Loch Eil Division Research Articles

Formation of gneisses of migmatite and diatexite appearance in Yell, Shetland, by solid‐state grain‐growth recrystallization

AbstractThe island of Yell in the north of Shetland is largely formed of rocks which have been correlated with the Glenfinnan/ Loch Eil Division of the Moine in mainland Scotland. The rocks forming Yell are dominantly granoblastic to schistose quartzo‐feldspathic psammitic rocks of greywacke composition. Over about a third of Yell these rocks have been transformed partially or completely to gneisses by a recrystallization, which results in a trebling, or more, of the grain size of the psammites, together with the loss of sedimentary lamination, banding and bedding‐like partings. The end result for mica‐poor granofels is a granoblastic gneiss of diatexite appearance, recognizable as a paragneiss due to its gradation to unaltered metasedimentary rocks. Mica‐rich granofels and mica schists interbedded or interbanded with the granofels also recrystallize and develop quartzo‐feldspathic leucosomes of typical migmatitic appearance. This gneissification by recrystallization takes place in a patchy and unpredictable manner throughout the psammite outcrop. The gneissified and the ungneissified metasediments are of similar bulk composition and metamorphic grade throughout the island, the grade being too low for melting to have occurred. In the absence of any published mechanisms of gneissification which could explain all these features, it is suggested that the grain‐growth recrystallization was caused by the pasage of a fluid through the rocks.

The Sgurr Beag nappe: A reassessment of the stratigraphy and structure of the Northern Highland Moine

The rocks of the Northern Highland Moine have suffered two phases of intense, mid-crustal orogenesis. Elucidation of original stratigraphic relationships within these rocks requires the application of techniques that can be applied to other terranes of this type, in which the regional lithostratigraphy has, on first inspection, been obscured by a complex deformation history. Mapping along the boundary between the Glenfinnan and Loch Eil divisions of the Moine has shown that the rocks of both divisions have a common structural, metamorphic, and igneous history. The Glenfinnan division passes upward by stratigraphic passage into the Loch Eil division. The usefulness of assigning rocks rigorously to one division or the other is therefore called into question. It is suggested that the Moine rocks in the Northern Highlands which lie above the Sgurr Beag thrust, that is, those previously assigned to the Glenfinnan or Loch Eil divisions, should be referred to a single tectonostratigraphic unit, the Sgurr Beag nappe. The Glenfinnan and Loch Eil divisions have the status of lithostratigraphic groups within the nappe. It is further possible to subdivide the stratigraphy of the nappe into a number of mappable formations, some of which are laterally continuous, and some of which die out laterally. The base of the Loch Eil Group may be diachronous. Stratigraphic considerations suggest that the Sgurr Beag thrust cuts down-section northward, exposing deeper levels of the nappe from Loch Quoich northward than those present above the slide farther south. The Morar division is also thought to have the status of a lithostratigraphic group, but its original relationship to the Glenfinnan and Loch Eil groups is normally obscured by the Sgurr Beag thrust.

Shallow marine sedimentation in the proterozoic moine succession, northern Scotland

Shallow marine deposits are recognised from the Loch Eil division of the Proterozoic Moine Succession. Three sedimentary facies are identified: siltstone, interbedded siltstone and sandstone, and sandstone. The bulk of the succession comprises sandstones which are commonly cross-bedded. Bipolar cross-bedding is indicative of tidal activity, and the facies is considered to have accumulated in an offshore shallow marine setting. The interbedded siltstone and sandstone facies and the siltstone facies are similarly interpreted as offshore sediments deposited in areas characterised by relatively high rates of silt deposition and starvation of sand. Calc-silicate rocks represent concretions formed during diagenesis. Palaeocurrents derived from the lower part of the succession mainly flowed to the north-northeast with a subsidiary mode to the south-southwest, and probably represent longshore currents flowing broadly parallel to a palaeocoastline.

The stratigraphy and structure of the Moine rocks of the Loch Eil area, West Inverness-shire

Synopsis The Moine rocks of the Loch Eil area consist mainly of psammites and quartzites which belong to the Loch Eil division. Sedimentary structures are used to establish a Loch Eil division stratigraphical succession. The Loch Eil division lies in stratigraphical and structural continuity with the underlying pelitic and banded gneisses of the Glenfinnan division. The deformational history of the Moine rocks of the Loch Eil area may be resolved into two phases of recumbent folding (D 1 and D 2 ) and three phases of upright folding (D 3 –D 5 ). The regional structure is dominated by the interference of D 3 and D 4 fold systems. D 1 , and possibly D 2 also, are considered to be Precambrian in age, whilst D 3 –D 5 are likely to represent the effects of Caledonian reworking.

The structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire

The Glen Dessary Syenite crops out in the steep belt of the Northern Highland Moine. It is enclosed within a psammite envelope which was previously assigned to the Glenfinnan Division, but which is here correlated with the psammites of the Loch Eil Division exposed to the E of the Loch Quoich Line. Three episodes of deformation have been identified in the Moine metasediments of the Glen Dessary area, but only the latest (D 3 ) is thought to have deformed the syenite, imposing on it a strong, steeply plunging mineral lineation. The syenite is thought to occupy the core of a large, intensely curvilinear, D 3 synform. The D 3 structures which deform the syenite can be correlated with upright structures, which reworked previously flat-lying, already crystalline rocks, recognized at Loch Quoich. Upright reworking must be younger than the Glen Dessary Syenite, dated by U-Pb methods at 456 ± 5 Ma. Two, hitherto unrecorded, small bodies of granitic gneiss, similar to the c. 1020 Ma Ardgour Gneiss, have been identified in psammites of the Loch Eil Division near the W end of Loch Arkaig. Their presence suggests that the Loch Eil Division has a Grenville history, and is part of the 'Old Moine'.

The geochemistry of the Strathconon amphibolites, Northern Scotland

Synopsis The Strathconon ortho-amphibolites were intruded into Glenfinnan and Loch Eil Division metasediments of the Moine Assemblage before the area was deformed in late Precambrian time. A chemical study has indicated that they consist of a single suite of variously fractionated tholeiitic sills and dykes which can be distinguished chemically both from the Y-poor amphibolites of the Morar Nappe and from more Y-rich amphibolites within the Central Ross Nappe spatially associated with the Quoich Line in Inverness-shire. The chemical distinction between amphibolites from the two nappes requires that major transport of the overlying Central Ross Nappe occurred after the intrusion of the basic sills.

The Loch Quoich Line—a limit of early Palaeozoic crustal reworking in the Moine of the Northern Highlands of Scotland

The Loch Quoich Line, a major structural line at least 60 km in length, within the Moine of the Northern Highlands of Scotland, has hitherto been variously interpreted as a major tectonic dislocation. the root zone of a nappe, and an unconformity. The work reported here was carried out at the type locality of the line, and was directed at resolving these ambiguities. The lithostratigraphic succession established by sedimentary structures shows that the rocks of the Glenfinnan Division are older than, but are stratigraphically continuous into, those of the Loch Eil Division. Rocks of the Loch Eil Division crop out extensively to the W of the Loch Quoich Line, and are not restricted in outcrop to the area E of the line, as was previously thought. The early (D 1 –D 2 ) structures were initially sub-horizontal. Steeply inclined, NNE-trending D 3 structures are weakly developed E of the axial trace of a major fold, the Beinn Beag Synform. In this area the metasediments remain essentially in their post-D 2 , gently inclined attitude. W of the axial trace of the synform, the earlier folds and fabrics were severely reworked into steep attitudes during D 3 , producing NNE-trending zones of 'platy' rocks, which mark regions of severe strain on D 3 fold limbs. Earlier (D 1 –D 2 ) structures are clearly preserved in D 3 hinge zones, which mark zones of low D 3 strain. It is concluded that the Loch Quoich Line coincides with the axial trace of the Beinn Beag Synform, and that it is the eastern limit of severe tectonic reworking (probably Caledonian) over-printing early (probably Precambrian) structures. Recognition of structural and stratigraphic continuity between the Glenfinnan and Loch Eil divisions, implies that the Loch Quoich Line cannot be correlated across the Great Glen Fault with the putative unconformity separating the Grampian (cover) and Central Highland (basement) divisions, in the Moine rocks of the Central Highlands.

Tectonic sliding within the Moinian Loch Eil Division near Kinlocheil, W. Inverness-shire: comment

Tectonic sliding within the Moinian Loch Eil Division near Kinlocheil, W. Inverness-shire: comment

The stratigraphy and structure of the Moine rocks of Eastern Ardgour

Synopsis The Moine rocks of Eastern Ardgour which mainly belong to the Loch Eil Division are sub-divided into five fault-bounded units, the Cona Glen, Glen Gour, Coire an Iubhair, Inversanda, and An Torran units. Sedimentary structures were used to establish the stratigraphic succession, the lower levels of which may be a transition into Glenfinnan Division rocks. Deformation of the Moine was polyphase and is described in terms of a D 1 –D 4 sequence. The D 1 phase is probably Precambrian in age with D 2 –D 4 structures formed during Caledonian deformation. The Glen Scaddle basic intrusive complex was intruded during the D 1 –D 2 interval.

