sive J-unphibolite Psarunite 200 ft. TA!Ilite SeriDelite Foliation by hornblende schist and that the hornblende schist contains
"inclusions" of massive amphibolite and migmatite. In the southeast corner of the map it can be clearly seen how the two schistose margins of the body merge where the body becomes thin.
Lenses of deformed migmatite (mentioned earlier on page 47) which can be seen to pass into hornblende schist and lie on the same strike as the Loch Shiel body, crop out in the little wood south of Loch Shiel.
Similarly in the exposures north of Loch Shiel, massive amphibolite can be seen in all stages transitional to hornblende schist. From these observations and in view of its similarity in composition and mineralogy to the unmigmatized and migmatized amphibolite, it is probable that the flaggy, banded hornblende schist is a tectonic derivative of these two rock types, The tectonic schist is folded by phase II folds and is affected by Caledonian migmatization. It must therefore have developed before both these events,
C. Azeof the Lewisianoid Rocks
Masses of rock, which to various extents resemble rocks from the
Lewisian Complex of the Foreland, have been recognised in widely separated parts of the Moine outcrop in Scotland. Their true age relation to the Moine schists that surround them has been debated for some time (Read 1931, 1934), but recent opinion has again swung in favour of these rock messes really being infolded or inthrust pieces of the Lewisian basement (King, in Ramsay 1958), (Sutton, in
P. Clifford 1960), (Sutton & Watson 1962). It is clear, however, from the "Lewisian Inlier" debate (Read 1933), (Charlu 1928)0 55
(Sutton & Watson 1953, 1955), (Fleuty 1957), that many of the criteria used by earlier works, such as the Survey officers, to identify a rock mass as Lewisian, are invalid at least in some situations. The evidence now demonstrates:
(1)In a region of polyphase folding, stratigraphic orders road from
folds, not proven to be first folds, are invalid,
(2)Lithologic similarity to the Lewisian may be a useful guide in the
field; but does not prove that a certain rock, or rock group, is
Lewisian,
(3)Lithologic heterogeneity and structural complexity may be a
useful guide in the field; but it is no proof that a certain mass
is Lewisian.
(if) Where the Moine Series is involved in Caledonian migmatization,
differences in the amount of migmatization is not a reliable
criterion for mapping the boundaries of a Lewisian inlier. The
pre-Moine aoo of the migmatite in the inlier must be demonstrated.
(5)A line drawn on the map to enclose all the occurances of Lewisian-
like rocks in a given area need not represent the line of
unconformity or tectonic contact, although it may be a crude
approximation.
Moreover, it appears now from more recent work (Ramsay 1958),
(Sutton a Watson 1959), (Fleuty, personal communication) that even some of the criteria used to demolish inliers during the "debate" are not always valid; for example:
(6)If the cover and basement have been deformed together, the fact
that an angular unconformity cannot be found does not necessarily rf!
mean it did not exist.
(7)Close interbanding of "cover" and 'basement" rocks does not
necessarily prove interbedding. It can be developed by close
infolding and inthrusting and by metamorphic convergence of
Moine granulite and Lewisian acid gneiss.
Furthermore, it must be borne in mind that if a body of rock is shown to be a piece of the pre-Moine basement, this by no means proves its correlation to the complex that is now known as Lewisian. If, however, a mass can be demonstrated to be Lewisian it is probably pre-Moine, as the pOst-Lewisian age of the Moine (though not its correlation to the
Torridonian) is now reasonably established on the strength of the
Glenelg conglomerate (Peach & others 1910), (Ramsay 1958) and radiometric dating (Miller, Barber and Kempton 1963).
There are thus two possible approaches to the "Inlier" problem.
(8)The rocks can by some means be unquestionably correlated with
the Lewisian.
(9)The rocks can be shown to record metamorphic and tectonic events
which predate any such events that affected the Moines, and in
addition must have been earlier than the emplacement of the
"Lewisianoid" rock mass within the Moine Series.
In deciding on the age of the Lewisianoid bodies mapped in the
Saddle Area, the following observations must be considered.
(10)The Lewisianoid bodies vary greatly in size and are interbanded with rocks of Moine type.
(11)Within bodies mapped as Lewisianoid there are rocks not unlike Moine-type semipelite. 57
(12)Some of the bodies consist, in fact, of a number of small lenses
and bands of various Lewisianoid rock types set in acid schist
that could well be interpreted as Moine psammite and semipelite.
(13)Lewisianoid hornblende-biotite schists apparently grade into Moine semipelite.
(1/4) The bodies as a whole are not made of one rock type, as are the
garnet amphibolite balls, or the typical Moine oak-silicate bands,
instead they include various rock types of sedimentary, basic.
intrusive, and migmatitic parentage. These rocks are found
together in the Lewisianoid bodies but not separately, as
individuals, in the main mass of the Moine Series.
(15)Generally, as shown later, page 64, the Lewisianoid rocks consist
of metamorphic mineral assemblages which are in equilibrium with
and of the same metamorphic subfacies as the Moinian rocks.
(16)The margins of the Lewisianoid masses are marked by zones of
fine grained, flaggy and strongly banded hornblende schist
possibly derived from the other Lewisianoid rocks by intense
movement. These tectonic schists are folded by phase II folds,
are recrystallized in the Main Metamorphic episode and are
affected by the earliest Caledonian migmatization recognized in the Saddle Area.
(17)At Loch Shiel there are two main Lewisianoid rock types besides
the flaggy hornblende schist: (a) unmigmatized massive
amphibolite, probably of doleritic parentage and (b) patchy
amphibolite migmatite. It is not certain whether the massive
amphibolite is an unmigmatized remnant or a post-migmatite ro0
intrusion. Some migmatite-amphibolite contacts are sharp,
others may possibly be gradational and the two rock types
interfinger on hand-specimen and map scale (map, Fig. 6). It
seems possible therefore that the amphibolite here is an
unaffected "parent" of the migmatite. It is clear, however, that
the migmatite-amphibolite contacts are truncated by the tectonic
hornblende schist and therefore must predate its development.
The schist, moreover, is affected by phase II folds, Main
Metamorphism and Caledonian migmatization. The Lewisianoid
migmatite must, therefore, have been formed during an episode
earlier than both the phase II folding and the migmatization
which developed the Caledonian complexes in the Saddle Area.
This is also suggested by the mineralogy and granulitic texture
of the Loch Shiel and other Lewisianoid migmatites.
(18) The Lewisianoid rocks, as their name suggests, closely resemble
rocks from the Lewisian inlier of Glenelg. This resemblance
is not restricted to certain individual types, but includes all
varieties such as marble, lime silicate, polite, amphibolite and migmatite„
Observations (10), (11), (12), (13) and (15) are readily explained if the Lewisianoid and Moine rocks were part of the same conformable sequence. It was evidence of this type which led to the "demolition" of the Lewisian inliers, an interpretation which is simple and which might well be accepted as the solution for Lewisianoid rocks of the Saddle Area. Observations (14), and (17); however, suggest that the Lewisianoid masses represent pieces of a complex older than the Moine Series and now tectonically intercallated with it. Point (16) might be taken to support the latter idea, but marginal shearing could equally well develop as a result of movement between two members of the same series but having different competences.
Unless it can be demonstrated that the massive amphibolite and
metagabbrollacenn'; o igneous intrusive origin, as their composition, texture and structure suggest; and that the relation of the early
Lewisianoid migmatite (point 17 above)totheLter Caledonian migmatite of Loch Shiel and elsewhere is a misinterpretation, then the simplest explanation is that the Lerilianoid masses are pieces of a pro-Moine complex. The occurance of limey and pelitic metasediments, basic intrusives and early migmatite all together in masses which form distinct zones, but never separately as individuals in the main mass of the
Moine Series, is then readily explF)ined.
If a pre-Moine age be accepted, then the lithologic similarity to the Lewisian strongly suggests, even though it does not prove, correlation with that complex.
It is to be expected that Lewisian acid gneisses when crushed; metamorphosed and migmatized along with and under the same conditions as the Moines during the Caledonian movement; will suffer a metamorphic convergence and be converted to a rock differing little from Moine scmipelito as both have a. composition near granite-granodiorite,
As pointed out by Read. (1957, page 264):
"Two equilibrium rocks that might have had profoundly 60
different earlier histories may by metamorehic convergence (Read 1937) come to be essentially alike in that they record a common concluding act."
Points (11), (12) and (13) therefore need not be contradictory to points (14) and (17). The interbanding of Moine and Lewisianoid could
be the result of infolding, inthrusting or, in part at least, a result
of misidentifying Lewisian acid gneisses as Moine psammite and
semipelite in the field. It is possible, therefore, that some of the
semipelite near Shiel Bridge, for instance, is reworked Lewisian acid
gneiss and that the sman Lewisianoid bodies in it are not individpn3
masses enclosed by a plane of unconformity or sliding but that they
are simply basic bands in a former acid-basic complex.
It cannot be sufficiently emphasized that even if the interpretation
of the Lewisianoid as Lewisian is correct for the main zones of the
Sadale Area, this is no support for any idea that hornblende is an "index fossil" to the Lewisian (Read 1934). It is quite possible, for instance, that some of the small bodies in the Strathconon fault
zone and northwest of it are not Lewisian. They have been grouped only
because of their similarity to other Lewisianoid rock in the Saddle Area.
D. petamoimhic Grade of the Rocks
The mineral assemblages and textures in the Moinian as well as
the Lewisianoid rocks of the Saddle Area can be ascribed, with some
exceptions, to one main metamorphic episode. This metamorphism will be
termed the Main Metamorphism. The mineral assemblages of the two rock
groups were presumably formed contemporaneously as the platy and prismatic minerals of both have the same geometrical relation to
61
phase II folds.
Relics of a metamorphic episode earlier than the Main one and phase II are locally preserved; and nearly all the rocks show some signs of late retrogressive metamorphism. However, for the moment,
these will be neglected, and only the Main Metamorphism will be considered.
The, main assemblages of each rock type are listed in Table III. The range of metamorphic facies through which each of the assemblages is considered stable (Turner and Verhoogen 1960) is also indicated. Underlying the facies classification of metamorphic rocks is the assumption that mineral assemblages approaching thermochemical
equilibrium will form under natural metamorphic conditions (Turner, in Fyfe, Turner and Verhoogen 1958) and the facies are defined in terms of equilibrium assemblages. It must be demonstrated, therefore, that
the assemblages used to classify a body of rock into its proper facies are indeed equilibrium assemblages. In practice, this is exceedingly difficult to do with any degree of certainty.
There are, at present, two standard methods available to the petrographer and neither is highly reliable.
(1)The texture of the rock is examined to see if there is clear
evidence for reaction between minerals and for decomposition of one or more of the phases, or if the minerals appear fresh and have simple boundfrry relation with their neighbours.
(2)The mineral assemblages are tested against the mineralogical phase rule for closed systems (Goldschmidt 1911)
< c (1) 62
TABLE III
Stable Issemblazaall211221E22Iaualaa1
AssemblaLas Lta0111Iy Eaus. A A A oligoclase biotite muscovite garnet quartz magnetite I 2 3 A A oligoclase biotite muscovite epidote quartz +K-feldspar l 2 A A andesine actinolite garnet epidote quartz calcite 1 2 andesine hornblende garnet quartz A A A I 2 3 oligoclase actinolite calcite quartz A A 1 2 oligoclase diopside epidote A A l 2 (oligoclase) hornblende epidote garnet quartz magnetite A A andesine I 2 3 A G G diopside forsterite calcite Al A2 A 4 I 2 diopside tremolite calcite quartz + phlogopite A A A A l 2 3 4
Subfacies
Al staurolite almandine A2 kyanite almandine - muscovite sillimanite - almandine - muscovite 11A3 sillimanite » almandine - orthoclase G hornblende granulite pyroxene granulite 63
or against its modification for systems with mobile components as developed by Korzhinsky (1936) and (1959) and Thompson (1955)
c m (2) where 0= the number of mineral phases
c = total number of components m = the number of mobile components. As discussed by Turner and Verhoogen (1960), the fact that an psemblage, which from its texture appears stable, recurs the world over in various situations does strengthen the probability that it is an equilibrium assemblage. Method (1) was used to select the apparently stable assemblages for Table III. Care must be taken, when dealing with polymetamorphic rocks, to distinguish between the textures developed during the phase under consideration and those developed earlier and later. For example, chlorite replacing biotite and garnet in the axial zone of crinkles that post-date the Main Metamorphism is not considered part of the assemblage formed during the Main Metamorphism. The biotite to chlorite replacement texture does not provide any information concerning the stability of biotite during the Main phase.
Method (2) was used to test further the stability of the assemblages given in Table III. ciA, is readily determined petrographically but c and m are much more difficult to estimate. As the mineralogical phase rule is here being used to test for equilibrium rather than disequilibrium the minimum value of (c-•m) must be estimated. Chemical data on Moine rocks in the vicinity of the Saddle Area
(T. Clifford 1957), (Lambert 1959) and (Dhonau, personal communication) 6L suggest that, at least outside the migma-tite complexes, these rocks
closely resemble their sedimentary parent rocks except for a loss in water. The lime silicate rocks have probably lost 002. It would seem reasonable, therefore, to count 002 and H2O as mobile components. The minimum number of components was estimated for each assemblage by grouping as one component all the oxides that did not clearly behave separately. For example, if we consider the assemblage
biotite muscovite - oligoclase - garnet - quartz - magnetite the main oxides are SiO2, TiO2, A1203, Fe203, FeO, MgO, CaO, Na2O,
K20 and H2O. These can be grouped as follows:- (Si,Ti)02 ; A12031 Fe203 0 • ; FeO, Fe203 ; FeO, Mg0 ; CaO, Na2 , Al2 03' K20, A1203 ; H2O (mobile). In this example c = 7, m ..1 and 0 = 6. Equation (2) < c (6) (7) (I) is satisfied and the assemblage may be stable. The fact that it is a common one throughout the world, perhaps lends some support to this conclusion.
All the assemblages of Table III satisfy equation (2) and they may therefore represent stable assemblages that can be used to determine the facies in which the rocks were metamorphosed during the Main phase. From the ranges given in Table III it is clear that the rocks of the Saddle Area were metamorphosed in the amphibolite facies. The association of epidote with oligoclase-andesine in many of the assemblages is characteristic of the staurolite-almandine and kyanite- almandine-muscovite subfacies (Turner and Verhoogen 1960). 65
The prevalence of the association epidote-oligoclase "An20 in semipelitic and some hornblendic schist may perhaps suggest that
only the staurolite-almandine subfacies was attained (deWaard 1959). The absence of kyanite from the Moine pelitic schists may also indicate that the kyanite-elmandine-muscovite subfacies was not achieved but it could equally well be a reflection of the chemistry of these pelites (T. Clifford 1958). As the more "sensitive" rocks such as lime silicate, amphibolite and politic schist are not evenly distributed over the area, it is difficult to be certain whether or not metamorphic gradients existed
across the area during the Main phase. No new index minerals or assemblages appear an the areais crossed, and one may consider it to lie entirely within one isofacial zone. Certain observations, however, may possibly suggest that during the Main Metamorphism there was a slight rise in grade from northwest to southeast across the area. (1)On the north slopes of Sgurr Mhich Bharraich, calcite-bearing
lime silicate rocks have plagioclase as sodic as An23. Southeastward on the north slopes of A'Mhuing similar lime silicate rocks carry plagioclase of composition An30 An35. The Moine talc-silicate bodies of Creag nan Damh, in the
southeast portion of the area, are probably less calcic than those from the Lewisianoid; nevertheless, they have a more
calcic plagioclase (Anil. 45) possibly suggesting that the rocks of Creag nan Damh reached a somewhat higher grade than those to the northwest on Sgurr Mhic Bharraich. (2)In the region southwest of the ridge formed by Sgurr na Sgine, 66
Sgurr a'Bhac Chaolais and Creag nan Dash, G. Tanner (personal communication) has drawn northeast trending isograds on the basis
of the mineral assemblages in cale-silicate rocks. According
to these assemblages, the grade of metamorphism became progressively higher towards the southeast. If Tanner is right, it is probable that a similar situation existed in the southeastern portion of the Saddle Area. If these rather tenuous conclusions are correct, they would, in a general way, correspond to the model suggested by Kennedy (1949) for the Western Highlands. 67
IV. MIGMATITE
Migmatite is common in the map area but it is mainly restricted in its occurance to three main belts:
(1)a great arcuate belt underlying an area of about 10 sq. miles within the Saddle Area and extending from the head of Loch Duich, across the area, to the head of Ohoire Sgamadail. Its extension into the Five Sisters of Kintail occupies another 4 sq. miles (Dhonau 1960).
(2)a narrow belt between the Strathconon fault zone and the Beinn Aoidhdailean fault occupying an area of about 12 sq. miles.
(3)a belt of weakly migmatized rocks striking parallel to the lithologic banding and having its northwest margin a few hundred yards northwest of the Am J3raoch Ohoire Lewisianoid. Its southeast margin lies outside the mapped area and remains unknown.
The volume percent of discrete granite1 in the rocks was measured in the field on suitable exposures by using a modification of the point-count method. Measurements were only made on exposures which were smooth and washed clean, usually these were located in stream beds.
1 Unless specified, "granite" and granitic are used to describe quartzo-feldspathic rocks of granitic texture rather than granite according to a precise mineralogic definition. It includes, in the Saddle Area, K-feldspar-quartz alaskite, leuco-granite (sensu stricto), leuco-quartz monzonite, leuco-granodiorite and trondjhemite. 68
Exposures with pitted, rough or weathered surfaces were avoided as they were found to give results which were inconsistent with nearby clean outcrops. The intensity of migmatization has been shown
(Fig. 7) by lines which were drawn on the map and connect points of equal granite abundance in semipelite. The patterns indicating the variations in the abundance of granite are fairly simple, and the drawing of the isopleths by "eye" is probably fairly accurate. While mapping in the field, exposures with migmatite were classified by inspection into one of four classes;
very weakly migmatized p - approximately 0 -10% granite
weakly migmatized - f - approximately 10%-30% granite
strongly migmatized F - approximately 3W0-50% granite
very strongly migmatized M - more than 50% granite and the contours were interpolated between measured outcrops using these qualitative observations as a guide. The actual measurements and the isopleths are shown on the map (Fig. 7).
