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.