Discussion on the Glen Kyllachy Granite and its bearing on the nature of the Caledonian Orogeny in Scotland

D rs A. L. H arris , A. J. H ighton A. M. R oberts & M. S. S toker write: A recent paper by van Breemen & Piasecki (1983) prompts some comments about correlation of lithostratigraphic units on either side of the Great Glen in Scotland, with its implications for displacements across the Great Glen Fault. Van Breemen & Piasecki (p. 47 and fig. 1) have followed the earlier work of Piasecki et al. 1981 in correlating the Grampian Division of the Grampian Highlands with the Loch Eil Division of the Northern Highlands, regarding both divisions as cover sequence to older gneisses which had undergone Precambrian (Grenville) deformation and metamorphism. Work by A. J. Highton in Upper Strathnairn, reported by Harris et al. 1981, has to a large extent confirmed the conclusion of Piasecki (1980) that a cover–basement relationship exists in the Grampian Highlands between older complexly deformed Central Highland Division gneisses of high metamorphic grade (basement) and younger, simply deformed, pebbly, cross-bedded Grampian Division psammites (cover). Both Piasecki (1980) and Mr Highton have concluded that a major ductile fault (the Grampian Slide of Piasecki, op. cit.) of unknown displacement separates the rocks of the 2 divisions. Piasecki (1980) regards the slid younger rocks as still overlying the older, while Mr Highton has demonstrated that older basement rocks have been thrust over the younger.

Tectonic sliding within the Moinian Loch Eil Division near Kinlocheil, W Inverness-shire: reply

Tectonic sliding within the Moinian Loch Eil Division near Kinlocheil, W Inverness-shire: reply

Microdiorite sheet intrusions as incompetent time- and strain-markers in the Moine assemblage NW of the Great Glen fault, Scotland

ABSTRACTRadiometrie evidence suggests a complex evolution for some parts of the Moine assemblage NW of the Great Glen fault in Scotland. Four generations of microdiorite sheet intrusions are distinguished and used as time- and strain-markers in this region. Two are found to record different tectonic episodes in the Morar and Glenfinnan divisions and their Loch Eil division cover during the c 750 Ma Knoydartian orogeny. These demonstrate that F1 and F2 (of the Grenvillian orogeny) and F3 folds (of the Knoydartian orogeny) in the Morar and Glenfinnan divisions are absent from the Loch Eil division. Two later generations of microdiorites record successive episodes (F4 and F5) of the c. 455 Ma Caledonian orogeny.The tectonic history of the region is considered after a discussion of the use of microdiorite sheet intrusions as strain-markers. This is because, rather than fold or boudinage, they deformed as incompetent single layers and foliated, mullioned or thinned uniformly instead. The attitudes of different elements of the microdiorites and their internal foliation are used to construct strain ellipsoids for four tectonic episodes on scales of 4–400 m and >4000 m. Three of these episodes are found empirically to have involved irrotational plane strain and all are close to coaxial even though the principal axes of strain appear to have swapped one for another in space as well as time.

Tectonic sliding within the Moinian Loch Eil Division near Kinlocheil, W. Inverness-shire

Synopsis Recent mapping within the Moinian Loch Eil Division of West Inverness-shire has resulted in the erection of a lithostratigraphic succession for the Kinlocheil area which has been subdivided into psammitic, quartzitic and semi-pelitic formations. A series of tectonic slides have been identified within the lower stratigraphic levels of the Loch Eil Division. These slides formed at an early stage in the structural history of the Loch Eil Division, preceding major folding during the Grampian event. Sliding in the Na-h-Uamachan-Gulvain area, north of Loch Eil, has resulted in the interleaving of Glenfinnan and Loch Eil Division rocks. Structural considerations indicate that the stratigraphic level at which sliding occurred was probably controlled by an early-formed “steep belt” within a broadly sub-horizontal succession. Within this structure “older” rocks have been thrust over “younger” and vice versa during westerly-directed movement along slides.