The zero line is taken as the outer boundary of the complexes, and it can readily be seen from the relation of the zero lines to the lithologic banding and the topography that the migmatite complexes are roughly concordant, sheet-like bodies. The main Central complex is seen to be a southwardly inclined, folded wedge about 2z miles thick in Glen Shiel, and tapering southwestward to less than one mile thick in Glen More. The occurance of unmigmatized rocks "underneath" the Central complex can be readily seen and can be demonstrated in
Glen More and its westward flowing tributaries. South of, that is above, the Central Migmatite complex are small migmatitic outcrops
69
Figure 7. M IGMATIZAT ION MAP. r 3
% Nt LEGEND. t 1S) .. • \
‘t 1 k \ \ r r
m 4; r r \ I r 5 5 5, • 50 60 Measurement Loco.' i ty.
55 Migmatizaion Is ople
Foul t. \;, I .51- i 1 ‘ I Vi r 1.0 r I I I ( c.s I ti ,4 1 / 54/ / , t / • Phase III AxiaL Plane .
I5 t 4. t7Ve...... :Li i t i l ‘... tr,i II, ..; .4 t Phase II Axial Plane. \ I I r: 1 135 , 0, t i i pc, t.10.\ 0 ‘ 1 i 65••••• one mile A — \ I I 0 1 I .6,tt J .i0 i ' I 5. % 50' 1 Lb Cre." q , I ; s S I:., I \ \ 10 I 1 1 - r Si ,.. • t I r I 50 I so" , 4 ; (4.3 5.3 1".71 ...- ♦ % / , , , I , ,,,---...1 .- - 40--- / / I ,, ,/ ..404,..! — —iti j...i: 5 : I• / ,./ / , „. / /ri4 , / t „ -- _. / .W e ,....1 •_ ,_ - •', ... / /1. , / / tI0;440 ,ko - • .,,,,,, _// / 0 1., . 5.1 ...... i, ' P • ,,c2% ... / / % # /♦% CI /I • • a0 0 ♦ •0
so % 70 which form isolated groups rather than a definite migmatite body, particularly in the Lair° Brice and Dhomhuill semipelite.
Study of thin sections of the host-rock suggests that little diffuse or permeation type granitization has gone on, and that the abundance of discrete granite in any one rock type gives a measure or can be used as a measure of the intensity of the migmatization process, whatever the mechanism of that process may be. On the basis of granite abundance (or migmatization intensity) which varies from zero to 80%, the complexes can be divided into three mappable zones.
(1) An outer zone which lies at the top and the bottom of the
migmatite sheet, and in which the abundance of granite varies
between 0 and 11i; equivalent to zone "p". The granitic material
occurs in the form of small augen (Hate 5) and feldspar
porphyroblasts in all rock types. The development of feldspar
porphyroblasts are nowhere quantitatively very important. As
the granite content increases across this zone the host-rocks
increase in coarseness; in particular their mica flakes became
larger. Quartz veins are common in certain portions of the
outer zones of all three migmatite complexes, but quartz veining
of the same age is also found in many other parts of the area.
It is therefore not clear whether the quartz veining in the outer
zones represents a zone of veins as Read described in Sutherland
(Read 1931), or whether these veins are not directly connected
to the development of migmatite. The upper and lower zones of
the Central Migmatite complex grade outwards into non-migmatitic
country rock; the only difference between the two transitional 71
Plate 5: Small augen of trondhjemite in polite. Glen Shiel, one mile south of Shiel Bridge.
Plate 6: Large augen of trondhjemitc in polite. Glen More, below Sgurr Leac nan Each. 72
boundaries is that the lower one is much sharper. The lower
boundary of the southeastern migmatite complex is a very
diffuse zone. (2)The next zone inwards is characterized by the amount of granite varying in semipelite, between 106 and 406. This zone overlaps between the field-zones f and. F. The granite content within
this zone shows a great variability in the same rock type from one exposure to the next. One band of semipelite may contain as much as 4C granite while a band of similar semipelite nearby may carry as little as 10%. The place of small augen and porphyroblasts, characteristic of the outer zone, is taken by
lenses, sheets and rods similar in form to those described by. G. Wilson (1953), but- of quartzo-feldspathic material. (3)In the internal zone, the abundance of granite varies between 35P and 80,P depending on rock type. All the rocks are coarse
grained and the granite appears in the form of rods, lenses, and sheets. Nevertheless, rocks which are permeated with granitic material, in contrast to carrying it in discrete bodies, are rare and are limited to the innermost zone. It is noteworthy
that even in this innermost zone migmatization has generally not obliterated the host-rock beyond recognition.
In the two internal zones (2) and (3), the granite appeals in somewhat different forms in the different Moine rock-types. In pelite it is present largely in the form of augen and rods; in semipelite as rods, flat lenses and bands; in siliceoUs semipelite and psammite, as flat lenses and bands and only rarely as augen or rods (see Plates 6, 73 7 and 8). In other words, the more micaceous the host-rock, the
more discontinuous are the granite bodies.
Conformity of the granitic phase of the migmatite with the
lithological laying of the host-rock is characteristic throughout
the area. The flat lenses and sheets lie exactly in the foliation.
It was, however, observed in a few localities, that in very schistose
pelite, the granite augen are locally tabular and they lie in the
phase II axial plane schistosity instead of lying on the foliation
plane. This can be seen in the hinges of minor phase II folds
(Fig. 8). The granitic material itself never cross-cuts the lithologic
bands (beds), but only augen have developed in, and lie parallel to
the axial plane schistosity, and they are restricted to particular lithological bands as is illustrated in Fig. 8.
The granitic or quartzo-feldspathic rods vary from 2 inch to one foot in diameter, typically having a diameter of 2 to 3 inches;
their lengths vary from 1 foot to 10 feet. The rods lie in the phase II
schistosity plane as well as in the foliation, and therefore they are
parallel to the axes of the phase II folds; they are phase
symmetrically defined b-lineations comparable to the quartz rods of
Ben Hutig (G. Wilson 1953). Locally, it can be observed that some rods
are really the crests of small phase II folds in which granite has become concentrated as shown in Fig. 8. They are analogous to saddle veins as suggested by Hallimond (in G. Wilson 1953, page 146). This, however, does not seem to be the explanation for most of the rods, nor
do they seem to have originated from the cutting up and pulling apart of granitic veins. More probably the rodding is a primary tectonic 71
Plate 7: Granite lenses in semipelite. Alt Undalain, Shiel Bridge.
Plate 8: Granite bands in psammite. Alt Undalain, Shiel Bridge.
75
Figure 8
IMIATIOIT OF . TO II FOLDS
•-"z.... --,, N.,...... --.....:,
....., ...... ,-", ::Z.,...' ',.... L \ .... '''"- ,.... •••:- -, / ."--= •"------, . -, --.... -N.'
--- ...... '- ....: -...... , ....--z -...,,, ..,... --....' 1 foot 4 ,.." N.
Granite lenses in schistosity at crest of fold. Glen no re.
Granite in "saddle veins" with micaceous selvages. Sgurr lade Dharraich. 76 effect dovelol)ed during the migmatization as a result of the concentration of granitic material concomitant with movement. The fact that every gradation from small elongate augen to long rods can be found in many exposures supports this view. Groups of rods all lying in one foliation plane and possibly representing a single disrupted or boudin6 vein are rare; and such granulation textures as do occur at the edge of rods can usually be ascribed to movement post-dnting the formation of the rods.
The evidence that the migmatization and phase II movements were penecontemporaneous is strong.
There is difficulty in comparing the migmatization intensity of the Saddle complexes with that of complexes elsewhere,as systematic ratios of granite to host-rock are rarely available. Indeed, in many complexes such measurements would have little significance as the picture is complicated by bodily injection of sheets, by diffuse permeation, and a loss of the identity of the host-rocks. However, from the clear and detailed descriptions, photographs and diagrams given of some of the classic migmatite complexes certain comparisons may be made.
(1)It is obvious, for instance, that in comparison to the great
complexes described in Sutherland by Read (1931) and Cheng (194A)
or to the enormous migmatitic areas in southwest Finland
(Sederholm 1923) and southeastern Canada (Quirke & Collins 1930),
(Adams and Barlow 1910), the 1 mile to 2 miles thick sheet in the
Saddle Area is a very small occurance indeed.
(2)The migmatites of the Saddle Area not only differ in size from the
examples cited, but also in the granite:host-rock ratio . 77 From Read's and Cheng's descriptions it would seem that Saddle
migmatites only correspond to the outer, less migmatized portions
of the Sutherland complexes. Nowhere in the Saddle Area does
the host-rock lose its identity, nor is more or less homogeneous
granite gneiss developed,
(3)In the Saddle colplexes, the granitic material nowhere breaks
across the banding of the host-rock and therefore pieces of
host-rock are not seen "floating" in the granite as is so well
illustrated by the photographs of Sederholm from southwest
Finland.
(4)In the migmatite complexes of southwest nnland, southeastern
Ontario and in those of Lewisian age in northwest Scotland,
amphibolitic rocks are strongly affected. In contrast, hornblendic
rocks are left almost unaffected by the Caledonian migmatization
of the Saddle Area.
Moan.] analysis of the exposures in the Saddle Area reveals that aside from the variation in the amount of granite of any one rock-type, from zone to zone, there are important local variations in the granite content of the different rock-types within the same intensity zone.
Typically, in any one zone, granite is most abundant in semipelite, less abunanut in pelite and least abundant in very quartz-rich psammite or mica-rich pelite. Examples from localities where two or three of the different rock-types occur together are plotted in Figure 9. It is seen that the granite content of semipelite has a crudely linear relation to the granite content of nearby pelite and psammite and, for example, that in the inner zone of the Central complex at a locality
70
l'iLure 9
REL.LTION OF GRALIT A 73 U DA CE
I 1:`, S E I: 1). :I:: I T PELIT 1: A D I: I T
,c'rallit oc,r1i2elite
100
•.,
1
cranite Iii psaramite iyrnitc 79 where semipelite would contain wrro granite, it is to be expected that nearby pelite would contain about 3C$ - 39% granite and nearby psammite 25% - Cheng (19414) found roughly the same relations in the Bettyhill migmatites.
The granite content in semipelite was used to map the
migmatization "intensity" because semipelite was found to be the most sensitive rock and because few large outcrops do not have some semipelite bands. An exposure where some of the semipelite contains
5% or more granite is considered to lie within the 50% zone, but that does not mean that all the semipelite bands in that rock will contain more than 5CPjo granite. It was found, for instance, on large stream- polished slabs along the lower portion of Alt Undalain, near Shiel
Bridge, that, though some semipelite bands contained75io granite, the average for the sequence exposed there for a thickness of several hundred feet and consisting of interbedded more and less quartzose semipelite is only 5c?. It must be borne in minds therefore, that the inner zone of the Central complex, taken as a whole, has a granite content of probably less than 5Cro.
Description the Granite
Some 50 samples of granite from semipelite$ pelite, hornblende schist and psammite were examined microscopically and of these 41 were found suitable for modal analyses. The sizes of the granitic bodies sectioned, varied from bands and lenses several inches thick, to small augen with a median diameter of 3 to 5 mm., much of the slide being composed of host-rock. The number of points that could be counted 80 per slide varied from 1,000 to 4,000. To gain some idea of the precision of the results, -two sections were cut normal to one another from two specimens. The results are given in Table IV. It would appear that the values counted from single slides for the major constituents are probably reproducible to the nearest percent.
The results of the modal analyses are presented in Table V and . plotted on a triangular quartz-plagioclase-K-feldspar diagram in
Figure 10. As all the granitic material is leucocratic, a triangular diagram of this type accurately depicts the composition of the rock plotted on it. It appears from the ternary diagram that the granite phase of the migmatite varies continuously from a potasic granite
(K-feldspar alaskite) through normal granite (sensu_stricto) and leucocratic granodiorite to trondhjemite. It is also evident that the composition of the host-rock has controlled the composition of the granite phase: psammite and semipelite carry granite (sensu stricto) and adameIltite, while pelite and hornblende schist carry plagioclase- rich granodiorite and trondhjemite,
Throughout the complexes, however, there is little variation in the composition of the granitic material in any one host-rock type.
The modes of all the samples cluster in a narrowly defined zone corresponding to the position of the cotectic line separating the quartz from the feldspar field in the diagram for the system quartz-albite- orthoclase-water (Tuttle and Bowen 1958). The scatter of the points, small as it is, is still too great to enable exact comparison with the cotectic line at any particular water pressure. The lines for 500 2 2 kg/cm. and 4000 kg/cm. water pressure have been put on the diagram 81 TABLE IV
IhRlicate Modepof Granite to Test Precision
YI 3)4 Y.J .34P* YI 21dis. YI _21 dB* Quartz 37.4 36.6 43.0 42.7 Oligoclase 60.0 60.2 53.1 55.7 K-feldspar 2.6 0.5 1.3 1.2 Biotite -- -- 0.1 0.2 Muscovite -- 2.7 2.5 0.2
*second slide cut normal to slide A 82
TABIE V
Modes of Granitic Portion ofjiWaatite
Rock No. Grid ref. Host apa plag.. C.-fels. Biot. Musc. Acc. & •-• •+. •+--, YSJF 87913 7 s.pel 28.1 46.2 25.3 0.0 0.4 0.0 RIV7a 922150 s.pel 33.2 28.4 36.8 1.0 0.0 0.6 RIV7b 922150 psam. 32.8 19.2 46.8 1.0 0.2 0.0 RIV1 0 9 28150 s.Pel 34.3 37.9 27.2 0.6 0.0 0.0 RIV11 9311 61 s.pel 35.0 32.8. 31.4 0.8 0.0 0.0 RII 21 a 9231 81 s.pel 27.8 28.4 39,6 2.8 1.0 0.4 RII22a 922183 psam. 28.0 29.5 42.0 0.5 0.0 0.0 RII25c 9 201 83 psam. 37.8 36.8 24.6 0.8 1.0 0.2 other band in same rock 28.4 50.0 18.4 0,0 0.0 0.0 RII26b 9 201 79 s.pel 33.1 36.9 27.3 0.5 0.1 2.1 RII26c 9 201 79 s.pel 28.7 46.4 21.5 0.6 1.4 1.4 RII23a 920178 s.pel 36.3 33.1 29.7 0.7 0.1 0.1 Rill 9a 9211 78 s.pel 37.0 33.7 24.7 2.9 0.9 0.8 RII19b 921178 s.pel 36.0 30.1 29.7 2.5 1.3 0.4 RII13f 9221 75 s.pel 36.9 24.3 37.0 1.3 0.1 0.4 RII25d 9201 83 sape1 33.4 1+2.3 18.5 0.8 5.0 0.0 YII 6 915170 s.pel 30.5 51.8 12.5 2.4 2.3 0.5 YVI5b 91 71 48 s.pe1 35.4 37.7 24.4 1 .4 0.9 0.0 YV4 890145 psan. 20.8 34.4 44.1 0.7 0.0 0.0 RIV34 9 221 61 psam. 43.6 11.4 45.0 0.0 0.0 0.0 YVI1 7 916143 s.-pel 29.9 44.1 26.0 0.0 0.0 0.0 Y137 914164 s.pel 26.8 57.4 15.8 0.0 0.0 0.0 1122a 91 21 79 sopel 36.2 37.2 24.2 2.4 0.0 0.0
Table continued on page 83. 83 TABLE V (contd.)
Rock No. Grid Ref. Host gtz. Flag. K-fels. Biott MU2C, Acc. Y134 912171 pal. 37.0 60,1 1.5 0.0 1.4 0.0 YI21d 912178 pel. 428 54.4 1.2 0.2 1.4 0.0 Y121b 912178 pa. 37.7 48.2 11.2 0.0 2.9 0.0 YI29a 917166 pel. 49.6 41.3 3.9 1.1 4.2 0.0 YV7 892136 pel. 35.2 63.6 0.2 0.0 1.0 0.0 YII8e 907177 pel. 41.2 54.6 1.0 0.8 2.4 0.0 YII11c 905177 pet. 51.4 47.8 0.5 0.0 0.3 0.0 Y1123 908173 pel. 43.4 48.4 5.6 1.3 1.3 0.0 YV6 889144 pel. 35.4 44.2 4.8 0.0 15.4 0.2 YI2 914176 pel. 26.2 65.7 6.1 1.6 0.0 0.0 R114 944180 pel. 30.1 64.6 1.6 0.2 3,4 0.1 R11112 943158 pel. 42.7 42.1 4.7 4.2 6.3 0.0 RI19 941167 poi. 43.1 50.6 1.7 0.9 3.6 0,0 RI21b 938184 arrph. 28.0 70.2 0.7 0.5 0.6 0.0 YV2c 892137 ar4Ph. 37.6 61.3 1.1 0.0 0.0 0.0 YVIlla 91914) ampla. 34.8 54.5 0.0 5.6 0.0 6.1 YI20 909180 auph. 38.3 58.6 0.0 0.0 0.0 3.1 Yi17b 908183 arph. 37.7 59.7 0.7 0.0 0.0 2.2
Accessories include epidote, sphene, apatite and hornblende. 8
• Figure 10
COP POSITI01; OF MAI:TT:v.: Il TIDE 1.IIGLIATITES
Or
• pelite host • psammite and semipelite host o amphibolite host
• 85
for reference. The moan) value Ab-Or-Qtz of a representative suite of Moine schists
was recalculated to 100 and plotted on a ternary diagram in Fig. 11 to see if the trend found for the granitic phase is in some way "inherited" from the parent or host-rocks. It is apparent from the figure that this is not the case as the modes for the schists cluster along the Ab-Qtz side of the diagram in marked contrast to the trend exhibited by granite they enclose; the average granite trend from Fig. 10 has been inserted for comparison.
Texturally, the different granitic varieties are very similar. They are even-grained, medium to coarse grained and have the typical
hypidiomorphic granular or granitic texture. guartz occurs in three ways; (a) as large composite grains made of unsutured, and generally unstrained individuals with differing optical orientations, (b) as small round grains which are also unoriented, enclosed within feldspar, and (c) as "worms" in myrmekite.
Myrmekite was observed only in those specimens containing K-feldspar. It typically occurs along the boundaries between plagioclase and Kfeldspar, either as straight rims on the plagioclase or in cauliflower shapes "sprouting" from a base in the oligoclase grain into the K.-feldspar (Fig. 12). The age relations between plagioclase and
K-feldspar are by no means clear. Only in two sections were "worms" found to continue into the K.-feldspar, beyond the limit of the
plagioclase. This is a texture which, according to Drescher-Kaden (1948), is good evidence for the K-feldspar having grown at the expense of the
plagioclase and myrmekite. Generally the K7feldaar. has very irregular 36
Figure 11
PLOT OP Q—Ab—O VALUES OP :on SCHISTS
• psanraite and sernipelite
+ pelite 07
Figure 12
TEXTURES IN GRANITE Myrmeidte replaced by K.-feldspar. 1;.E. slope of Sgurr chic Dharraich. K-feldspar
quartz worm extending into K-feldspar
oligoclase with a little sericite
am. 4 Myrmekite strip. Sane specinen as above.
