Moine Succession, introduction and lithology, Northern Highlands of Scotland

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From: Johnstone, G S and Mykura, W. 1989. British regional geology: the Northern Highlands of Scotland (4th edition). (Nottingham: British Geological Survey.)

Introduction

The Moine Succession, which takes its name from the peninsula of a’Mhoine (the Peat Bog) in northern Sutherland, is a group of metasedimentary strata, the oldest of which may have been deposited on the Lewisian basement between 1500 and 1025 Ma ago (Brook and others, 1976) and whose youngest members, on current definition (Johnstone, 1975), pass upwards in the Grampian Highlands into Dalradian strata, whose base is taken to be around 700 Ma. Isotope ages suggest that the succession comprises two groups of rocks. One has been subjected to two late-Proterozoic tectonothermal events (the Grenvillian Orogeny at c.1000 Ma and the Morarian episode at c.750 Ma), and overprinted by the Caledonian Orogeny; the other has only been affected by the latter event. Harris and others (1978) consider that those ‘younger’ Moines are best referred to the Dalradian Supergroup.

Within the Northern Highlands, Moine rocks are found only east of the Moine Thrust, but in southern Skye a small outcrop of ‘Tarskavaig Moines’ lies within the thrust zone. They are intermediate in character between the metasediments of the overlying Moine Nappe and the Torridonian rocks of the underlying Kishorn Nappe (P915471).

The general equivalence of the Moine and Torridonian successions has been proposed, rejected and, more recently, again considered as a possibility.

Lithology

The Moine rocks comprise a limited number of lithological types, representing a metamorphosed series of arenaceous and argillaceous sediments containing a very minor proportion of calcareous and dolomitic material; beds of limestone are extremely rare. The present mineral composition and condition of the rocks depends upon their metamorphic grade and the extent to which they have been migmatised.

Psammitic schists and granulites

These vary from quartzose schists to massive or flaggy grey quartz-feldspar ‘granulites’ with rare quartzites. The granulite is so called because of the granoblastic, equigranular texture of its principal minerals, not because of any association with the granulite facies of metamorphism.

By far the dominant type is the psammitic (sandy) granulite, representing sandstones, locally pebbly, in which the constituent minerals are quartz, feldspar (typically potash feldspar and sodic plagioclase) and a variable, though small, proportion of mica (which can be muscovite or biotite, or both). Garnet is a common, though inconspicuous, constituent, and zoisite or epidote, apatite, zircon, sphene and iron ore occur as accessories, some being abundant in certain groups. In the typical granulite the micas are dispersed and, although they may show a pronounced orientation, the rock as a whole is not schistose; it is commonly quite massive. In certain areas thin pelitic bands (probably representing original bedding planes) are sufficiently schistose and closely spaced to impart a pronounced flaggy aspect to the rock, which tends to split along these micaceous surfaces. In other localities flags of similar appearance result from the fissility imparted by more or less micaceous laminae developed along cleavage surfaces, which are usually axial planar to the dominant folds of the area.

Cross-bedding can be seen from place to place and is well displayed in areas which have escaped intense tectonic deformation, especially in the western part of the district around Morar and Arisaig (Richey and Kennedy, 1939). In that area too, numerous pebbly bands can be found with grains up to 8 mm across and, rarely, even up to 25 mm. Elsewhere in the outcrop pebbly bands are uncommon.

Undoubted conglomerates are rare. They have been recorded as a basal conglomerate of Moine overlying the Lewisian basement at Attadale on the south shore of Loch Carron (see p.28); in a uniquely preserved area of low deformation around the Carn Chuinneag pluton in Ross-shire (p.92); and in the north of Sutherland, west of the Kyle of Tongue at Strathan (Mendum, 1976). In the last area there are many excellent exposures; the thick lenses of conglomerate have quartzite pebbles (originally possibly as much as 20 cm across) and rarer granite pebbles. The rock here has been so strongly deformed that individual pebbles have been reduced to thin discs, and the original nature of the beds is barely recognisable. In several exposures it now resembles a lit-par-lit gneiss.

In certain areas, and especially in some groups thought to represent stratigraphical units, heavy mineral bands are developed. These are up to 15 mm thick, and some of the minerals (including zircon, garnet, sphene, epidote, allanite, ilmenite and magnetite) retain their original detrital form. A conspicuous development of these bands is found in the Lower Morar Psammite (see p.68 and P915509). Biotite-sphene schists (which may also represent metamorphosed placer concentrations) have been recorded, especially from the Upper Morar Psammite of Ardnamurchan (Butler, 1962).

Throughout most of the Northern Highlands the Moine psammites show little megascopic variation resulting from changes in regional metamorphism. Recrystallisation of a quartzofeldspathic sandstone under grades of metamorphism up to amphibolite facies results in a very similar quartzofeldspathic metasandstone (psammitic granulite). It seems unlikely that the commonly-occurring granulite of the Moines is other than a metamorphosed sandstone, even though recrystallisation of the larger pebbles in the pebbly bands of Morar is progressive for a few kilometres eastward from the assumed position of the Moine Thrust, and the resultant granulation of these pebbles under strain and rising temperature when traced eastwards into the area of more intense deformation and metamorphism can produce a rock of even-grained ‘sandy’ appearance. Slightly deformed crossbedding and the sparse occurrence of almost non-deformed pebbles well within the Moine outcrop suggest that the medium-grained, even texture of the rocks reflects that of the original sediment.

