Foreland Lewisian, the Hebrides, Northern Highlands of Scotland
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
South Rona and Raasay
Lewisian rocks outcrop in South Rona and the northern tip of Raasay (P915460). They are biotite-hornblende gneisses, locally migmatitic, with basic and ultrabasic bands, lenses and clots. The gneisses are very similar to the Laxfordian rocks around Loch Torridon and are generally considered to be part of the same assemblage. The generalised trend of the foliation is NW–SE, the dip is highly variable. It is steep in the north of Rona but becomes nearer horizontal in the south. It is relatively steep in Raasay.
Lyon and others (1973) obtained a whole rock Rb-Sr isochron of 2790 ± 210 Ma from the gneisses, which supported the view (see p.19) that the Laxfordian assemblage is of relative antiquity and has been subjected at least to part of the Badcallian metamorphism. They also dated a granite sheet, emplaced at a late stage in the structural history of the rocks, at 1680 ± 170 Ma and suggested that this recorded an event very close to the end of the Laxfordian. This date is considerably older than that suggested by Moorbath and Park for the mainland (see p.19). Park (in Lyon and others, 1973) has, however, argued that Laxfordian deformation persisted for much longer on the mainland than on Rona, the late deformation dying out southwards.
Small outcrops of Lewisian gneiss outcrop within the main ring fault of the Tertiary igneous complex on Rhum. The main rock type is biotite-hornblende gneiss. The original minerals have been considerably modified by thermal metamorphism, notably in the growth of pyroxene and clouding in plagioclase. A Rb-Sr whole rock age of 1880 ± 40 Ma has been obtained from the rock, suggesting significant Laxfordian effects.
Tiree and Coll
In Tiree and Coll the main rock type is biotite-hornblende gneiss, locally migmatitic, with various basic and ultrabasic inclusions. In south-west Tiree the gneiss is pyroxene-bearing, whereas in north-east Coll it is highly sheared and retrograded (Drury, 1972). Metasediment bands are particularly abundant in these islands. They contain garnet-biotite schists, siliceous schists, and a variety of calc-silicate impure calcareous rocks. Among the purer marbles is the Tiree marble, which has a generally pink appearance; it has been used as an ornamental stone.
In western Tiree the pyroxene gneisses are cut by a series of sharpedged basic dykes with two-pyroxene and hornblende assemblages. In eastern Tiree and Coll these dykes have amphibolite facies assemblages with, in some cases, clinopyroxene and garnet; they are strongly deformed, although they still retain some discordant contacts (Westbrook, 1972; Drury, 1972). The gneissose foliation on Coll and Tiree is disposed in a monoclinal synform (Drury, 1972) whose axial trace runs N–S. The greater part of Coll lies on the gently inclined limb of this structure; the foliation varies around NE–SW and dips moderately to the south-east. Western Coll and Tiree lie on the steep limb of the synform, the foliation striking roughly N–S with nearly vertical dips.
A simple history of the area can be given as follows: a period of high-grade granulite-facies metamorphism; intrusion of basic dykes; deformation associated with amphibolite-facies metamorphism; and late deformation and localised greenschist-facies metamorphism. Correlation with other Lewisian areas is uncertain. Westbrook (1972) correlated the basic dykes with the Scourie dykes and regarded the history of events in eastern Tiree as roughly the same as that of the Outer Hebrides. Drury (1974), however, noting the two-pyroxene assemblage in the dykes, suggested that they predate the granulite-facies metamorphism, which he regards as the equivalent of the Scourian. The amphibolite-facies metamorphism he referred to the Inverian; the Scourie dyke swarm and Laxfordian deformation he regarded as largely absent in the area. However, high-grade assemblages are found in the probable equivalents of the Scourie dykes in the Outer Hebrides (see below), although the dykes only intrude amphibolite-facies gneisses. There it is argued (p.25) that the high-grade assemblages relate to relatively dry conditions in the dykes and not to regional granulite-facies metamorphism. It is, therefore, possible that the dykes of Coll and Tiree do post- date the main granulite-facies metamorphism, and that Westbrook’s correlations are correct.