Selvage between tiro granite augers. Moire a GharG Gharaidh.
z- K-feldspar epidote quarts biotite vith epidote, sphene and apatite
t muscovite 88 rartlines and fills interstices. The last two features suggest that the
K-feldspar grew at the expense of the plagioclase. The protrusion of the myrmekite, on the other hand) suggests it grew into K..-feldspar which was already present.
The K-feldar is generally clear. Grid twinning is not common, as in the host-rocks already described, and perthite is rare. It is probably a microcline with poorly developed twinning.
The aagioclase is fresh to partly altered oligoclase, and is generally twinned. In the more potassic rocks it has locally a little
K-feldspar in the form of spindles parallel to the mineral cleavage or in irregular patches. The composition was determined from the extinction angles and the refractive indices on cleavage flakes. The results are plotted as a histogram in Fig. 13. It is apparent that the plagioclase of the granite phase has about the same composition as the plagioclase in the host-rocks. This could be verified in a few slides where host-rock and granite were present together. Similar observations were made by Mehnert (1962) on analogous rocks from the Black Forest.
In contrast to the host-rock schists, which have a well developed foliation and lineation, the granite phase is massive and homogeneous.
The rods are elongated parallel to the phase II lineation, but they are not themselves lineated.
A narrow selvage 1-3 mm. thick in which biotite, muscovite) garnet, epidote, apatite and sphene are concentrated occurs at the margins of the granitic bodies. Only rarely are these minerals found scattered within the granitic material) but where they are, they have the same optics and shape as in the host-rock and selvage. It would 39
Fi, are 13
.._;oisITIor OF PLAGIOCLAST: FR= GPAITE
r1.14..Le 4— in ?elite tame 2sall'ait9_* and seniclite Go
5 t
• • • en rc
Pe _ - 3 . . . )...0 samples 10 samples ency
u in psa.mmite UI. pelite
eq aritl.semipelite - - - -
Fr . -
1C 15 20 35
; J. 90 appear that they were stable in the presence of the granitic phase
during the migmatization episode. Epidote maintains its zoned habit
and crystal shape, and biotite retains exactly the same pleochroic
colours that it has in the host-rock and selvage.
Modes from selvages in semipelite and. pelite are given in Table VI.
It is evident that the minerals in the host-rock which did not enter
the granite phase, including some quartz, are concentrated at its
margin.
There are two alternative explanations for these "basic fronts"
(Reynolds 1947), "basic behinds" (Read 1951) or "melanosomes" (Mehnert
1962). According to Reynolds, Na, K, Al, Si are added to the parent
rock from an outside source, and Ca, Mg, Fe, etc. migrate outwards to
concentrate in the basic front. However, as suggested by Read, and as
has been rather neatly demonstrated by Mehnert, identical basic zones can be formed if the salic comments of the host-rock concentrate into
a body and the cafemic components "left behind" concentrate in a zone around that body. The following observations suggest that the second interpretation is probably the more correct one for the Saddle migmatites.
(1) The selvages are not only basic in comparison to the host-rock,
but the quartz:feldspar ratio of the selvage tends to be higher
than in the host-rock, especially in those rocks in which quartz
exceeds the amount required to combine with feldspar to make
granite (see Table VII). Some augen have noticeably quartz-rich
rims. Thus, in those very rocks in which quartz is to be expected
as a "leftover" it appears in the selvage. 91 TABLE 111
Modes of Selvages Around Granite in Semiq1kte and Pelite
(1) (2) (3) (4) quartz 22.9 20.0 36.4 o.6 oligoclase 8.9 20.7 12.2 0.4 K-feldspar 0.0 0.2 0.8 0.0 biotite 31.5 24.6 34.7 50.7 muscovite 32.1 20.8 6.3 19.0 garnet 3.6 9.3 0.0 0.0 epidotc 0.0 0.0 9.0 26.3 apatite 0.8 0.9 0.8 0.8 magnetite 1.0 1.1 0.0 0.0 sphene 0.0 0.0 0.0 2.2
Selvage around small augen in pelite YIII1c Selvage 1 mu. thick, around trondhjemite augen 3 mm. x 6 mm. in pelite YI2. Selvage 1.5 mm. thick, along granite vein 7.2 mm. thick in psammite YV4.At the other side of granite vein is a quartz selvage 2.4 mm. thick. "Selvage" 1 mm. thick, in semipelitic gneiss band YVI17 consisting of 1T'o "selvage" and 90A granite. From highly migmatized portion of internal zone of Central complex 92
(2) Although it was found difficult to obtain samples in which
the selvage was clearly enough defined from the host-rock for
modn1 analysis, it appears that granite plus selvage crudely
approximates the composition of normal host-rock.
B. OrAgin of the Migpatite0omplpx
The most noteworthy result of the modal analysis of the granitic
ph,,IRe in the migmatite complexes is the finding that the compositions
of the granites are not erratically scattered, even though augen only
a few millimeters in diameter were analysed. Instead, they fall in a
narrowly defined zone which corresponds to the quartz-feldspar field
boundary in the Q-Ab-Or-H20 system in a manner similar to the Q-Ab-Or
values of the experimental anatectic melts produced by Winkler and
Van Platen (1961).
This close correspondence between the trend of the modal values
of the granitic material and the trend predicted from experiments in
the granite-water system indicates that the composition of the granitic
material was controlled by crystal-liquid equilibria, and it suggests
that the granitic material passed through a melt stage.
One apparent divergence from the result of Winkler and Van Platen
(1961) is the fact that the pelites yield a trondhjemitic rather than 1 an alaskite "melt" in spite of their high mica and therefore high K20
content. A possible explanation of this phenomenon is that it is not
1None of the micas analysed by Lambert (1959) in Morar show a large anomalous Na20 content. The white mica is muscovite, not paragonite. 93 the chemical composition of the "total" host-rock that exerts a direct control over the composition of the granitic phase, but rather the composition of the aggregate of phases in the host-rock that are available for the migmatization reactions.
The following points suggest that the micas were stable during migmatization, locking their K20 away from the reaction and leaving the quartz plagioclase aggregate to dictate the development of trondhjemite in pelite.
(1) The mica "caught" in trondhjemite apparently remains stable.
The flakes do not appear corroded and biotite that has reddish
brown pleochroic colours in the host pelite retains exactly
these colours in, and adjacent to, the trondhjemite. Hayama (1959)
has shown that the colour of biotite reflects the value of the
ratio TiO 2 + Fe205 + FeO. Therefore if the pleochroic formula Fe 0 2 3 remained exactly the same, possibly the value of the ratio also
remained constant.
(2)Micas, along with other minerals, such as apatite and garnet,
which do not seem to have taken part in the formation of
trondhjemite, are accumulated in the basic selvage or melanosomes.
(3)Minerals such as sillimanite, kyanite and cordierite which in
many geologic settings (Cheng 19)101 (Shaw 1962), (Turner &
Verhoogen 1960), (Mehnert 1962) as well as in experimental
systems (Winkler and Van Platen 1961) develop as a result of
the breakdown of muscovite and biotite, are not found in the
Saddle Area. 94 (i.) Muscovite and biotite maintain their simple texture in the host-
rock, and in fact became coarser within the migmatite complexes
during migmatization.
(5) The abundance of mica in the host-rock directly controls the
abundance of granite, very micaceous and feldspar-poor rocks
having little or no granite.
The isolation of many of the granite bodies and the rigid control
exerted by the host-rock over their compositions and shapes, preclude
bodily injection of the granite as magma, or even as a more "dilute" alkali-silicate hydrothermal fluid or "pegmatitic" fluid. Hence, if
the conclusion that the granite phase of the migmatite has passed through a melt stage is correct, then it must have formed through partial fusion of the host-rock.
The conclusion that the granitic fraction of the migmatite complex was formed by such partial fusion does not directly lead to a decision whether metasomatism was or was not involved in its formation.
Migmatization involving metasomatism as well as the presence or development of a granitic magma has been recognized in a number of regions, (Goldschmidt 1921), (Sedcrholm 1923), (MacGregor and Wilson
1939), (Engel and Engel 1952, 1958, 1960) and the possibility must be borne in mind that within the Caledonian migmatite complexes of the
Saddle Area, the Moine schists may have been metasomatized as well as partially fused. A decision on this point could only be made after an extensive programme of sampling and analysis, which is beyond the scope of the present study. Certain observations, however, may shed light on the question. 95 The composition of the granitic material varies from rock-type
to rock-type: it is sodic in the polite and in the hornblende schists in which the available potash is either locked up or is lacking, but it is potassic in semipelite and psammite which do have available potash.
In a few localities, such as on the north slope of Sgurr Mhic
Bharraich and near the schoolhouse at Shiel Bridge, where intensely,
migmatized semipelite is adjacent to Lewisianoid hornblende schists$ potash feldspar and biotite are developed in the hornblende schist, six inches to two feet in from the contact. This perhaps gives some idea of the small scale on which K-metasomatism took place during migmatization.
If, during migmatization, no significant quantities of matter were added to, or removed from a politic unit within the migmatite complex, then the composition of the pre•-migmatization parent should equal the combination of trondhjemite and host.-rock in the proportions in which they are now found, or
P nT + H(100 - n) (3) where P is the composition of the pelitic parent, T is the composition of the trondhjemite, H is the composition of the host-rock (including melanosome) and n is the percentage of trondhjemite in the rock.
Equation (3) should be satisfied for each element or oxide, and for each mineral providing identical minerals make up the component parts of the migmatite.
As trondhjemite of the Saddle Area has a fairly restricted composition around 62% oligoclase and 0% quartz, and because the same 96 minerals of approximately similar chemical composition appear in all the component parts of migmatized pelitic units, the composition P and H may be expressed in terms of trondjemite or "potential trondhjemite" T, and "rest" R. The values of T and R calculated from the modes of polite and associated sandy polite given earlier in Table II, are presented in Table VII. The amount of quartz present in the polite, in excess of that required to combine with oligoclase to make trondhjemite, was included in R and is also tabulated.
Two features are directly apparent from Table VII.
(1)All the pelites have excess quartz. (2)30% to 70% trondhjemite could be melted from the polite leaving a rock of composition R, consisting of garnet, mica-schist with 1% to 30i,; quartz. The type of metasomatism considered here would be the introduction of quartz- and feldspar-making substances, i.e. trondhjemite-making substances. Therefore, it is the "potential trondhjemite" content of the rocks that is of particular interest. Equation (3) may be rewritten:
11 , nT (100 - n)TH (4) where Tl, is the amount of potential trondhjemite in unmigmatized polite, and (100 - n)TH is potential trondhjemite "left" in host polite in which n percent trondhjemite has formed. If no alkali metasomatism has taken places equation (4) should hold.
There is no unique composition of unmigmatized polite or of migmatized pelite; rather, the compositions of these rocks vary within certain limits and tend to cluster around a most common value or mode. 97 TABLE VII
Potential Trondhiliite and Rest Pelitic Rocks
291.:men._ Quartz _0)1.accase K-feldspar T R EocayaLtz. (inside complex) Y118e 22.9 27.3 0.0 45.7 54.3 4.3 Y118c 22.3 31.0 0.0 51.7 48.3 1.6 1124e 21.9 19.8 0.0 33.0 67.0 8.7 YI2 25.3 30.0 0.7 50.9 49.1 5.1 Y127 30.3 20.3 0.0 34.1 65.9 16.5 YI20a 36.5 18.9 0,1 31.7 68,3 23.8 YI19a 40.1 16.5 0.0 27.5 72.5 29.1 RII Jun 25.5 28.1 0.2 47.2 52.8 6.6 YII11 e 33.7 20.2 1.3 34.6 65.4 19.3
(outside complex) YVII2 30.6 28.1 0.0 46.8 53.2 11.9 =1 4 34.9 36.2 0.4 60.6 39.4 10.5 YVIII2 15.8 11.7 0.0 19.5 80.5 8.0 YV113 29.1 27.9 0.4 48.2 51.8 9.2 YV7 31.5 40.2 0.2 67.1 32.9 4.6 Yv6 41.8 24.2 0.0 40.3 59.7 25.7 IVII1 32.4 25.4 12.1 62.5 37.5 7.4 R11118 27.3 38.2 0.2 64-.0 36.0 1.7 RIV19 28.1 27.2 0.3 245.5 54.5 9.8
98 The composition of the pelitic rocks must therefore be tested. against equation (4), bearing these compositional variations in mind. To do
this the values of TP (from pelites outside, or on the outer edge, of the migmatite complex) and TH (from pelites within the inner zone of the complex) were plotted separately as a histogram in Fig. 14. It is seen from the histograms that the most common value of TH lies between
3C and 144 and that Tp lies most commonly between 4.0 and 70%. It was shown earlier, that in the inner zone of the Central complex, where semipelite contains about 3( granite, polite contains about 30A trondhjemite. We may therefore set n = 30 and equation (4) becomes: 4.0 .(- Tp 70= x 1 oo + 35 x 100 100 and 40 = 54.5 We cannot say from this result that equation ( ) is exactly satisfied and that no metasomatism has taken place; but we may conclude that the available data suggests that equation (4) 1.a.y be satisfied, or is a29.2,....z'ol D1v satisfied, and that trondhjemite could have formed by partial fusion of polite and associated sandy politic rocks without it being necessary for metasomatism to have taken place.
C. Possible Teinture and Pressure I i mat iz at ion If the conclusion that the migmatite was developed by partial fusion be correct, the rocks must have been at temperatures as high or higher than those at which a hydrous granitic melt can form. In
Fig. 15 the Ts PH2O curve (A) for the "ternary minimum" in the granite water system as determined by Tuttle and Bowen (1958) is drawn. 99
-ore 111.
DI STR= UTIC;1'.7 CF :IT)Cf:,7-17, t2Lai TP.07 1)11.711!'.I3E
117 sciis
5
4. • 0.
I' • • L • _ 0 10 20 50 50 SO 70 00
Percc-- -Dote. ¶1:r0:
Dr.G:ned Dotted — ,1_,ua eii to -, 8 101
The presence of fluxes in addition to water, such as fluorine or boron would tend to shift this curve towards lower temperatures, i.e. to the left, by about 65°C (Wyalte and Tuttle 1961). If, contrary to the writer's interpretation, iron from the biotite had also somehow entered the reaction, the temperature of melting may have been depressed even more (I. Carmichael, personal communication). We can, however, say that the rocks of the Saddle migmatites must have been at temperatures lying to the right of a curve approximated by that given in Fig. 15, and from the present data it would seem that temperatures of about 600°C may have been reached to produce the granite. It is clear from the data of Tuttle and Bowen (1958) that the melting of trondhjemite would require temperatures some 30° to 50° C higher.
Some idea concerning the pressure may perhaps be gained from two criteria in the rocks.
(1)It is seen from the Q-Ab-Or plot of the granitic phase that in
spite of the scatter, the modal values cluster closer to the 2 4000 kg/On. line than to the 500 k cb.2 line. Moreover, the
values are probably a little high in quartz because some of the
quartz in the augen and rods is probably xenocrystic (Mehnert 1962).
(2)Evidence has already been presented suggesting that the micas
remained stable during migmatization. The rocks, therefore, must
have been at temperatures and pressures lying to the left of a
curve which represents the breakdown of muscovite in a muscovite +
quartz + feldspar + water system. The only approximation available
for such a curve is the one plotted in Fig. 15 as curve (B)
determined by Segnit and Kennedy (1961) for the quartz + muscovite 102
+ water system. Work presently underway (W. Fyfe, personal communication)
suggests, however, that this curve should be drawn further to the left,
approximately where (1960) has calculated it to be (Fig. 15, curve (0.
If the rocks of the migmatite comulexes have to be at temperatures
and pressures to the right of the granite melting curve, and to the
left of the muscovite breakdown curve, then the pressures must be above
the point of intersection of the two curves, that is, higher than
4000 bars.
The widespread occurrence of the apparently stable assemblage
epidote-oligoclase, also suggest pressures of the order of, and perhaps
exceeding, L000 bars (Fyfe, Turner and Irerhoogen 1957), (Pistarius & others 1962).
If water pressure approximately equals total pressure, and if
the total pressure is not greater than the lithostatic pressure (no
overpressure), burial of the migmatite to a depth greater than 10 is indicated. As will be shown later, this is not unreasonable in terms of the structural succession established within the Saddle Area.
D. LA-L[211k. Granite pegmatite in the form of sills, dykes, pods and irregular bodies is found throughout the entire area and in all rock-types. The pegmatite bodies in most localities can be distinguished from the granite phase of the migmatite, from which they have been differentiated on the map by their coarse, irregular texture. In contrast to the migmatitic granite which is never cross-cutting, the pegmatite bodies, even when dominantly in lit-Tar-lit sheets, have discordant relations to their 103 country rock at some part of their contact. The bodies vary in size from rows of scattered feldspar crystals and pods (Plate 9), and dykelcts a few inches thick to irregular, sheet.-like masses 100 feet thick and continuous far 4 mile along strike. Pegmatite bodies are most commonly sheet-like masses a few feet thick and at many localities they lie approximately in the phase II axial plane schistosity but are cross-cutting in detail. Elsewhere, they lie in the axial planes of phase III folds or, as on the Sgurr a Gharg Gharaidh ridge, they are dykes oriented roughly parallel to phase III axial planes (Plate 10).
On the ridge of AJMhuing and on Druim na Firean$ pegmatite layers a few inches thick and concordant with the foliation "go around" phase II minor folds, but it is not clear from the field relations there whether the pegmatite "mimicked" the fold or whether it was really folded during phase II.
The pegmatite varies in composition from trondhjemite to granite
(sensu siricto) like the granitic phase of the migmatite complexes.
However, the host-rock does not seem to control the composition of the pegmatite as strictly as that of migmatite granite, though trondhjeuite pegmatite is largely restricted to pelite and amphibolite. Granite pegmatite is by fax the most common variety and it commonly occurs in pelite and amphibolite as well as semipelite and psammite. This less strict host-rock - pegmatite control may be related to the discordant structures of many of the masses which may have been emplaced some distance from their source of origin.
The pegmatitic rock is leucocratic except for a few small muscovite and biotite "books" found in the larger masses. It consists of quartz, AnL
Plate 9: Row of small pegmatite pods with phase II fold. Shiel Bridge garage.