The remarkable deformation of the Sutherland conglomerates referred to above does, however, indicate that care must be exercised in the interpretation of any section. It cannot be assumed, either, that the apparent bedding of the Moine psammites is everywhere an original feature. The flagginess can result from the reduplication of bedding in isoclinal folds, accentuated (as already mentioned) by the development of a new schistosity parallel to the axial planes of the folds. Langford (1980) attributes much planar banding in the Moines east of Attadale to extreme flattening of the limbs of folds.

The psammitic granulite normally remains unaffected by the regional migmatisation found in adjacent pelitic rocks (see below). Certain bands seem, however, to be liable to develop augen and lits (elongate pods) of quartz and microline. Such susceptible bands are commonly feldspathic and often micaceous, sometimes, but not always, approaching semipelite in composition. These lit-par-lit or augen psammites are referred to as psammitic gneiss (Clough, 1910). The quartzofeldspathic lits rarely exceed 10 cm in width and usually have a conspicuous biotite selvedge (see Chapter 8).

Semipelitic schists and granulites

With increase in mica content, both muscovite and biotite, the psammitic granulites pass into semipelites. While the increase in mica content is usually taken to reflect the higher clay-to-sand ratio of the original sediment, it could be a function of the composition of the sand grains making up the rock and their alteration by diagenetic and metamorphic processes. Semipelites are commonly fissile with a pronounced foliation of more or less micaceous layers which show a well developed schistosity. More massive varieties with uniformly distributed mica and a less pronounced schistosity are also found. Where they have both muscovite and biotite those rocks commonly have a ‘pepper and salt’ speckled appearance. Semipelitic rocks show all gradations between psammites on the one hand and pelitic rocks on the other and, because of this, the extent to which they have been separated from these types during mapping depends somewhat on subjective interpretation. Like the psammites (above) and the pelites (described below) the rocks become lit-par-lit gneisses on migmatisation.

Pelitic schists

These are metamorphosed shales or siltstones. Felts of muscovite and deep brown biotite form the bulk of the rocks, which appear black or silvery depending on the predominant mica determining the foliation surface. Feldspar and quartz are the other main constituents. Over much of the Moine outcrop in the Northern Highlands, however, these schists lie within the area of migmatisation and are represented by quartz-feldspar (oligoclase)-biotite gneisses which show a pronounced foliation on account of the segregation of quartzofeldspathic bands and lenticles. In the areas of more intense migmatisation it can be impossible to decide whether the hostrock was originally a pelite or semipelite (Plate 17).

The pelitic rocks contain additional minerals according to the grade of metamorphism. Garnet-bearing varieties are almost universal, as by far the greater part of the Northern Highlands Moine outcrop lies above the greenschist-facies of metamorphism (Chapter 7). Sillimanite-, kyanite- and staurolite-bearing schists are found, the latter two being sparse (a function of whole-rock composition rather than metamorphic grade). The former presence of sillimanite is commonly indicated by knots of muscovite nucleated on retrogressive shimmer aggregate. Within the area of migmatisation, ‘books’ of well formed muscovite up to 2 cm across give a spangly aspect to the rock.

Striped and banded schists

These comprise groups of rocks made up of psammite (commonly quartzitic) and pelite in alternate bands and laminae too thin to be individually mapped, the bands being 3 cm or so to several metres in thickness (P883253). These schists represent alternating sand and mud deposition and are especially common in the rocks of the Glenfinnan Division. They, too, give rise to lit-par-lit gneisses when migmatised.

Calc-silicate rocks and marbles

In all the above rock types, but especially in the psammites and semipelites, there are some thin discontinuous ribs and lenticles of calc-silicate minerals (plagioclase feldspar, epidote, zoisite, hornblende, pyroxene, garnet). The actual minerals present at a particular locality depend partly on the chemical composition of the rock (notably the CaO/Al2O3 ratio) and partly on the metamorphic grade. These ribs, which may be up to 30 cm thick, are quantitatively insignificant but are characteristic of certain stratigraphic formations (see P915509 and P915472) and their mineralogy forms the basis of metamorphic zonation in the psammitic rocks (see Chapter 7).

They are white or dark grey compact rocks, speckled with red garnet and usually with a visible green calc-silicate mineral. Banding occurs frequently as a result of the parallel disposition of garnetiferous, hornblendic and biotitic layers; all the minerals are granoblastic except hornblende, which tends to assume a poikiloblastic habit. In some of the semipelitic schists the ribs form part of a rhythmic unit comprising semipelite → psammite → calc-silicate rib → psammite → semipelite. The calc-silicates in most of these rhythmic units probably represent original marly bands; others, however, particularly the scattered and discontinuous occurrences, probably represent calcareous concretions in the original sediments. Epidotic bands and epidosites (which carry pyroxene or amphibole) have been recorded among the pelitic schists but are not common.

Marbles which apparently form integral parts of the Moine succession are extremely rare. At Rebeg and Blairnahenachrie (10 km and 6 km WSW of Inverness) crystalline limestones are associated with hornblende schists; they vary from pure-white or pink coarsely crystalline marbles to calcsilicate rocks. The rocks of these areas have riebeckite, a soda-rich amphibole, associated with them. Marbles and calc-silicate rocks are found on the east side of the Glendessary Complex (P915479), well within the Moine outcrop. In Ardgour rather similar rocks lie close to the Glen Scaddle Complex (P915476). In this area, however, the adjacent rocks includemica schists and black schists which resemble Dalradian strata of the Appin Group, and abut on the Great Glen Fault. The true stratigraphic affinities of these marbles are thus uncertain and will be further discussed on p.72

Selected bibliography