Geochemistry of the western part of the Moinian assemblage

Synopsis Comparison of chemical analyses of the principal units of the Morar Division suggest that the inter-relationship between psammites and pelitic rocks is dominated by mechanical rather than chemical processes. This conclusion is drawn from similar ratios of Fe 2 O 3 /MgO, K/Rb, K/Ba and Ca/Y, and dissimilar ratios of Ti, Nb and Zr to Fe. Rocks of the Glenfinnan Division differ from comparable members of the Morar Division in having higher Nb/P 2 O 2 and Nb/Y for any given Sr or Fe 2 O 3 /Ni ratio, and higher Sr/Y for any given SiO 2 level. These distinctions may reflect consistent differences in the ratios of quartz and feldspar, ferromagnesians, and accessory minerals. Samples from the Loch Eil Division tend to have higher Zr/P 2 O 5 , Y/P 2 O 5 , CaO/P 2 O 5 , Rb/Sr and lower Ca/Y and K/Rb than their counterparts in the Morar and Glenfinnan Divisions. The Morar Basal Pelite has an immature chemistry showing affinity with the Lewisian basement upon which it rests. It can be distinguished from other Moinian pelitic units by its high Sr/Y. Certain individual psammitic horizons can be distinguished by their contents of P, Ti, Nb and Zr, and sometimes by Cr/Fe. The Morar Division has a chemistry consistent with deposition in a basin close to its supposed Lewisian source. In contrast, the Loch Eil Division may have been partly derived from a (post-Lewisian) sedimentary source, and may rest unconformably upon the underlying Moinian rocks.

Time, space and intensity relationships of the Precambrian and lower Palaeozoic metamorphisms of the Scottish Highlands

Summary Isotopic evidence suggests that a complex sequence of events, collectively known to us as the Morarian event, affected the Morar and Glenfinnan divisions of the Moine from 1000 to 750 Ma. A regional thermal anticline, with a minimum age of 750 Ma, extends north from Ardgour to northern Ross-shire: it probably extends southwards under the eastern Dalradian. This thermal anticline is cut across and shifted westwards south of Monar by the Sgurr Beag slide, on which up to 25 km thrust movement is estimated. The Loch Eil Division lacks old isotopic ages; it appears to be structurally and metamorphically simpler than the Morar and Glenfinnan divisions and it also differs from them geochemically. The simplest explanation of these facts is that Loch Eil Division rests unconformably on the metamorphosed Glenfinnan Division. The Central Highland Group (including the Monadhliath Schists) is presumed to occupy the same tectono-stratigraphical niche as the Loch Eil Division. Grampian events (510-480 Ma) produced three thermal highs of sillimanite grade: one in the Monadhliaths, the second in the NE Grampians, and the third affecting E Sutherland. Throughout most of the area under consideration, Grampian age isograd surfaces appear to dip at low angles.

The ‘Central Highland Granulites’: cover-basement tectonics in the Moine

Summary The Central Highland Granulites have been split into a cover assemblage of medium grade, little disturbed metasediments named the Grampian group, separated by a zone of sliding referred to as the Grampian Slide, from an underlying basement assemblage of high grade gneissose and migmatitic metasediments called the Central Highland Division. The Central Highland Division rocks are correlated on lithological, tectonic, metamorphic and radiometric grounds with the high grade Grenvillian rocks of the western Highland Moine, and are believed to represent their extension southeast of the Great Glen fault. The discovery of Precambrian ( c .720 Ma) pegmatites sheared by Caledonian (<570 Ma) sliding in the Central Highlands, enables the separation of Caledonian from Precambrian orogenic events in the rocks of both assemblages; their Precambrian orogenic history is so different, that the Central Highland Division rocks are considered to form a Grenvillian basement to the younger (Morarian) cover succession of the Grampian group. The western Highland Moine region is examined in the light of the tectonic cover-basement relationship postulated for the Central Highlands. The Loch Eil Division contains rock assemblages indistinguishable from the Central Highland Division, and others similar to the Grampian group. Evidence is put forward for a post Grenvillian, Morarian age for the Morar Division. The distribution of metamorphic zones in the Moine would seem to reflect the distribution of high grade basement and lower grade cover rocks, and may be in part determined by movements on a series of Caledonian slides, though this has not been demonstrated.

A review of the structure and stratigraphy of the Moinian of Ardgour, Moidart and Sunart—Argyll and Inverness-shire

Synopsis The structural geometry is the result of four episodes of deformation (F 1 –F 4 ), with most major folds belonging to the second or third episodes. Major F 1 folds are probably absent (on stratigraphical evidence) but slides of the F 1 episode are not ruled out. Complex F 2 geometries—with curvilinear axes, and antiforms which turn into synforms when traced along traces—are found where F 2 folds are ‘folded’ by F 3 folds, and also remote from F 3 fold hinges. Intense inhomogeneous flattening explains these features, and it is considered that this flattening may be part of the second, rather than the third deformation. In the prevailing high-temperature conditions, the amount of flattening seems to be a function of rock-type and it is possible to distinguish between the simpler structures in thick psammites and the more complicated, highly flattened, structures in pelitic/striped rocks. The major rock units probably represent a stratigraphic sequence, from bottom to top, 1. Morar Division, 2. Glenfinnan Division and 3. Loch Eil Division.