•••
• V' 414b1+04ta.:." • #.110...11: • `••••• • %WO..illhai .t..41161 211 ;
Plate 10: Pegmatite dyke parallel in attitude to IIIG axial plane. Sgurr a Gharg Gharaidh. 105 oligoclase and microcline (largely with grid twinning), all highly variable in grain and abundance. The largest feldspar crystals observed were one foot long. Typical pegmatitic minerals such as fluorite, apatite, tourmaline and beryl were nowhere found, nor was
Obvious zoning of the bodies recognised. Thus, the pegmatites clearly fall into the simple unzoned class (Turner and Verhoogen 1960).
Though found everywhere, the abundance and size of the pegmatite bodies vary from one ].)art of the area to another. The main pegmatite zones are;
(1)along the south, i.e. upper boundary of the Central migmatite
complex, inside as well as outside the complex
(2)a belt extending from the summit area of Sgurr Mhic Bharraich to
the Sgurr a Gharg Gharaidh ridge, as far south as Choir° a Gharg
Gharaidh
(3)on the north slopes of Sgurr a Bhac Chaolais and Sgurr a Chuilinn.
The large masses north of Biot an Fhithich and the many small sheets north of Sgurr a Bhac Chaolais occur in rocks that are massive, medium to fine grained, with well preserved current bedding and little or no normal granite. Pegmatite is common throughout the area. It occurs in large bodies outside as well as inside the migmatite complexes; and it would appear, therefore, that there is no simple, direct relation between the abundance of pegmatite and the intensity of migmatization as measured by the abundance of normal granite. However, the clustering of large bodies of pegmatite near the upper boundary of the Central migmatite complex suggests that there may be some genetic connection between the migmatite and pegmatite. 106
V. ZOST-METAMORPILIC DYKES
Some twenty undeformed, narrow, igneous dykes of acid, basic
and ultra'oasic composition have been observed in the Saddle Area.
These dykes are all younger than the pegmatite bodies described in
the previous chapter, and each variety was found at one place or
another to cut the pegmatite. They show chilled margins where they can be seen in contact with the country rocks, and their typical igneous
textures are preserved. They are therefore of post-metamorphic age.
In the field they were grouped into the following classes:-
(1)brick-red weathering lanprophyre
(2)dark lamprophyre
(3)felsite
(4)basic bodies
The brick-red weathering dykes are by far the most widespread within the area, and are found to vary in width from six inches to four feet. Only rarely can they be traced along their strike for more than a few feet or tens of feet and, in oarticular, the thinner ones have very irregular trends.
The longest dyke is the one that extends from the upper part of
Choire Chaoil, across the Sgurr na Creige ridge and into Choir' Uaine.
At its widest, southeast of Sgurr na Creige, it is four feet thick, and there it contains numerous inclusions of its country rocks; psammite, pegmatite and vein quartz. In addition, it has inclusions of amphibolite not unlike some of the hornblende schists of the Sgurr na Creige Lewisianoid sheet. The presence of these inclusions has 107 already been commented on (page 37) and was taken as evidence for the down dip extension of the Sgurr na Creige sheet.
In the western portion of the area, many of the dykes have a roughly east-,west trend, similar to the trends of the dykes west of
Glen Aoidhdailean (Clough, in Peach & others 1910). East of Glen
More, however, the trend of the brick-red dykes is northwesterly.
The colour of the "brick-red dykes" which is intensified by weathering varies from reddish brown and reddish grey (particuThrly in the finer grained rocks) to bright red. The brick-red colour of the surface is, however, the most common.
The main minerals of these lanrprophyres are plagioclase) biotite + augite ± hornblende. Quartz, apatite and magnetite together constitute as much as 15; of the rock, while calcite, chlorite and epidote appear to be, typically, alteration products of plagioclase and the mafic minerals. The texture of the rocks is panidiomorphic, as is common for lamprophyres in general.
The alaaiocjase, which is present in large, partly idiomorphic grains with interstitial quartz, forms the groundmass. It is highly altered to epidote and calcite and is dusted with haematite. Positive identification of the plagioclase is difficult, but it appears to be andesine.
The biotite, which is mainly fresh, occurs as disoriented, hexagonal flakes only locally corroded at their edges. It is pleochroic in dirk reddish brown and light brown and its absorption is stronger at the margins than in the centres of the crystals; this is probably the result of a higher iron content. 108
The pyroxene which forms phenocrysts in the rock is now largely altered to chlorite and calcite and its original shape is indefinite.
The mineral has no marked pleochroism; its extinction angle ZAc is approximately 45° and the optic angle is about 500.-60°. It is probably augite. In the fine grained rocks from the chilled margins, the same
minerals are present; but calcite and apatite are less common, and the plagioclase is distinctly finer grained than the other minerP7s thus forming a true groundmass. The augite still occurs as large phenocrysts indicating that it has already crystallized before the dykes were emplaced.
The dark variety of lamprophyra3was observed to form two dykes only in the Saddle Area. The one is more than one mile long and was traced over Sgurr Mhic Bharraich on the northwest slopes of which the dyke has been eroded into a spectacular gorge, to the Stratheonon fault
zone where it is apparently cut off. The other dark dyke could only be traced for a few hundred feet on Bad an Fhithich Mhoir. Both dykes
have the same 14.N.W. trend and the same composition. They differ from the red lamprophyres described above in that they contain abundant hornblende and less plagioclase and biotite, and pyroxene is absent. The hornblende is pleochroic in pale yellowish green, green and dark
brownish green. It has -217.-,75 and ZAc ,-'220, and is thus a common hornblende.
All the lamprophyres are plagioclase- and biotite bearing and may therefore be classed as kersantites. Those which, in addition, carry augite or common hornblende, may be termed spessartite-kersantite 109
and odinite-4-cersantite respectively.
The felsites occur as dykelets a few inches thick in the ground
between the Ratagain igneous complex and the Strathconon fault. They
are most abundant in the immediate vicinity of the igneous complex
and, as suggested by Dhonau (1960) for identical felsites in the Five
Sisters, they are probably genetically related to the complex. The
rocks are very fine grained, light red to pink and consist of an
oligoclase, K-feldspar and quartz groundmass with a few small quartz
and oligoclase phenocrysts.
The age relation of the felsites to the lamprophyres is not
known, but it is clear that the felsites are earlier than the faulting
on the Strathconon fault zone; and that they, like the lamprophyres,
are chilled against the pegmatites and migmatite granite of the Moine
country rock. Their close association and resemblance to rocks of the
Ratagain complex indicates that they are of approximately the same age.
Basic bodies consist of two very small bodies of dolerite and one
of serpentinite, all of Ithich were found in Glen More. The serpentinite forms an isolated exposure surrounded by grass, but lying within the
outcrop of the Sgurr na Creige Leuisianoid sheet. It is a dense fine grained rock composed of fibrous serpentine which has a preferred
orientation cut by irregular veinlets of serpentine. It is quite possible
that this rock is part of the Lewdsianoid, but the field relationships are indefinite.
There is no evidence within the area as to the relative or real ages of these minor intrusions, save that the lamprophyres and felsites are cut off by the Strathconon fault. Clough (in Peach & others 1910) 110 considered that the felsites and lamprophyres are all generally associated in time with the Ratagain complex and so are possibly of Lower Old Red Sandstone age (Nicholls 1951). The dolerites may be Tertiary, but here again definite evidence is absent. 111
VI. STRUCTURE
At least three distinct phases of folding and one episode of post-metamorphic faulting have been recognized in the Saddle Area. The three folding phases all involve the Moinc Series as well as the
Lemisian and Lewisianoid rocks, and predate the faulting of Strathconon age. They may be, therefore, referred to the Caledonian orogeny.
Structures clearly younger than the Strathconon fault have not been recognized.
As the younger movements - Strathconon age faulting and the third
Caledonian folding - are largely responsible for the obvious geometry of the ground as it can be read from the maps; they will be discussed first. Folds formed in the earlier movement phases are so tight that a structural succession of sub-parallel fold limbs was developed; this responded to the third phase movements essentially like a normal stratigraphical succession of parallel beds.
A. Faults
The most important faults in the area are those related to the Strathconon fault. This fault, one of the great Scottish post-
Caledonian wrench faults, passes through the northern edge of the area.
In the slopes west of Loch Duich, the Strathconon fault splits into two sub-parallel fault planes 500 feet to 1000 feet apart. Much of the ground in the fault zone is poorly exposed and the position of the two planes can only be readily determined in a few localities. One of the fault planes, possibly the main one at that locality, is well exposed in the side of a ravine in the lower slopes west of 112 the head of Loch Duich. The rock of the fault zone is here a breccia consisting of angular to rounded fragments of migmatized semipelite, psammite, and vein—quartz set in a fine clay paste stained red with haematite and locally cemented by calcite. The breccia is massive, but does have a rude planar structure dipping southward at 750 to 800 which indicates the local dip of the fault. Near Loch Duich the fault touches the south contact of the Ratagain
Igneous Complex (Nicholls 1951) and the rocks of the complex are cut by shear zones parallel to the fault. The complex must therefore have been emplaced before the faulting. The fact that a dark lamprophyre dyke with a Iff.N.17% trend is apparently cut by the Strathconon fault northwest of Sgurr Mhic Bharraich suggests that the faulting also post—dates some of the lamprophyre dykes. The more southerly plane of the Strathconon fault splits again northwest of Sgurr Mhic Bharraich. The one branch continues parallel to the main fault zone, but the other branch strikes away to the southwest, passes just north of Beinn AoidhdrIllean and then forms the southern boundary of the Lewisian inlier of Glen Aoidhdailean. This fault will be called the Beinn Aoidhdailean fault. It rejoins the Strathconon fault west of the Saddle Area (Geol. Surv. Scot. Sheet 71) and thus completely isolates a lens of rock between itself and the Strathconon fault zone.
The Strathconon fault is considered to have a sinistral displacement of two to three miles (Peach & others 1910, 1913), and there is some local evidence on this point. Bodies of diorite and dykes of felsite related to the Ratagain igneous complex have been 113 found in the Five Sisters northeast of Glen Shiel and in Glen Lichd on the northeast side of the Sisters (Dhonau 1960). In the Saddle Area diorite has not been observed, felsite dykes are rare and have only been found north of the fault. This suggests that the Ratagain complex has been separated by some two miles from its associated minor intrusions in the Five Sisters.
Because of its curved strike, it is most -probable that the
movements on the Beinn Aoidhdailean fault were dominantly vertical.
Correlations of structures north and south of the fault in the ground west of and outside the Saddle Area suggest that the block south of the fault is down-thrown relative to the north and that the combined vertical displacement of the Strathconon and Beinn Aoidhdailean faults is of the order of 5000 feet (J.G. Ramsay, personal communication).
Vertical displacement of this order of magnitude is in accordance with a reasonable interpretation of third phase fold geometry in the
Saddle Area, as will be discussed later.
Another fault of the Strathconon set passes through the southwest corner of the Saddle Area, at the head of Glen Aoidhdailean (Ramsay and
Spring 1962). According to G. Tanner (personal communication) it continues into the Kinloch Hourn fault near the head of Loch Hourn
(T. Clifford 1957). Horizontal (Misplacement on this fault is doxtral.
Numerous lines of crush on which displacement could not be proven have been observed in the Saddle Area. Near the Strathconon fault zone, they are mainly parallel to the fault. Further south, as in the region between Creag nan Damh and Sgurr a Chuilinn, there are a number of vertical crush zones which trend N.N.W. but apparently have no displacement. They are parallel, and may be related to the Kinloch
Hourn set.
B. Folding The large folds, which are responsible for the obvious structures of the ground, fold the axial planes of at least two earlier sets of folds. For convenience tho fold sets, and the movement phases responsible for them, will bo numerated as follows:- III - the youngest folds. These are the fairly open, and therefore obvious folds, which are seen from the deflections of the lithologic bands and foliation trends on the map. It will be shown that all the folds of this style are probably coeval. II - the folds older than III. These are tight to isoclinal folds and can only locally be identified from the deflection of strata. I - all the folds which can be shown to predate phase II. The existence of these folds is not obvious at all from the map and hence their recognition does not depend on geometry alone but, in part at least, relies on an interpretation of the stratigraphy.
In spite of local uncertainties, these three fold generations can be distinguished with some confidence in the entire area, and their separation is justified. a) Folds of Phase III
The large phase III folds in the area may be conveniently grouped into three tectonic units;
(1) the pair of antiforms which combine to make the large antiformal
structure that dominates the entire area southeast of the 115
BeinnLAoidhdailean fault between Glen Aoidhaailean and Glen
Shiel
(2)the pair of antiforns between Faochag and the Saddle ridge
(3)the trio of folds that make up the lens between the Beinn
Aoidhdailean and Strathconon fault.
The dominant structure of the area is a large antiformal box fold whose two parallel axes plunge southeast and with two axial planes dipping towards each other. The term "box fold" (Bills 1953) is used here simply in reference to the shape of the folded surfaces. The box folds of the Saddle Area have some features in common with the conjugate folds described by Johnson (1956) and Ramsay (1962) and fit
Ramsay's more general definition. The term is avoided, however, because it may imply genetic notions including the brittle state of the rocks and shearing movements on the axial plane to some geologists.
The antiforms at either side of the fold are analogous to Busk's (1928)
"anticlinal bond".
The shape of the fold is best outlined on the map by the Sgurr na
Creige Lewisianoid zone and the southern (upper) boundary of the
Central migmatite complex. The lower bouncInry of the complex is also clearly folded by the western half of the box fold in Glen More.
The axial plane of the western antiforn approximately follows the course of the Glenmore River and dips east. For convenience, this western portion of the box fold will be referred to as IIIG. In spite of limited exposure in Glen More, the IIIG axial trace can be located fairly accurately. The axial plane dips at approximately !0° to 50o to the east, but the relief is not sufficiently great to permit a more 116 accurate direct dip determination.
On the east side of Glen More, the beds have a moderate dip towards the southeast and their trace can be recognized in the hillside.
At the head of the Glen the strike changes from about 40°, through
900 to 120° and the dip of the beds becomes vertical. This curvature of the foliation, which marks the hinge of the fold, can be followed by walking along the lower contact of the Sgurr na Creige pelite, or the Lewisianoid around the corrie, from the east to the west side. As the beds are traced northwestward, along the west side of Glen More, the strike changes from 120° to 150° and the beds dip eastward, i.e. they are overturned towards the west.
The northeastern antiform is best outlined by the Undalain psammite.
It might be noted here that the apparent curvature of the psammite as seen on the map, away from its southwest trend towards Sgurr a Gharg
Gharaidh, is only a result of the intersection of the topography with moderately dipping beds. The axial trace of the antiform can be followed from the Strathconon fault zone, which truncates the fold, across the northwesterly facing wall of Choire nan Laogh, across Alt
Undalain and over AlMhuing to Glen Shiel. This northeastern portion of the box fold will be referred to as IIIB. As shown in Fig. 16, the fold is clearly seen from the ridge east of Alt Undalain, looking at the slope of Sgurr Bharraich. North of Choire nan Meirleach, the beds stand vertically. On the south side of the corrie„ the beds, here on the "roof" of the box fold, dip at a moderate angle towards the observer. East of A'Mhuing, and structurally upwards, the radius of curvature of the fold hinge increases and the fold broadens out. 117
Figure 16
HINGE OP IIIB IN CHOIRE NAL LTIRLEACH
IIIB Axial Plane - ••••••
Undalain Psammite
•••• ••••
/1 k
to -•• / 1 7,"
, pr •
\ \ IIIB Axial Trace
View from the east. After field sketch by G. Wilson. 118
In the Five Sisters, cast of the Shiel River, the fold does not exist
(Dhonau 1960). Because of the broadening of the fold hinge the axial plane cannot be precisely located east of Allithuing. The attitude of the IIIB axial plane was determined by the "three point method" as having o o a strike of 900-95°, dip 40 -45 S, between AtMhuing and. Alt Undnlain, o o and a striko of 100°-.110°, dip 45 -50 S. in Choire nan Laogh. The axial plane of a "bulge" whose trace was napped by Dhonau (1960, page 46) in the Five Sisters just north of Shiel Bridge is probably the axial plane of the synformal complement of IIIB. It is approximately parallel to the IIIB axial plane, and like IIIB this synform dies out structurally upwards.
The plunge of the IITR and IIIG antiforms in the region below the
Sgurr na Creige polite, where the two folds exist, can be calculated in two ways without recourse to the minor folds:
(1)plotting the loci of the Tis-poles of the planes of foliation or bedding on a stereogram and determining the pole of the great
circle so formed: which gives the attitude of the axis, Fig. 17 (2)by determining the plunges of the lines of intersection of the
two axial planes with the general strike and dip of the foliation forming the roof of the box fold, Fig. 18. The results of both methods agree well and show that the two antiforms are approximately coaxial, plunging 30°-40° towards 125°-135 . The line of intersection of the IIIB and IIIG axial planes has the same plunge as the axes and the structure as a whole has orthorhombic symmetry.
119
TFit5ure 17
It POLES OF FOLIATION Am) SCIIISTOSITY BELOW SGURR NA ()REIM fELITE
. foliation x phwle II axial plcne schistosity
• • • „I. 11.
'
axes of box fold. ,/ ; / •
This and all other "stereoLTarz" are "enual area" nrojectionn„ 1 20
INTIMSECTION OF IIIB AND IIIG AXIAL MATTES WITH SGURR NA CREIGE PELITE 121
The shape of the large box fold is best seen in the profile. The following points are clearly demonstrated.
The Sgurr na Creige Lewisianoid and the Underlain psammite outline a structure with a relatively short and nearly vertical I.E. limb, a midale limb or "roof" 4 miles wide which buqs a moderate S.E. dip increasing from 20° to 60° southeastward, and a western limb of unknown length that is structurally overturned to the west.
Structurally upwards, the northeast (IIIG) antiform and its complementary synform die out and the structurally higher levels do not outline a box fold, (profile) Fig. ). Only the IIIG antiform which can be traced south across Loch Hourn (G. Tanner) personal communication) continues structurally upwards. In fact, above the
Sgurr na Creige pelites the IIIG fold hinge becomes tighter as if some decollement had taken place in a zone near the upper boundary of the pelite. All the rocks of the Saddle Area and of the Five Sisters which are structurally above the Sgurr na Creige pelite are on the upper limb of the Glen More, IIIG fold, and within the area they bend over to form the steep lower limb of the fold. It would seem that this fold is one of the major structures in this part of the Northwest Highlands. This great structural succession, however, is not folded over by the =TR antiform.
The outer boundaries as well as the zones of migmatization intensity of the Central complex are also folded by the phase III folds, and they conform to the general structure.
Four types of minor folds are apparently associated with the large box fold: 122
monoclines of "plastic" appearance congruent with IIIG. Micas in the pelites are crinkled as described on page 30, but thin
psammite bands and granite lenses are folded without rupture. The folds do not have axial plane cleavage or axial plane schistosity except in pelitic bands where the axial planes of the crinkles may be considered as a variety of "strain slip cleavage" (Wilson 1961, fig. 28A), (Figs. 4. and 19). There is considerable variation in the style of these folds. As illustrated in Fig. 19, a, for certain bands the fold is tighter and departs
from the more typical monoclinal shape. Regular spacing of the folds on large dip slope exposures gives the rock a mullioned appearance; this is well seen on the roadside exposures just southeast of Shiel Bridge.
(2)monoclines congruent with IIIG of "brittle" appearance. Granite lenses are crushed and broken, feldspar is reddened and micas
chloritized, particularly in the axial zone. These folds also do not have an axial plane cleavage, but a few fractures have
developed in, and parallel to, the axial plane zone (Fig. 19). Locally slickenside striations are found on the foliation plane approximately normal to the axes of the folds.
(3)monoclines congruent with IIIB of plastic appearance
(4)monoclines congruent with IIIB of brittle appearance. The last two (3) and (4), are mirror images of (1) and (2) above respectively, and are identical to them.
The brittle type of minor folds are believed to represent folding under more "rigid" conditions than those prevalent during the "plastic"
123
Figure 19
MINOR PHASE III FOLDS
G ins (a) IIIG plastic with (b) IIID plastic in (c) Plastic IIIB,G box pegnatite. Glen Glen Shiel. fold. OhoirtUaine. More.
3 feet i foot 2 feet (d) Discordant pegmatite (e) Brittle IIIG fold (f) Brittle IIIB,G box folded and sheared by with "a" striations. fold refolding brittle IIIG fold. Sgurr Mhic Bharraich. phase II fold. Choire nan Laogh. Sgurr Mhic Bharraich.
(g) Plastic MB fold with ( h) IIIL fold with mullions. dgcollemeni. In Lewisianoid. Ridge of the Saddle. AlMhuing. folding and so to have been formed slightly later. They are,
nevertheless, grouped with phase III and thought to have contributed to the formation of the major box fold. The reasons for this
conclusion are as follows:- (1)IIIB and IIIG "brittle" have approximately the same geometry and orientation as have IIIB and IIIG "plastic" respectively (rig. 19 and Fig. 20). (2)The distribution of the brittle and plastic minor folds in different portions of the major box fold, qualitatively, if not
quantitatively follow the same rules. For example, both IIIB brittle and IIIB plastic arc most common on the eastern, upright limb of the box fold, but are rare in the "roof" and western limb. Minor folds of both types are common in the hinge zones. Both IIIG brittle and IIIG plastic are common in the "roof" and western limb.
(3)In Choire nan Laogh on the north slope of Sgurr Mhic Bharraich, the psammites and semipelites are crushed and reddened by
haematite in the axial zone of the major IIIB fold, Therefore,
some of the phase III folding did take place under rigid conditions. This alteration dies out away from the axial zone and would appear to be related to it.
(4)Every gradation can be found between brittle and -Mastic folds,
the brittle ones being best developed in psammite, and their appearance approaching that of the plastic folds as the rock becomes more micaceous.
125
Figure 20
AXES AND AXIAT., HAMS OF IIIB AID MG IfINOR FOLDS
Axes of Folds . plastic ; o brittle Axial Manes of Folds — x plastic ; A brittle
A 4. %p • A 'N A ga,, Al. Y. A 1.1. X t, Y. , A A* v. \ 1. )4 Sr A A* A 14. \ 1. Y. * i.4 "A•Alk % A I.** 1. \ A 1. 7.. . appA % y"yA,- \ GI Y. \ ‘ '.• \ Ne” ‘)..A V1211--' 11p 1 OD \ 1 \ —1—. \ A 4 AA 14 I • x • X.w.. .,,,,,,,,awsxsay. AA 4 A • 1 .e• N.. XN AAY`X x % X 0 1 / N. X i. AN A irs, 0 • 1 '.. ' N. 4 Ar X A .. X Ooncion III& and 0 0 0 • o• ". A •• •. 0 • 00' • e* 'IN ,A x A x o o .000 0 0 ...." V. 0 11. • axis .••cr -•90. • Axial Plane • • • :3' • • /* • / 0 / *0 • 126
It is considered that the two antiformal bends with their opposed axial planes, which make up the large box fold, developed simultaneously. This conclusion is based on the following observations:-
(1)Locally, as in the axial zcnes of the major structure, the minor folds of IIIG and IIIB types occur together in the same exposure, where the axial planes of the one set can be seen to cross the
axial planes of the other set. Neither axial plane is folded by the other structure as would be expected if they were of different ages; instead, minor box folds are formed (Fig. 19).
(2)Pegmatite lenses and dykes lie in the axial planes of plastic IIIG folds as well as in those of IIIG minor folds.
(3)The micas in politic rocks which are affected by IIIG minor folds are crinkled in the same way as is seen where IIIG minor folds deform polite. Few mica flakes lie in the axial plane, instead the mica flakes outline the folds. Locally some of the mica flakes are chloritized near the axial plane. Both (2) and (3) indicate similar metamorphic conditions during IIIG and IIIG folding, which in turn suggests that they have a similar age.
Another box fold forms the ground between the ridge of the Sadello and that which joins Faochag to Sgurr na Sgine. There is no single marker band that outlines the "box", but the "roof" is defined by the semipelitic band of Faochag. The structure is made of two antiformal bends with axial planes which dip at 50° and 60° towards each other and have two approximately parallel axes. The more westerly axial plane passes in a N.K.E. direction through Lochain Choire Mhalagain on the watershed 127 and is lost in the south face of the Saddle ridge where the fold dies out in a series of small folds with axial planes parallel to the major axial plane and some hundred feet apart. This portion of the box fold will be referred to as IIIL. The more easterly axial plane extends in a N.W. dlrection from the summit of Faochag to the south face of the Saddle.ridge where, like the IIIL axial plane, it is lost in a series of large, but relatively minor folds. This eastern portion of the box fold will be called IIIF. It is probable that the axial trace of a synformal bend, complementary to IIIF, lies some 2000 feet to the north of the IIIF axial trace, but it is too gentle a. fold to be located accurately. The strike of the IIIL axial plane can be read from the map as being about N.25°.E., but the relief along the axial trace is too low to permit accurate direct determination of the dip. It was, however, calculated indirectly in two ways: ( ) by plotting on a stereogram (Fig. 21) the axial planes of minor folds apparently congruent with the major IIIL fold. This method gives a mean value of strike N.325.E. and dip 56°S.E. (2) by plotting on the stereogram (Fig. 21) the strike of the axial plane and the plunge of the axes. The great circle that includes the two ends of the strike line and the pole of the fold axis
represents the axial plane. With a strike of N.25°I.E., the dip is 60°S.E. which is in fair agreement with that determined by method (1). The attitude of the IIIF axial plane was determined by the "three point method!' as having a strike of approximately 100° and dip 400-550S.
123
.:54 :tiro 21
THE AXIAL PLANE OF IIIL
x oidal planes of minor folds
t ►
X /0 P i td 4?, /0 1 C• • ..... i..,... • '2./ • I 'sr 1i I,s4i • / ett 1 1.4.o .....,/ r0' • ,m,..; i /AY' dit)• ♦• ,, / ... 1 ( -9 • Jr 1. kdi I i di +1, \ 'sr i 1 0 °*•Y i ..t 'Ne 4:// 6"// 10'
44-,16;;•.. ,...... -.... Cd, / Axis From 7;•70 .., ..... ••-• 4)/ Pig. 2.. ...- -- ' k•-i>1 -- -* -V, _. •••••• dr. , ..d.dii ill, CI ' ., iIi ./, ./ , / do 129
The uncertainty in the dip results from the uncertainty of the point where the axial trace crosses Alt a Choire Malagain. Only two minor folds were found that were apparently congruent with IIIF. The axial planes of two minor folds are not considered enough to define the axial plane of the major fold, but it can be seen from Fig. 22 that their attitude agrees with that of the major axial plane as determined by the three point method. The attitude of the axis of the IIIL antiformal bend was determined from the attitude of the minor folds and by plotting the Ilipoles of the bedding and schistosity planes on a stereogram and determining the pole of the great circle so formed (Fig. 23). Both methods give results that agree well within their margin of error. The attitude of the axis is taken as having a trend of 164° and a plunge of 50°. The attitude of the axis of the IIIF antiformal bend could not be determined from the minor folds. Instead, it was determined firstly by plotting the fe'poles as above, and secondly by finding the line of intersection of the "roof" and the axial plane (Fig. 22) as in method (2), page 127. The mean positions from both methods gives a trend of 170° and a plunge of 46°.
The line of intersection of the two axial planes has a plunge of
168°/47°. Thus the line of intersection of two axial planes and the axes of the two antiformal bends are parallel. The box fold therefore has true orthorhombic symmetry with the symmetry b axis lying in the bedding plane.
In the profile (Fig. at back), the shape of the box fold is outlined by the flat "roof" of Faochag semipelite and the nearly vertical
130
Figure 22
CALCULATION OF AXIS AND ATIAL PLANE OF IIIF
Foliation x Phase II axial plane schistosity o Minor IIIF axial planes
N
0 0 • •Y.. • •
.:A • • • . • • • • • • • . 4. • )1. x x . i% ci'l • •:( • x • .1.° . . • . w x i101.. . ik. \ ..
\ ,4151 / •••• % • • + ‘. •.• .i./. ,.. ;'41. /1 ‘ ‘ 1 p kil Q 47e • 4•-• .• •,.. a.Z p Possible Positions d, , • .-9 ... /4, ,c• .., + fi, of Axis from , •,. 4, ... • di • 0'. s. : ...„, , 717). iff....li: poles / .••• ,••• ••• .... • '',, 0..,ik, /‘ • -... ••• ,... \.,•‘,J '••,
.... 04 •••.•:/ ftiz ,/ s 41‘.`1`.... #.*- •••• 1... .ez - .... - ,•A/ • • position N, • , ;:--"...... " C
4:._ ..._ . , .—• --..-- *"...— ..- .- -- ..., 't — %,...... :
Calculated Axis 131
Figure 23
CALCULATION OF IIIL AXIS
. Foliation x Phase II axial plane sohistosity 4 Axes of IIIL minor folds
..-.•••1116 132 foliation planes in the underlying psammite. The plunge of the small box fold is steeper and more southerly than that of the large box fold. It was drawn on the profile by projecting up curving plunge lines, on the assumption which may or may not be warranted, that the rocks and the structures on Faochag once extended to a point some 2 miles above Sgurr Mhic Bharraich and that there their attitude was once the same as that of the rocks now seen on Sgurr Mhic Bharraich.
The IIIF antiformal bend can be well observed on the north slope of Faochag. _The near vertical beds and phase II axial planes of the east limb of IIIF are well exposed on the floor of Choire Mhalagain.
The IIIL antiformal bend can be seen just south of Lochain Choire
Mhalagain but neither the hinge zone, nor the west limb of IIIL are well defined within the map area. The western bend in the Faochag semipelite has been drawn on the profile using data gained by
G. Tanner (personal communication) during reconnaissance mapping.
The zone in which the two axial planes come together is well exposed on the south face of the Saddle ridge. It is not marked either by a single plane of decollement as Heim (1921) envisaged at the base of some of the Jura box folds (Kofferfblten), nor by a tight fold with upright axial plane as Paterson and. Weiss (1962) obtained in their experiments. Instead, the folding is gradually dissipated into a series of folds with a wave length of 100 to 200 feet and a style similar to the IIIL minor folds (Fig. 19).
The minor folds associated with the box fold are mainly of one type in all parts of the structure. They are "plastic" monoclines with rounded hinges, congruent with IIIL rather than with IIIF. 133 They have no cleavage or axial plane schistosity. Psammite beds and
granitic veins are folded by them without rupture or crushing, and one small fold in Choire Mhalagain has a pegmatite dyke lying in its axial plane. A coarse type of crumples or small fold mn115ons (Wilson 1953, fig. 3) are developed locally. The occurance of (here incongruous) IIIL minor folds on the steep east limb of IIIF can be
seen very well on the floor of Choire Mhalagain. Only in two localities have minor folds of IIIF type been found. At the one point, south of Loch Choire Mhalagain, the IIIF minor folds are crinkles in micaceous semipelite, identical to IIIG and IIIB crinkles described from the Sgurr na Creige pelite.
The age relations of the two axial planes of the box fold are difficult to verify, mainly because there are so few IIIL minor folds.
The two antiformal bends are believedl however, to have formed approximately at the same time, largely in analogy to the large box fold in which IIIG and IIIB can be demonstrated to have developed
together. There are, moreover, two lines of evidence which support this view.
(1)IIIL minor folds, on the east limb of ITU, are not refolded.
They must therefore be younger than IIIF or of the same age. (2)In the hinge zones of the minor folds of IIIF and I= type, mica lies in the folded phase II schistosity rather than in the phase III axial planes and is not crushed. It would seem IIIF and IIIL formed under similar metamorphic conditions and possibly at the same time. 134
The small box fold between Faochag and the Saddle is considered to have formed at the same time as the large box fold and therefore to be a phase III fold for the following reasons.
(1)The Faochag box fold and the large box fold have a similar style
and a similar orientation.
(2)They both refold phase II folds and are therefore both later
than phase II.
(3)They have similar relationships to pegmatite and mica growth
and therefore were formed under similar metamorphic conditions.
The lens of rock between the Beinn Aoidhdailean and Strathconon faults is made of three major phase III folds, two of which combine to make one synformal box fold. The box fold is best outlined on the map and profile by the Fhithich Mhoir pelite. The complimentary antiform to the southwest is best outlined on the map and profile by the
Sgamadail psammite.
The axial trace of the eastern synformal bend can be followed about
1000 feet southwest and parallel to the Glenmore River from the
Strathconon fault to the Beinn Aoidhdailean fault. This eastern portion of the box fold will be referred to as IIIN. The axial trace of the western synformal bend which will be called IIIM passes from the mouth of
Choire Sgamadail over the summit of Bad an Fhithich Mhoir and is truncated by the Beinn Aoidhdnilean fault before it reaches Glen More.
The complimentary antiform will be called IIIA. Its axial trace can be followed from the head of Choire Sgamadail in the southeast, down the stream of the same name, and over AIChranrngto the Strathconon fault zone where it is cut off. 135 The attitudes of none of the axial planes can be determined directly by the "three point method" and have had to be calculated indirectly. The plunge of the axes was determined from plot of the
lipoles of bedding and schistosity from the limbs and hinges of the folds as has already been described (Fig. 24). The axial plane is then represented by the great circle that includes the strike line of the axial plane as read from the map and the pole of the fold axis
(Fig. 25). The attitudes of the three axial planes and axes are
tabulated below.
Fold Strike and Dip of Axial Plane Plun e of Axis IIIN 160° 4.5°E. 128°/30° IIIM 100° 70°S.W. 110°/30° ILIA. 122° 6o°N.E. 100°/30°-35°
The two axial planes, IIIM and IIIN, of the box fold intersect
along a line having a plunge of about 115°/35° and thus differing little from the plunge of the IIIM and N axes. Like the two other box folds of the Saddle Area, this box synform has roughly orthorhombic symmetry with the symmetry b axis of the structure lying on the foliation plane.
The axial plane of IIIA, the complimentary antiform of IIIM, intersects the IIIM and N axial planes in two lines which are only roughly parallel to the three fold axes and to the line of intersection of the IIIM and IIIN axial planes (Fig. 25). To what extent the lack
of parallelism is real is uncertain. As can be seen from the scatter
of 1I' poles in Fig. 24, the points of intersection of the axial plane i36
ngure 24 r AXES OF IIIN, M ArD A FROM Tram OF FOLIATION SCHISTOSITY
• Poles of foliation and schistosity of limbs of IIIN ft $1 X " IIIM o a if n n n a IS " IIIA 137
[Figure 25
CALCULATION OF Int:, 11 ATM A AXIAL MANES
The strike lines are tal:en from the map. The axes are obtained from Fig. 2). 138
great circles could be brought closer together. The shift of the
axes from N to A in Fig. 24. does seem, however, to be real at least
in part, and to be connected with the westward shift of their" poles
in the northern portion of the stereogram. On the map, the more
easterly plunge of the western portion of IIIA can be recognized
from the orientation of the strike of the vertical to steeply
northward dipping Sgamadail psammite in Glen Aoidhdailean.
The profile (Fig.38) illustrates the shape of the three folds.
The IIIN and IIIM antiformal bends combine to form a synformal box
with a "floor", which is horizontal in profile and dips moderately S.E.p
a nearly vertical east limb, and steep eastward dipping, west limb.
The Sgamadail psammite outlines the structure with a "floor" more than
one mile wide. Structurally upwards, however, the two axial planes
come closer together, the "floor" becomes narrower and the box fold
dies out. The western synformal bend is sharp below Bad an Fhithich
Mhoir in Moire Sgamaani1, and is well outlined by the upper boundary
of the A'Chrannag semipelite. As the axial trace is followed over
the summit of Bad an Fhithich Mhoir the hinge zone becomes more rounded and broader as the fold dies out.
The IIIA fold is tight at the head of Choire Sgamadail where it
is truncated by the Beinn Aoidhdailean fault. This can be seen from
the nearly parallel attitude of the two limbs on either side of the
corrie. Structurally downwards, the fold becomes more open and near
the Strathconon fault it is dissipated in a series of small folds.
It can be seen from the map and profile that the outer boundaries and internal intensity zones of the small migmatite complex that occupy 139 part of the ground between the two faults are also folded by the three major folds, which lie between the Beinn Aoidhdailean and
Strathconon faults.
The three folds are grouped as being of one age because they do not refold one another, but form a series of complimentary antiforms and synforms. Moreover, they all fold the migmatite sheet and the phase II schistosity, they have a similar style, and are all truncated by the Strathconon age faults.
The minor folds apparently associated with the IIIM, N and A folds are very similar to the ones associated with the IIIG and IIIB folds. They are mainly monoclines of plastic appearance with the steep limbs facing southwest and axial planes dipping northeast. The steep limbs are usually a few inches to one or two feet long, but in politic rocks crinkles are developed of roughly the same shape as the monoclines and very similar to the crinkles described from the Sgurr na Creige pelite, on page 30.
A small number of monoclines with west-dipping axial planes as well as symmetrical folds with vertical axial planes were also found.
They too are mainly of "plastic" type, and in the more micaceous rocks, are associated with crinkles.
"Brittle" monoclines do occur, but they are far less common than
"plastic" ones. Many of the minor folds near the Strathconon fault zone have a "brittle look", but in fact the rocks were folded by them without rupture, the crushing and reddening which can be seen having been imposed on the rocks later as a result of the nearby fault movements. There are also some "brittle" monoclines near the i 40
Strathconon and Beinn Aoidhd..ailean faults, but these are probably related to the faulting and were observed to refold phase III folds.
The axial planes and axes of the minor folds are plotted as poles on a stereogram (Fig. 26). From this plot, the following observations may be made.
(1) All the minor folds have axes that plunge E.S.E. at 25°-35°
regardless of which way they face or whether they are "brittle" or
"plastic".
(2) Minor folds, from the region or sub-area occupied by each of the
major folds, plunge parallel to the axes of the major fold,
regardless of which type of minor fold is plotted. Thus the
small shift in the major axes from N to A in Fig. 24 is partially
reflected in the shift of the concentration of the minor fold
poles in Fig. 26,
(3) As already described, the axial planes of the minor folds fall
into three groups: (i) exactly parallel to the axial plane of IIIN,
(ii) nearly vertical and with a strike of 115°, and (iii) with a
strike of about 70° and a dip of about 40°S. Groups (ii) and (iii)
are not parallel to the axial planes of any of the three major folds.
(4.) The three sets of axial planes intersect approximately in one line.
The group (i) and (iii) axial planes are symmetrically disposed
about the vertical axial planes, malting an angle of about 60° with them.
(5) The line of intersection of the minor fold axial planes which
trends 120° and plunges 30°, approximately coincides with the Figure 26 AXES AND AXIAL PLANES OF MINOR FOLDS ON THE LINES OF IIIN, IIIM, AND IIIA
Axes . IIIA ; a IIIM ; a IIIN Axial planes - x IIIA ; o IIIN ; e IIIN 142
plunge of the minor folds (116/30 - 13(F725) as well as the plunge
of the major folds and the line of intersection of the IIIM and 0 the IIIN axial planes which was determined as being 116 /349.
Thus the three sets of minor folds together are congruent with, and share the orthorhombic symmetry of the major structure.
The major and minor folds just described are grouped with the phase III folds of the Saddle Area because of their similarity in shape and orientation to the other phase III folds, the fact that they were formed under similar metamorphic conditions and that they, like the other phase III folds, deform the migmatite complexes and the phase II axial plane schistosity.
A number of features of the phase III folds considered as a whole leads to the conclusion that they arc of concentric type and formed largely by flexure.
(1)Typically, competent layers such as psammite or granite have
roughly constant thickness, as measured normal to the foliation
plane, throughout the fold. This is seen by inspection on many
outcrops and was checked by measurements on folds in outcrops
yielding true profiles and on hand-specimens. The results are
shown as a series of graphs in Fig. 27.
(2)Locally, there are slickenside striations on the foliation surfaces,
roughly normal to the fold axes. These are more common in the
"brittle" folds, but they were also found on typically plastic
IIIB minor folds in a large road cut, 1 mile south of Shiel
Bridge. The striations are probably a type of "a" lineation
143
Ficure 27
TIIICKHESS OF BAIDS FOLDED BY PHAS 1.1 III FOLDS
i 1.0 { I - it.— 71.--n -7---T .0, , . i I V 1 1 0 ., 1• . I E." 1 1 1, 1 I I I
,-1 5 10 l5
4- L_ 9.
ds ban r o ss e kn
. Thic 7. 8 t o
A"
4 b 9 I o
Distance along beirlinc, i)lane in inches. 144
resulting from bedding plane slip (Nieuwenkapp 1928).
(3)It is a characterisitic feature of the major as well as the
minor folds that their amplitudes vary rapidly -with depth in the
structure. This is a feature related to the "room problem" in
flexural folding (de Sitter 1956).
(4)Although a simple d6'collemont plane was not observed at the base
of the major antiforms, such decollements are not uncommon -with
minor folds of plastic type. Fig. 19 shows a good example of
such d6.collement at a locality where actinolite schist layers
between more massive amphibolite bands act as lubricating planes.
There is also evidence that in some folds a shearing movement has taken place along the axial plane. In some of the monoclinos of brittle type, the axial plane is marked by a fay/ closely spaced cleavage planes.
In the vicinity of these planes the rock is crushed and reddened and marker bands are displaced.
In very open folds, whose limbs are at more than 1200 to each other, the axial plane is everywhere at such a high angle to the foliation, that there is little difference between the true thickness and the thickness measured along the axial plane. It is thus difficult to distinguish concentric from similar folds and it is therefore quite possible that some of the more open plastic folds are at least in part "similar".
If the inward dipping axial planes of the box folds represent planes of actual or potential shear related to the shearing stresses, then the orientation of the principal stress axes of the system which caused the folding may be calculated as shown by Johnson (1956) 145 and Ramsay (1962, b). Because the box folds of the Saddle Area have approximately orthorhombic symmetry they conform to Ramsay's (1962,b) simplest case where P intermediate = b (symmetry) and B (kinematic). In Fig. 28 the axial planes of the three box folds are plotted together and the principal axes of stress are deduced from each pair. P max is nearly horizontal and in a N.E.--S.W. direction. P int = b plunges moderately S.E. and P min plunges steeply N.W. It must be emphasized that these calculations would be invalid if the axial planes of the box folds were simply geometric constructions, whose position is dictated by the shape of the flexed beds. There are, however, a number of features which may possibly support the view that the axial planes are real structural planes related to the orientation of the stresses.
(1)Minor folds with axial planes of both inclinations occur throughout each structure. The minor folds were thus ubiquitous throughout the region, and aro not simply drag folds on the limbs of major structures but were generated by penetration stresses.
This is particularly well illnstrated by the abundance of IIIL
structures on the steep limb of the IIIF antiformal bend, and the occurance of minor folds in the region north of the Beinn
Aoidhdailean fault, whose axial planes are not parallel to any of the major axial planes there, but instead, are roughly
parallel to the IIIB axial plane. (2)In each of the three box folds one of the axial planes is almost exactly parallel to one of the axial planes of another box fold. (3)The axial planes are real planes of weakness in the rock. 146 11+7
Pegmatite was injected or secreted into them during the plastic stage, and during the rigid stage, shearing movement
took place along them. (4) The brittle type of monoclines combine to form box folds identical to those described by Johnson (1956) and Ramsay (1962, b).
As pointed out by Ramsay (1962, b), oven though the axial planes arc planes related to the maximum shearing stress, the resultant movement may be largely taken up by flexure of the beds.
The phase III folds of the Saddle Area can probably be correlated with Ramsay's (1960) phase 3 folds in the Arnisdale (Loch Hourn) Area and his phase 2 (now phase 3) folds of the Glenelg Area (Ramsay 1958), (Ramsay and Spring 1962). This correlation is based on the following evidence.
(1)The Loch Hourn-Glenelg third folds, like the Saddle phase III folds, follow a period of migmatization and high grade metamorphism. (2)In the Glenelg Area, abundant pegmatite bodies are shown to have
developed late in phase 2 (now phase 3) (Ramsay 1958) which is perhaps comparable with the pegmatite dykes occupying phase III axial planes in the Saddle Area. (3)In the northern portion of Glen Aoidhdailean, minor folds which
the writer groups with phase III on the east bark of the river,
are identical to minor folds on the west bank of the river which were mapped and grouped by Ramsay with his phase 3. (4)Like Ramsay's major third folds, the phase III major folds of the Saddle Area are truncated by the faults of Strathconon age, and congruent minor folds associated with the main folds are
crushed near the fault zone (5) The steeply dipping west limb of the Glen More (IIIG) antiform is the same as the steeply dipping east limb of the Ben Sgriol (Beinn Sgritheall) synform, described as a major phase 3 fold by Ramsay (1960). The Glen More fold probably is the antiformal
compliment of the Ben Sgriol fold and isr therefore, also a phase 3-Ramsay-fold. In their general shape and orientation the large third phase folds of the Loch Hourn Area are not unlike the IIIG and the IIIN folds in Glen More, and the IIIL fold south of the Saddle. Their axial planes dip east at about 45() and the folds have a nearly monoclinal shape, with steeply dipping, west-facing limbs and nearly flat east limbs. They differ, however, from the Saddle phase III folds in that they are of "similar" rather than of concentric type and, according to Ramsay
(1960) were formed by shear along their axial planes with a, the o direction of tectonic transport, being a line plunging at 40 towards
122° Unfortunately, a comparable a direction could not be determined in the Saddle Area because the phase III folds do not deform the earlier lineations, but the plunges of the phase III (3) folds in the two areas is approximately the same.
The phase 3 axial planes in the Loch Hourn Area might perhaps be considered as having the same relationship to the principal stress axes as have the east-dipping phase III axial planes of the Saddle Area, but there the west-dipping axial planes are absent. In the 149 Saddle Area the eastward dipping axial planes are more numerous and more prominent than those which dip westwards. At Loch Hourn this difference is developed to the cogplete exclusion of the westerly dipping planes. The reason for this weak development or absence of one of the predicted planes of shear or potential shear is a difficult problem in geology. A. possible explanation may perhaps be found in the suggestion made by Ramsay (1962, page 521) which was substantiated by the experiments of Paterson and Weiss (1962), that if one of the planes of shear or potential shear is near the foliation plane, it will not cypear as an axial plane. Therefore, if the pro-phase 5-111 orientation of the foliation was such that it was nearer to the westward dipping than to the eastward dipping potential axial planes, then it is to be expected that the eastward dipping axial plane would be more strongly developed.
Ramsay (1960) concluded that the steeply dipping limbs of the third folds are nearer to their rare phase 3 orientation than the "flat" limbs.
By contrast it is more reasonable in the Saddle Area to consider the "flat" "roofs" of the box folds to be nearer their pre-phase III orientation than are the steep limbs. Three possibilities have to be considered if the phase 5=III correlation is accepted. (1)Ramsay is right, the writer is wrong. (2)Ramsay is wrong, the writer is right. (3)Both Ramsay and the writer are right.
The writer can find no alternative to Ramsayls (1960) interpretation of the fact that the pre-phase 3 structures of the steep limbs are less 150 deformed than those of the "flat" limbs. Also he cannot envisage that the "roofs" of the box folds were steeply dipping prior to phase III folding. Possibility (3) is therefore accepted, and as a tentative explanation it is suggested that a great arch had developed sometime prior to phase III, and that the western limb of this arch was steep; and it was upon this major structure that the phase III folds were developed. There is considerable difficulty in correlating the axial planes of the third folds northward across the Strathconon—age faults. The Ben Sgriol axial plane is cut by the Beinn Aoidhdailean (B.A.) fault just north of Ben Sgriol. In the lens between the B.A. fault and the Strathconon (S.C.) fault, the only fold that could possibly be correlated with the Ben Sgriol fold is the IIIN synform just west of the Glenmore River. If this correlation is correct, the apparent horizontal displacement of the axial trace is about 3 miles dextral. Both segments of the axial plane dip eastward at about 45° and their apparent dip on the plane representing the mean attitude of the curved
B.A. fault is about 40°. The Kinloch Hourn fault may have dextral displacement of nearly mile so that the displacement to be taken up by the B.A. fault is 21. miles. This could be taken up by dextral shift of 2:1- miles or a vertical shift — north side up— of 2 miles
(Fig. 29). A little dextral movement on the B.A, fault would lessen that figure. If the II= fold is not the northern segment of the Ben Sgriol fold then the displacements on the faults have to be even greater.
I 51
F i u•re 1.9.
DISPLACEMENT OF PHASE III AXIAL PLANES BY F- AU 1:1- 5
A
1 — , - _ ‘,-) -- — - \3 . \ — Ni,1 Ci' -. 4\1 14 „. Sc) \--6, l' - - -\•, ?.• \ 4- c'''. -4' s oss Lis ci3 Voc' ck fle 0 e ox,--- 1 4C/ _ , _ -- 1 ____---rt....- ---- ,_,.,:, t_li-o.c.e...... -- / ffl k 6:-/,--, ...._ -:;--:"r.. ,_I x ; e rD it, • 4_ o , "•%, 0 ui& 3rt oi ii-- ul i Pi i 14t(h N Q t,
A\ Q f. DextralDe 6 Shi ft o rs
Bourn. Fault ...>" ' no
A ('Ai Cr S ,A N • •i•6,- 1 -e....l s L., V ‘• , 'A.e cii• 1 1;1 • C1' / 1:0 °/ — ------% " J •, Trace of III G
s One Mile i\ / 152
If the movement on the Strathconon fault were entirely horizontal, it would be expected that the line of intersection of the IIIG and IIIB axial planes would intersect the surface to the north of the S.C. fault, just west of the Ratagain Complex. From Ramsay's map (1958) and from sheet 71 of the Geol. Surv. of Scotland, it appears that a box fold does not exist in that vicinity. In fact, with the exception of a possible pair of small folds west of Torr Beag, the southeast limb of the Beinn A'Chaoinich-Beinn Mhialairidh antiform (Ramsay 1958) continues unfolded along the southeast side of the Ratagain Complex to Loch Duich. This suggests that the ground north of the S.C. fault is structurally lower (with respect to phase III structures) than the intersection of the IIIG and IIIB axial planes of the large box fold.
The two axial planes dip towards each other at about 45° in the fault plane, and probably intersect one mile below the surface just south of the fault (Fig. 29). Therefore, if the box fold does not exist on the north side of the fault, that side must have moved up at least one mile relative to the south side. b) Folds of phase II The phase II folds arc nearly isoclinal recumbent folds, whose axes plunge southeastward approximately parallel to the axes of the third folds. Hence, though the axial planes of the phase II folds are refolded, the plunge of their axes remains relatively constant, There is little point in attaching the name "synform" and "antiform" to the phase II folds because they change from upward closing to downward closing as they are traced over the hinges of phase III folds. Mere 153 they are recumbent "neutral" folds plunging down the dip of the foliation, as on the "roof" of the large box fold, slight changes in their -plunge have the sane effect.
Nearly all the minor folds that predate phase III in the Saddle
Area are grouped with phase II. In some localities the grouping has to be done arbitrarily for lack of evidence to the contrary. But it can be demonstrated on many exposures that these minor folds indeed are congruent with the larger phase II folds and that they were formed during an episode earlier than phase III, yet post-dating even earlier fold movements. On the northwest slope of Sgurr Mhic Bharraich and on the west slope of Choire Chaoil, minor folds identical in style and orientation with those demonstrated to be refolded by phase III folds, themselves refold even earlier folds (Fig.35). It is apparent, therefore, that the minor folds grouped as phase II are indeed older than phase III and younger than phase I, an even earlier phase of folding.
The main features of the style of these folds are illustrated by
Plates 11, 12, 13 and 14- The folds are nearly, but not quite, isoclinal and most of them would be classified as "similar" but not
"identical" according to Gill (1953). They vary greatly in size, from folds with adjacent axial planes enlyilf inch apart to folds with axial planes 30 feet apart. Even where exposure is good, however, folds with wave lengths intermediate in size between the latter (30 feet) and the large phase II folds shown on the map, whose axial planes are
2,000 to 5,000 feet apart, are not found. It was also observed that, in any particular exposure, there was not a complete gradation in the size 154- of the folds from large to small, but that the folds occurring
together could be grouped into a limited number of size classes. For
example, in a cliff of politic rocks. on the northwest slope of Sgurr
Mhic Bharraich, the amplitudes of folds are of the order of one inch,
3 inches, 12 inches and 3 feet. Some mica lies in the axial plane direction of the phase II folds
in all parts of the area, and in the migmatite complexes virtually
all the mica lies parallel to the axial plane giving the micaceous rocks a pronounced schistosity. The line of intersection of this
schistosity with the foliation constitutes one of the common phase II
lineations. Other typical phase II lineations are granite and quartz rods, the flutings on quartz veins and the alignment of acicular
minerals. All those lineations are parallel to the axes of the phase II
folds.
It has already been shown on the stereograns in the previous
section that the axial planes of the phase II minor folds are refolded
by the phase III folds, This can also be clearly seen in the field,
for instance, in the hinge zone of the IIIB fold in Choire nail Laogh,
the progressive change in the attitude of the phase II minor fold
axial planes, from vertical on the northeast limb to gently
southeast-dipping on the "roof", can be followed along the south wall
of the corrie. The refolding of phase II minor folds by phase III
minor folds was also observed on exposure scale in widely separated
parts of the area and is illustrated in Fig. 19.
To prove conclusively that the phase II minor folds are really
congruent with the major folds, here ascribed to phase II, is not easy, 155 largely because the orientation of major fold axes cannot be determined accurately without recourse to the minor folds. The axial planes of the minor and major folds are parallel, and at two localities on the southwest slope of Sgurr an t-Searraich (Dhonau 1960) and on the west slopes of Choire Chaoil the plunge of the major folds can be shown to roughly parallel that of the minor folds. Also the "regard" of the minor folds is here in agreement with the rlirection of closure of the major fold.
The only major phase II fold within the Saddle Area that can be. readily identified from the map is the westward closing fold that involves the boundary between the Dhomhuill semipelite and the Main
Ftammite on the west slope of Choire Chaoil. In the psammitic rocks, near their junction with the Dhomhuill semipelite, the foliation is distinct from the axial plane schistosity and individual beds can be followed around the fold hinge. Small parasitic isoclines whose axial planes are folded by the phase II folds were recognized in these beds. By contrast, in the semipelitic rocks, the smooth hinge is replaced by a series of tight minor folds with axial planes 10 to 20 feet apart. The actual position of the axial trace is lost and it is continued on the map across Choir tUaine by extrapolating it from
Choire Chaoil parallel to the phase II axial plane schistosity, aided locally by the distribution of "S", "W" and "Zr' folds. In the same may the axial trace was drawn over Biot an Fhithich and into Glen Shiel.
The complimentary, eastward closing fold, whose axial trace is drawn on the map roughly mile to the south, is recognized on the following evidence:- 156
(1)the termination of the semipelite on the west side of Choire
Chaoil
(2)the convergence of the foliation trends of the psammite north and south of the semipolito tongue that is thought to represent
the core of the fold ( 3 ) the distribution of "S", "W" and "Z" folds near the tip of the semipelite tongue in Choire Chaoil.
The axial trace is difficult to locate accurately and it has been
drawn over the Saddle and Biot an Fhithich largely by extrapolation
parallel to the phase II schistosity trend. Only locally, as on the
east side of Biot an Fhithich, was its -9osition checked by the
consistent direction of "regard' of large minor folds.
Whether the semipelite tongue just north of the westward closing
fold is a phase II fold, a phase I fold, or a facies change is not
certain, and no axial trace has been marked on the map. It would seem reasonable, however, that the tongue of semipelite - the Dhomhuill
semipelite as a whole - is at the core of a large northeastward closing
fold and that the pair of folds just described are only large
digitations.
Except in the hinge zones, the phase II major folds are so tight,
that their limbs are virtually parallel and a reliable estimate of
their axial plunge cannot be made from a stereographic plot of the
foliation, as was done for the phase III folds. The orientation of
the foliation and phase II axial planes is largely controlled by the
geometry of the phase III folds. The plunge of the axes can, however,
be determined from the attitude of the abundant minor folds. The 157 plunge of the minor phase II fold axes, from a zone one mile wide and straddling the pair of phase II axial traces drawn on the map, was 0 plotted on a stereogram in Fig. 30. The mean .-plunge is about 110 towards 165°, but there is a considerable spread which appears to result, in part at least, from a variation of the attitude of the minor fold axes within the phase II axial plane direction.
No major phase II folds were recognized south of Choire Chaoil but, because the monotonous lithology of the Main Psammite does not lend itself well to the detection of tight folds, there may well be unrecognized folds in that ground. Phase II minor folds arc also less abundant south and southeast of Meallan Odhar (at the head of Choire
Chaoil) than elsewhere in the area, even so they are widespread. Their axes are plotted in Fig.30 and they may be compared with the Choire Chaoil folds. In both groups there is considerable scatter, but the folds from the southeastern part of the area have a distinctly steeper plunge towards the southeast. This is related to the steep dip of the bedding and schistosity in that part of the area. It is also seen from
Fig. 30 that the plunge of the phase III (IIIF,L) folds falls within the limits of variation of the phase II fold plunges. The two sets of folds are thus roughly coaxial, but in detail some minor phase II folds diverge by 30° from the phase III fold axes. This was observed on exposures on the west side of Choire Chaoil.
In the Five Sisters which lie northeast of Glen Shiel, Dhonau (1960) demonstrated the presence of a nearly isoclinal, overturned fold in the polite band forming Scrum. an t-Searraich. It can be seen on the map and profile (Fig. 38 at back) that the axial plane of that fold is 158
Figure 30
AXES OF PHASE II MINOR FOLDS ON THE LIMBS OF THE MOIRE CHAOIL FOLDS AND IN GROW) TO THE SOUTHEAST
. Minor axes with Choire Chaoil folds x Minor axes in ground to the southeast
N 1
„ x Ax' x% . 74/ X % ;44 Area Within Which Fall • *,',`,1"x)(s. x% " • 14 It IL and F Axes 7‘ • IL ), I %. 11..”4.; • • • • • • . • •• •
•
gem *Ell, •
• • 159
refolded by phase III folds. Moreover, the minor folds which Dhonau
(1960) demonstrated to be congruent with it are identical in geometry
and orientation and in their relation to schistosity and migmatization
to the minor folds grouped with phase II in the Saddle Area. The
Sgurr an t-Searraich fold is, therefore, a phase II fold.
In Glen Shiel the axial plane of the Sgurr an t-Searraich fold of
the Five Sisters abuts against the Sgurr na Creige Lewisianoid sheet.
Therefore a slide has been introduced on the map and profile as the
upper boundary of the Sgurr na Creige polite. Further evidence for
the existence of that slide is found in the gradual appearance of
additional beds at the upper boundary of the polite and in the snr111
but abrupt change in the plunge of the phase II minor folds on either
side of its supposed position (Fig. 31).
The Sgurr na Creige pelite is identical to the Sgurr an t-Soarraich
pelite and both contain Lewisianoid1 lenses. They can probably be
correlated. If this correlation is correct, there are two possibilities
to be considered.
(1)The Sgurr na Creige pelite is on one limb of a phase II fold that
closes eastward and whose axial zone is replaced by the slide.
(2)The Sgurr na Creige polite is in the core of a phase II fold
closing westward and which is the underthrust continuation of the
Sgurr an t-Searraich fold.
1 The two lenses in the Sgurr an t-Searraich pelite were considered Moine cafe-silicate bodies by Dhonau (1960) and indeed they contain lime-silicate rocks. But, in addition, the writer found them to contain flaggy hornblende schist and amphibolite. They are thus identical to the Lewisianoid rocks of the Sgurr na aceige zone. 160
PHASE II MINOR FOLD AXES ON EITHER Si'K OF THE SLIDE : AIEHUING There is very little evidence to make a decision as to which interpretation is correct, but (2) is tentatively accepted here for the following reasons.
(1)The next phase II fold north of the Sgurr na Creige polite is eastward closing and it is more reasonable for a westward closing
fold to be in the pelite than for the polite to be on the north
limb of an eastward closing fold.
(2)The Lewisianoid lenses occur in two horizons in the Sgurr na
Creige pelite, suggesting the presence of a fold.
(3)The Sgurr na Creige pelite widens as it is traced northeastward,
psammite appears in the middle and the two Lewisianoid horizons
draw farther apart.
The axial trace of a fold which is similar in style and orientation to the Sgurr an t-Searraich fold but which closes in the opposite direction, can be traced from Sgurr Mhic Bharraich, over Sgurr a Gherg
Gharaidh to Glen More. This fold is presumably the northern
(structurn13y lower) complimentary of the Sgurr an t-Searraich
Sgurr na Creige fold.. Like the other phase II folds of the Saddle Area, the axial trace is drawn for most of its length simply by extrapolation parallel to the trend of the phase II schistosity. In Choire nan Laogh, the Undalain and Choir° nan Laogh psammites, which are identical, thicken and come very close together. It is most probable that they are one and the same. On the "roof" of the large box fold the two psammites are separated by nearly a mile of rock. It is considered, therefore, that they are one psammite, on two limbs of a fold, the axial trace of which is shown on the map and profile. 162
Both the Sgurr an t-Searraich - Sgurr na Creige fold and the one just described, which will be called the Sgurr Mhic Bharraich fold, are refolded by the phase III folds. On the stereogram (Fig. 32) are plotted the axes of the phase II minor folds and related axial lineations from all the ground between the upper boundnry of the Sgurr na Creige pelite (the slide) and the Strathoonon fault. The main features of this diagram are the strong clustering of the poles about a plunge of 30° towards 125° and the fact that this concentration coincides well with the axes of the IITB and IIIG folds. Both features testify to the fact that in the area here considered the phase II and III folds are coaxial. This can be seen clearly in the field on outcrops where phase II minor folds are refolded by phase III folds or where phase II axial lineations, such as rods, coincide with the hinges of phase III folds (Plate 15).
A phase II fold trace is tentatively drawn in the Glen Aoidhdailean pelite-semipelite unit largely because all along its length there occur large and relatively open "W" phase II folds. In the Laire
Brice semipelite, between the Sgurr na Creige pelite and the possible fold trace, the minor folds on many of the outcrops consistently indicate an antiform to the west. This antiform; however, could be either the Beinn nan Caorach - Beinn a Chapuill antiform (Ramsay and
Spring 1962) or the one suggested in the Glen Aoidhdailean pelite. If the latter is accepted, then a synform is to be expected between the
Beinn nan Caorach and the Glen Aoidhdailean antiforms.
In sharp contrast to the ground north and east of the Sgurr na
Creige polite, where the phase II minor folds are relatively constant .163 164
Plate 15: Phase II granite rods parallel to axes of IIIG folds. Sgurr Mhic Bharraich. 165 in their plunge, the minor folds in the ground west of the polite
(west of the slide) are much more variable in their orientation. This can be observed in individual outcrops and is well illustrated by the plot of the minor fold axes on the stereogram (Fig. 33). The poles are spread out in a diffuse girdle that approximates the phase II axial plane schistosity with a strike of about 160° and a dip of 300 o to 50 east. This great circle includes the position of axes of the
Sgurr na Creige and Sgurr Mhich Bharraich phase II folds and that of the poles of many folds which plunge eastward at 30° to 50°. The reason for this variability in the axial directions is not clear and there are at least two possible explanations.
(1)The folds were "born" with such irregular orientations because
they were superimposed on beds which were already folded and
had variable dips.
(2)The folds were deformed in such a manner during phase III
folding that the lineation was twisted in the phase II schistosity
plane without the schistosity itself being folded, in a manner
analogous to that described by Ramsay (1960).
The faulted continuation of the trace of the Beinn alChapuill antiform, a pre-phase III (3) fold, is to be expected in the middle of the pelitic and semipelite rocks (the A0 Chrannag semipelite) entering the area from the west in the northern portion of Glen Aoidhdailean.
The semipelite and the axial trace in it are again faulted by the
Strathconon fault south of Torr Beag, but its continuation south of the fault is to be expected in the semipelitic rocks found in the
Glenmore River east of the IIIN axial trace. As the Beinn nan Caorach 166
Figure 33
PHASE II MINOR FOLD AXES IN GLEN AOIDHDAILEAN
N 167 antiform is its equivalent (Ramsay and Spring 1962) the axial trace has to be faulted westward by the Beinn Aoidhdailean and Kinloch Hourn faults for more than three miles. In most of the ground between the
Strathconon and Beinn Aoidhdailean faults the phase II minor folds, probably congruent with the Beinn alChapuill fold, are of the "W"
type. They plunge moderately to the southeast and are coaxial with the phase III folds (Fig. 34.). This relationship is analogous to that found by Ramsay northwest of the Strathoonon fault (Ramsay 1958, page 498, sub-area 13).
Style and Mechanism of Foldin
The phase II minor folds are dominantly of similar type, inside as well as outside the migmatite complex. This is illustrated by
Plates 11 and 12 and was checked by measurements on several outcrops
In the field. The thickness of a bed or group of beds as measured parallel to the axinl -Dlane remains relatively constant across the fold. This is particularly true for the less competent semipelite and polite bands. Those also locally show the thickening at the hinge discussed by Ramsay (1962). An example of this is seen in Plate 13, above the hammer. Bands of rock, which under the metamorphic conditions of phase II appear to have been much more competent than the semipelites, do not everywhere share the "similar" geometry of the neighbouring, less competent beds. Very competent bands appear to retain a nearly constant true thickness right around the fold.
This behaviour is well illustrated by the quartz veins in the three folds to the right of the hammer in Plate 14. 163 169
Plate 11: Phase II fold. Sgurr a Gharg Gharaidh.
Plate 12: Phase II fold. Druim na Firean. 170
Plate 13: Phase II folds. Glen Aoidhdailean. Note thickening in the hinges.
Plate 1A Quartz veins folded by phase II folds. Glen Aoidhdailean. Note constant thickness of veins. 17i
The similar geometry of many of the folds, as well as the strongly developed axial plane schistosity, suggest that perhaps the folds formed by a mechanism of shear parallel to their axial plane schistosity
(Becker 1882), (Ramsay 1962), The behaviour of many of the more competent beds, however, suggests that the folds were originally localized and their development initiated by the concentric or flexural folding of the more competent struts,
Relation of phase II to Metamorphism Migmatization and Other Folds
It is concluded. that the Main Metamorphism, during which the
equilibrium mineral assemblages of the stnurolite-almandine subfacies crystallized, was roughly contemporaneous with and possibly outlasted
the development of the phase II folds, This conclusion is based on
the following facts,
(1)Mica flakes define a schistosity parallel to the axial planes
of the phase II folds,
(2)Acicular minerals such as amphiboles and epidote lie with their
long axes parallel to the phase II fold axes.
(3)Quartz occurs as spindle-shaped aggregates elongated parallel to
the phase II fold axes,
W As shown in Fig, 4, garnet was already present during the early,
plasic stage of phase III crumpling of the pelites as it
inte:s.feres 1W7h thc development of the individual crinkles.
The minerals are not crushed. or bent by phase II movements and the strong mineral lineation and schistosity associated with the phase II
axes must ha-,re developed during or before active growth of the it: minerals. The lines of granulation occurring in some of the schistose rocks (page 51) can be ascribed to bedding plane slip during late
stage phase III folding. To establish the time relation of migmatization and folding in
more than very general terms, is always very difficult, but it is
believed that it can be shown for the Saddle Area, that the Caledonian
migmatite complexes formed approximately at the same time as the
Main. Metamorphism and the phase II folding, and possibly outlasted the
latter. The evidence for this conclusion may be summarized in the
following points,
(1)The migmatite sheets are folded as a whole by the major phase III
folds and therefore must have formed before phase III.
(2)The lower boundary of the Central migmatite complex clearly cuts
across the phase II axial plane schistosity trend and the axial
plane of the northeastward closing Sgurr Mhic Bharraich phase II
fold; the lower boundary of the internal zone and the isopleths
of progressively outward decreasing abundance of granite behave
in the same manner, yet none of these lines (planes in three
dimensions) are folded. The zones of migmatization intensity
must have therefore been established late in or after the phase II
folding.
(3)Pegmatite which is in part contemporaneous with and in part earlier than phase III has discordant contacts with the granite
layers. The granite must therefore have formed before phase III.
In pelite, semipelite and hornblende schist, the granite occurs
as rods oriented parallel to the fold axes of the phase II folds. 173
There is no textural evidence that the granite was deformed into
rods after its development, and it is more probable that it
formed during or late in phase II as bodies preferentially
elongated parallel to the line of intersection of the foliation
and schistosity,
(5)At the margins of granitic lenses, layers and rods, feldspar
crystals locally protrude into the host-rock, across the
schistosity planes, yet they are not in any way deformed.
(6)Outside the migmatite complexes the mica is finer grained and
only a portion of it lies in the phase II axial plane schistosity.
Inside the complex the mica is coarse grained and all of it lies
in the axial plane schistosity. This shows that the phase II
stress-system was operative during migmatization and that more
intense recrystallization, perhaps because of higher volatile
pressures prevalent inside the complexes, permitted the more
perfect development of the schistosity.
(7)The mineral assemblages developed during the Main Metamorphism
were stable and grew during migmatization. Therefore, the
migmatization and Main Metamorphism proceeded under similar
metamorphic conditions and probably at the same time.
The fact that granite layers "go around" the minor folds is no evidence as to their age. Granite nowhere bodily left its source bed and therefore if a bed were folded and then partially fused, the granite layer would still "go around" the fold. Two observations, however, lead to the conclusion that not everywhere does granite simply mimic already existing folds, but that it formed while the fold was 174 actually developing.
(-1 Granite locally occurs as saddle veins (Hallimond, in Wilson 1953) suggesting it was emplaced during folding. (2) The granite layers generally thicken near the fold hinges like the semipelites around them which are thrown into "similar" folds (Plate 16). Locally, however, the granite layers behaved slightly differently from the enclosing rocks. Where this has happened,
the fold as outlined by the granite layer is "concentric", or at least partly so; whereas the adjacent bands of host-rock outline
a "similar fold. It would seem that here the granite was present during folding and behaved as a layer more competent than the adjacent semipelites (Plate 17).
The phase II folds of the Saddle Area are correlated with the phase 2 folds of the Loch Hourn Area (Ramsay 1960), and, with the first phase (now phase 2) of the Glenelg Area (Ramsay 1958). The facts on which these correlations are based are as follows. (1)The phase II (Saddle) and phase 2 (Loch Hourn-Glenelg) folds have
the same relationship to metamorphism, migmatization, and pegmatite, and are earlier than the phase III=3 folds. (2)The axial plane of the Be inn a'Chapuill antiform, Glenelg, a
phase 1 (now phase 2) fold (Ramsay 1958) can be traced into the
northwestern portion of the Saddle Area. The minor folds associated with it there are identical in geometry and orientation to phase II folds elsewhere in the Saddle Area. 75
Plate 16: Granite layers in "aimilar" folds. Sgurr a Gharg Gharaidh.
Mate 17: Granite layer in partly "concentric" fold. Sgurr a Gharg Gharaidh. 176 c) Folds of Phase I
The presence of structures in the Saddle Area which predate the phase II folding and sliding, is not obvious from the maps and profile.
Nevertheless, there are a number of facts which suggest that phase II indeed cannot be the earliest tectonic phase, and that there must have been at least one movement earlier than phase II.
(1)Eyed folds probably resulting from the interference of phase II
folds with earlier structures were found on the northwest slopes
of Sgurr Mhic Bharraich, on the south side of Alt a'Ghleannain
and on the northwest slopes of Sgurr a'Chuilinn.
(2)Minor folds of phase II age are seen to refold pre-existing
folds and mineral lineations in several localities: on the
northwest slopes of Sgurr Mhic Bharraich, on the west side of
Choire Chaoil, and in Glen Shiel, below Biot an Fhithich. Examples of these minor structures are illustrated in Fig. 35. (3)It has been shown (page 54) that the Lewisianoid sheets already
formed a part of the Moine structural succession when that
succession was folded, metamorphosed and migmatized in tectonic
phase II. Now, if it is accepted that the Lewisianoid rocks
really represent portions of the Lewisian basement, then they
must have'been emplaced into the present structural succession
by thrusting or folding earlier than phase II.
(4)The correlation of phases III and II of the Saddle Area with phases 3 and 2 respectively of the Loch Hourn Area (Ramsay 1960)
is fairly certain, as it rests on a number of independent facts.
In the Loch Hourn Area, Ramsay (1960) has demonstrated the
177
Figure 35
PHASE I FOLDS
1 foot.,
Phase I fold, refold by Phase I fold, refolded phase II fold. SEurr a by phase II fold. Gharg Gharaidh. Choire Chaoil.
•
6 inches
Eyed fold resulting from interference of folds of 7thases I and II. 178
presence of major phase 1 folds; it is reasonable, therefore,
to expect in the Saddle Area phase I folds which could be
correlated with phase 1 (Ramsay) near Loch Hourn.
If the Lewisianoid rocks are portions of the Lewisian basement,
then each of the Lewisianoid zones indicates the presence of a major phase I structure but the evidence is obscure. The only Lewisianoid zone in which some idea of the geometry of the phase I structure maybe gained is the one on the northwest slope of Sgurr Mhic Bharraich.
The relationships are clearest from the profile when this is taken in conjunction with a section parallel to the plunge of the phase II folds (Fig. 36). The individual strips of Lewisianoid bodies are enveloped by pelite, which is in turn all but surrounded by the
Choire nan Laogh psammite. On the slopes north of, and above, Alt at Ghleannain the "prongs" of pelite extending from the main mass of the body and each containing Lewisianoid lenses, extend partly down the slope and then pinch out down dip, i.e. up valley. In this portion of the area the trend of the phase II fold axes is very near to the trend of the contour lines on either side of Alt a' Ghleannain, therefore phase
II folds would not close down'dip and "up valley", but the rocks in their cores would appear on the sides'of the valley as strips of constant thickness. The prongs of pelite with their Lewisianoid cores and psammitic envelopes are thus either sedimentary lenses or phase I folds.
The latter seems to be the more reasonable interpretation here. The trend of the hinges of the phase I folds may be crudely estimated by joining the tips of the prongs on opposite sides of the main polite lens. The hinges are seen to have a roughly N.N.E. trend and very gentle plunge.
179
Figure 36
SUCTION PAr a TO PITAM II AXIAL
Biot an Phithich Glen Undalain • Sgurr Hhie Bharraioh
„•-• /j' if , ‘ , / /// ....' .' ..--- • ; • / . ---”" .--' • •_,- ./ •-•"'" •
••
ps arza.i.te s eraipeli.te pelite Lewisianoid .20(' phase II axial plane 180
As illustrated in Fig. 36, the politic lens with its core of
Lewisianoid bodies and envelope of psammite and semipelite, may be considered to be a digitated recumbent fold whose "nose" or "front" is in Sgurr Mhic Bharraich. Its projection onto the profile is
thus, strictly speaking, inaccurate.
It has already been shown that the Undalain psammite is equivalent to the Choire nan Laogh psammite (page 161). Furthermore, the pelite with Lewisianoid lenses pinches out between the upper and lower occurrence of the Choire nan Laogh psammite as the phase II axial plane is approached from the west in Choire nan Laogh. Therefore, the axial trace of the first fold just described must run in the
Undalain psammite on the upper limb of the Sgurr Mhic Bharraich phase II fold. The Undalain psammite thus represents the nose of the phase I fold. At one place in Glen Undalain, two streams have eroded through the psammite nose, into the semipelite that envelops the psammite. On
Sgurr a Gharg Gharaidh the psammite "opens up" and a small Lewisianoid body is found at the core of the fold. It would appear that the phase I fold in the Undalain psammite closes southeastward and has a hinge with nearly no plunge and a N.N.E. trend, nearly at right angles to the trend of the phase II axes.
If the Lewisianoid at the core of the fold is really Lewisian, and roughly autochtonous with respect to its pelitic envelope, then the phase I fold seen repeated on the upper and lower limbs of the phase II
Sgurr Mhic Bharraich fold is a recumbent, downward closing anticline rooting to the W.N.W. 181
The next Lewisianoid zone up the structural succession is the
Sgurr na Creige zone. Here a sheet of Lewisianoid rock, enveloped by polite identical to the one on Sgurr Mhic Bharraich, appears on two limbs of a westward closing phase II fold. It is reasonable that in the same manner as the Lewisianoid on Sgurr Mhic Bharraich, the
Lewisianoid of the Sgurr na Creige zone is at the core of a phase I fold whose hinge trends roughly N.N.E. and closes to the southeast.
Unfortunately, a secure correlation of the Sgurr na Creige
Levrisianoid with the Lewisian sheets mapped by Ramsay (1960), (Ramsay and Spring 1962) in the Loch Hourn region was not possible. But it is reasonable that the phase I axial plane, represented by the Sgurr na Creige Lewisianoid and folded eastward by the Sgurr na Creige
Sgurr an t-Searraich phase II fold, is folded westward again by the phase II fold described in Choire Chaoil and that it passes into Glen
Aoidhdailean. Thus it is probable that the Sgurr na Creige Lewisianoid is equivalent to one of the sheets folded by the Beinn nan Caorach fold; which sheet it is, however, is not known. To correlate the Sgurr na
Creige Lewisianoid with the Lewisian of Eastern facies (Ramsay and
Spring 1962) is very tempting because of the strong lithologic similarities between the two. Such a correlations however, assumes that the plane separating the Eastern and Western facies of the
Lewisian was oarallel to the axial plane of phase I folds at the beginning of folding; as yet there is little support for this assumption.
The highest Lewisianoid sheet in the Snddle Area is the Am Fraoch
Choire sheet. It differs from the other sheets just described in 182 that it lacks the politic envelope and, in fact, any symmetry of the lithologic bands around it. The psammitic rocks below it are grey in colour, current bedded, finely laminated, and have abundant bands and lenses of calc-silicate. Current bedding in the psammitic rocks suggests that perhaps they are facing upwards. Immediately below the
Lewisianoid sheet is a thin, flaggy semipelite which possibly represents a zone of movement; while above the sheet is a pink coarsely banded psammite with no current bedding, but containing bands of heavy minerals.
This psammite is followed upwards by a thick striped pelitic unit not represented below the Lewisianoid. The relationships between the Moines above and below the Lewisianoid sheet are not symmetrical, and the
Lewisianoid sheet is therefore better interpreted as a slice of basement carried into the Moine succession on a slide-plane than as the core of a recumbent fold. The possible slide movement zone was later migmatized, and the migmatite itself is unaffected by serious movement. The sliding must therefore have taken place prior to phase II, i.e. during phase I.
A first fold axial plane has been recorded in the middle of the
Lewisian north of the Beinn Aoidhdailean fault (Ramsay and Spring 1962), and it is probable that the continuation of this axial plane is to be found in the Bad an Fhithich Mhoir pelite which takes the place of the
Lewisian east of Choire Sgamadail and possibly represents the Basal
Pelite (Ramsay and Spring 1962). If that correlation is correct, the first fold in the Bad an Fhithich Mhoir polite certainly has its roots to the northwest in Glenelg.
The idea that the Lewisian inliers have their roots in the west, near the Moine thrust rather than in the east under the Moines, the 183
Old Red Sandstone and the North Sea, was suggested in 1960 by Spring (in Fleuty 1961). As a result of his detailed work in Glenelg and the
Loch Hourn Region, Ramsay (in press) found evidence to support this idea. The data presented above, on the phase I folds of the Saddle
Area, can be satisfactorily explained by this hypothesis; and if the, interpretation of the phase I fold on Sgurr Mhic Bharraich is correct, the Lewisianoid rocks in its core must root to the northwest, possibly
in the Lewisian of Glenelg.
Only negative evidence is available concerning the metamorphic grade of the rocks during phase I, largely because of the intensity of
the metamorphism accompanying phase II.
(1)No phase I migmatization was anywhere observed. The phase mineral lineations are quartz rods, not granite or quartz kyanite segragations. (2)No phase I high grade relic minerals were found in the Saddle Area, although such relics are present elsewhere, e.g. Glen Affric (0. Tobisch, personal communication). (3)The emplacement of the Lewisianoid during phase I was accompanied by intense shearing and granulation only. It would appear, therefore, that the grade of metamorphism during phase I
was lower than during phase II, and has been completely wiped out during the latter episode. 184
VII. STRATIGRAFEY
The true stratigraphic succession of the Moine rocks has not been deduced in great detail for the Saddle Area. In the absence of distinct markers and abundant 'way-up" determinations, conclusions concerning the stratigraphy are unlikely to be on a sound basis until the earliest structures are completely understood in every detail.
Such detail knowledge can hardly be claimed for the phase I folds of the Saddle Area. The structural succession, dating back to the end of phase II, cannot simply be converted into a stratigraphic column by means of the few "way-up" determinations available because that structural succession is not only reversed by phrle II axial planes, but also by a number of phase I folds and slides whose exact position, or even whose existence, is not certain.
Bearing these difficulties in mind, an attempt may however be made to outline individual successions in different tectonic units of the area and to correlate some of these units with those of the succession erected by Ramsay and Spring (1962) to the west of the Saddle Area.
In the ground between the Beinn Aoidhdailean and the Strathconon faults, the following succession may be ready if the presence of a phase
axial trace in the Bad an Fhithich Mhoir polite is accepted. The numbers imply a possible correlation with the Ramsay and Spring (1962) succession.
Saddle Area Ransu& Spri4lE AlChrannag semipelite (3) Sgamadail psammite and psammite south of Bad an Fhithich Mhoir polite (2) Bad an Fhithich Mhoir polite (1) Lewisian (A) 185
The AtChrannag semipelite and the Sgamadnil psammite are direct equivalents of Ramsay and Springts (1962) nuMber 3 and number 2 units respectively.
If the interpretation of the phase I and II structures between
Sgurr Mhic Bharraich and Choire Chaoil is correct, then the following succession may be erected for that part of the area.
Saddle Area Ramsay & Spring
Dhomhuill Bhric semipelite, Glen Aoidhdailean pelite (3) A part of Main. Psammite, La-ire Brice semipelite, Gharg Gharaidh semipelite, Undalain,,..Choire nan (2) Laogh psammite Sgurr na Creige = Sgurr an t-Searraich pelite, Sgurr Mhic Bharraich polite (1) Lewisianoid (A)
The psammite of the zone of abundant tale bands can be traced continuously (G. Tanner, personal communication) into the number 4 unit of the Ramsay and Spring succession south of Loch Hourn. It is thus reasonable to correlate the structurally higher parts of the
Main Psammite with number 41 the Barrisdale psammite and to consider it the highest stratigraphic unit so far recognized in the Saddle Area.
The succession is then broken by a slide of phase I on which rests the following structural succession:-
Sgurr Beag polite with psammite member
Reidh psammite
Am Fraoch Choire Lewisianoid
slide
Unless the Lewisianoid is bounded by two slides and "injected" into the succession, it is reasonable to consider the immediately overlying 186 coarse psammite with heavy mineral bands, as the basal unit of the
Moine succession in this locality. If the succession is not further broken by slides the Sgurr Beag polite, with its psammite beds, may be taken as the second unit of the Moine succession. The Sgurr Beag polite is the basal portion of a thick politic unit and was correlated by T. Clifford (1957) with the Ladhar Bheinn polite in Knoydart (Clough, in Peach others 1910) which is the number 5 unit of the Ramsay and Spring (1962) succession. Clifford's correlation was accepted by Dhonau (1960) who, reading downwards, equated all of the Main Ftammite with number 1. of Knoydart and the Sgurr an t-Searraich polite with the number 3 unit of the Knoydart succession. Sutton and Watson (1962) also accepted this correlation. Recent work by officers of the Geol. Surv. of Scotland and by G. Tanner (personal communication) shows however that this correlation is wrong.
South of Loch Hourn the Sgurr Beag polite and the Ladhar Bheinn polite (number 5) do not come together, but are separated by a unit of psammite and possibly a slide. Therefore, any correlation in which one assumes that the Sgurr Beag pelite is the eauivalent of number 5
(Ramsay and Spring 1962) is incorrect, and with our present state of knowledge the Sgurr Beag polite cannot be correlated with the "standard"
successions. 187
TABLE VIII
Correlation Table
Saddle Area Structural Units Ransay and Spring Succession Choire Glen S amarinil Aoidhdailean Glen Shiel Sgurr Beag
5
Main Psammite
AChrannag Glen Dhomhuj11 Bhric polite ? -4 3 semipelite Aoidhdailean semipelite polite
ecier.r.becsraam ar-r-acs Sgamadail Glen Sgurr a Gharg psammite AoidlOnilean Gharaidh semipel. Reidh ? ----42 psammite and psammite Sgurr na Lnire Undalain psamb Brice semipel. kr1=11,JerAe=111.0.ati ... Bad an Sgurr Mhic Fhithich Bharraich and 1 ' Mhoir pel. Sgurr na Creige pelites
A L L L A 188
VIII. SUMMARY OF CONCLUSIONS
A number of different, though related, problems have been dealt with in the study of the Saddle Area and it may be useful to bring
the main conclusions together here in summarized form.
The rocks of the Saddle Area may be classified into four distinct
groups:
(4.) post-metamorphic and post-folding acid, basic and lamprophyric
igneous dykes, possibly of Lower Old Red Sandstone age
(3) migmatitic granite and later, but related, pegmatite
(2) the Moine Series; a succession of qunrtzo-feldspathic and
micaceous schists derived from elastic sedimentary rocks, with
minor calc-silicate bands and pre-metamorphic basic intrusivos
sheets and lenses consisting of assemblages of basic intrusive,
lime-silicate and migmatitic rocks identical with the Lewisian
of Glenelg. At least some of these bodies can be shown to
represent pieces of the pre-Moine basement tectonically
interlayered with the Moines and are correlated with the
Lewis is
The Moine succession and parts of the basement were involved in
three distinct phases of folding, all predating the intrusion of the post-metamorphic igneous dykes and the movements on the Strathconon and related faults.
During phase I, portions of the Lewisian basement were tectonically intercalated with the Moine succession by thrusting and infolding. It is tentatively concluded that the nappes which carried Levrisian rocks had axes trending Y.N.E. with gentle plunges and 189 had their roots to the W.N.W. of the Saddle Area. The structural succession constructed during phase I was then refolded during phase II into tight folds with axes plunging southeast, nearly at right angles to the trend of the phase I axes. As a result of the phase I and II movements a structural succession at least 8 km. (5 miles) thick was built up within the Sad(91e Area alone.
This thick structural succession was further refolded during phase III into large, open and disharmonic folds whose axial plunges are parallel to those of the phase II folds. Several of the major phase III folds are box folds in the antiforms of which the two axial planes dip towards each other. The symmetry of these major structures and their related minor structures, is roughly orthorhombic, -with brat-lathe intermediate axis of stress whichlies in the plane of foliation. P max was nearly horizontal and had N.E. -S.W. orientation.
After folding had ceased and the country rocks had become brittle, the rocks of the Ratagain igneous suite, as well as some of the lamprophyric and basic dykes, were intruded before the main movements on the Strathconon fault system took place.
The Strathconon fault has a sinistral displacement of 2 or 3 miles as well as vertical displacement, northwest side up, of at least one
mile, Associated with it was the Beinn Aoidhdnllean fault, to the south, which has a vertical displacement, northwest side up, of possibly 2 miles. This figure would be lessened if a little dextral movement on the Beinn Aoidhdailean fault were admitted..
Fig. 37 graphically presents the writerts conclusions concerning the relationship between the tectonic phases and the metamorphic events. 190
PiL7cre 37
PCX3SnIE TUT FCE.LiZIONS BErf,,T.'EN LOVEIEIN AND
800
doo
2 t known l no a rv te in e
Tim 4-2-5 E.Y. ?? Phase I Phase II Phase III .Pritt19 . taziiatite Pegmatite
Time 191
During phase I the metamorphic grade was lower than during phase II; probably greenschist facies. The time interval between phase I and II is not known, nor is it known whether the grade slowly rose during that interval or whether it dropped before rising in phase II.
Towards the end of phase II the rocks were metamorphosed to the staurolite-almandine subfacies and granitic material was locally developing in them as a result of partial fusion at about 60000 and 2 at least 4.000 kg/cm. water pressure. This resulted in the development of a main and subsidiary zones of migmatization. Unless significant tectonic overpressures developed in the rocks, this water pressure represents a depth of burial of about 12 km. Such a depth is quite reasonable for the Saddle Area in view of the fact that the structural pile of rocks in the Saddle Area alone is 8 km. thick.
Movement of alkali ions over short distances was certainly involved in the migmatization processes, and it is possible that some influx of alkali from a source outside the Saddle Area also took place.
The evidence available, however, shows that the granitic phase of the migmatite complexes could have formed from the materials available in the host-rocks without any influx from an outside source.
The migmatites form sheets which are roughly conformable with the foliation of the Moine schists. The upper boundary of the main migmatite zone is transitional and gradually fades into unmigmatized rock. The lower boundary is much more abrupt, and though still a zone of transition, it can be clearly delineated. The migmatite sheets were folded by the phase III movements. 192
After phase II pegmatite bodies developed in the rocks and continued to form until after the phase III axial planes had become planes of weakness. In the early part of phase III the rocks were. plastic and micas and quartz could recrystallize as they were being deformed. Phase III movement continued, however, until the rocks had become brittle and temperatures were too low for the crystallization of feldspars and mica. Clearly, the minimum amount of time that had' to elapse between the climax of migmatization at the end of phase II
and the end of phase III, when the rocks had cooled and had become brittle, is the minimum time required for the phase II structural
pile, 81cm. thick, to cool from over 600°C to, say, 200°C. From the curves prepared by Lovering (1935), this minimum time interval may be roughly estimated as being of the order of 2 to 5 million years. The time interval could, of course, have been much greater.
Tectonic phases I, II and III of the Saddle Area are correlated with phases 1, 2 and 3 respectively of the Loch Bourn Region (Ramsay 1960), and phase II is taken as equivalent to the Main fold phase of the Five Sisters (Dhonau 1960). The climax of metamorphism and migmatization
which, in the Saddle Area, is coeval with phase II is correlated with the high grade metamorphism and migmatization accompanying phase 2 (Ramsay) and Main fold phase (Dhonau).
The time relations between the tectonic phases here described and
the Post-Cambrian Thrust movements cannot be deduced within the Saddle Area. However, if the correlations with the Glenelg-Loch Bourn
Region, where phase 3 predated the thrusting (Ramsay 1958), are correct then all three tectonic phases in the Saddle Area predate the 193 last obvious and brittle post-Cambrian movements on the Moine Thrust.
A well substantiated stratigraphic succession was presented by Ramsay and Spring (1962) for the ground between Loch Nevis and Loch Duich, and a secure correlation with this "standard" succession would be highly desirable. Unfortunately, this was not achieved. It is suggested, however, that the politic units with Lewisianoid lenses are to be equated with the No. 1 Basal polite. The psammitic rocks in the zone of abundant calc-bands are probably equivalent to the No. 4 unit of the "standard" succession. All the other psannitic and semipelitic units north and west of the zone of abundant talc-silicate bands are equivalent to No. 2 and No. 3 of the Ramsay and Spring succession. Figure 38 LEGEND
Psammite !Foliation 'oine Semipelite 2K Phase II Axial. Plane Mo kne Pe -*sr 1 Phase 311 Axial Pjane Low s ariol d. klPossible Phase H Slide a Outer Boundary of Migmatite Complex tBoundary of Internal Zone of Complex
0 p 0 3
"V "P "1 = ol 71 3- 1 0
V -
--41 1 , 65
11 p " V 1 1 'MS - —44— -4—H-- —4_ a 4A
Li I **
Ae e! a
I- ---,
, 2112 ----iii*-----1.4 ----"------tt---44 -II - -it-- -H— -44--- --ii-2--‘ --ii__ / •... --,---:___' -C"'"<- > -.1...-0 ----- ° ' \ en. Level b, --:"- 0 ---- _-.- 0 ------(3 / o
N.W. 0 F B. AOIDLN. FAULT PROFI LE LOOKING . DOWN PLUNGE OF PHASE 11 AND PHASE 1.11, FOLDS 195 BIBLIOGRAPHY
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