Iona consists of biotite-hornblende gneiss with some interleaved metasediments. The gneisses are cut by a series of amphibolite dykes, which may be members of the Scourie dyke suite. The metasediments include garnet-mica schists, psammites and the well known Iona marble. The marble forms a narrow (6–8 m) band in the south of the island; it is white with a green serpentinous mottle. Also noteworthy is the ‘white rock’, a conspicuous band of feldspathic rock up to 400 m wide, which runs northwards from the south coast for some 2 km. Associated with the white rock in two localities are broad bands of magnetite rock, the origin of which is uncertain.
The foliation throughout Iona is very variable, but generally strikes NE–SW with dips to the SE. The rocks have been affected by movements on the Moine Thrust Zone.
The Outer Hebrides
The Outer Hebrides, sometimes collectively known as ‘the Long Island’, consist almost entirely of Lewisian gneiss; as such they constitute by far the largest area of such rocks in the British Isles. The term Lewisian was proposed by Macculloch (1819) from the predominance of the gneiss in the Isle of Lewis.
The islands consist of a mixed group of biotite gneiss and hornblendebiotite- quartzofeldspathic gneiss with a variety of basic and ultrabasic inclusions, the latter collectively termed the Older Basics and Ultrabasics (P915462). Pyroxene- bearing gneiss is found in a small area on the east coast of South Uist and more extensively on the east coast of Barra. The area as a whole is broadly equivalent to the Laxfordian zones of the mainland. The gneiss is believed to have been derived from a complex suite of largely calc-alkaline igneous parents intruded in the period 2800–2900 Ma (Moorbath and others, 1975), the rocks being subsequently deformed and migmatised during the Scourian event (c.2600–2700 Ma). The metamorphic grade of the gneiss during the Scourian is not thought to have exceeded the amphibolite fades over most of the area. Granulite-facies conditions were attained only to the south and east, as evidenced by the pyroxene gneiss and the extreme depletion of uranium which occurred in South Uist at that time (Moorbath and others, 1975), this latter feature being typical of high-grade gneiss terrains.
Lying within the gneiss are a group of exotic lithologies believed to be supracrustal sequence of metasediments and metavolcanic rocks. These are found as isolated occurrences throughout the islands (P915462B) but reach their largest and most varied development in the Langavat and Leverburgh Belts which flank the South Harris Igneous Complex and with it form the South Harris Complex (P915463). These major belts contain a mixture of pelitic schist, locally graphitic, quartzite, marble and Finely banded amphibolite. Elsewhere the supracrustal sequence is commonly represented by garnetbiotite schist, which is locally kyanite- or sillimanite-bearing. The extent of the sequence is masked by the long history of deformation and metamorphism; in consequence only the more exotic lithologies can be readily identified. There are, at a number of localities, finely striped hornblende-bearing gneisses and quartzose gneisses of uncertain parentage and affinity.
Areally associated with this supracrustal sequence, and believed to be of broadly the same age, there is a suite of characteristically banded basic rocks. Occurrences of the suite are largely confined to the southern isles, although the anorthosite at Ness in the north of Lewis (Watson, 1969) is believed to be part of the group. These basics show compositional banding on a centimetre to metre scale, the composition of individual bands ranging from ultramafic to felsic. Although the mineralogy and textures are now entirely metamorphic, the banding is believed to reflect original igneous layering. Chemically these rocks are similar to modern tholeiites.
The supracrustal sequence and associated banded basics are thought to be broadly equivalent to those of the Scourian region of the mainland (p.15) and the kyanite-gneiss north of Loch Maree (p.19). They are believed to be the oldest rocks of the Outer Hebrides, having been intruded by the igneous parents of the gneisses; this igneous event was so extensive that the earlier rocks survive only as subordinate relics.
A group of late-Scourian intrusives dated at c.2600 Ma, has been extensively recognised in South Uist and Barra. Although possibly present throughout the islands, this group is best seen in areas where the effects of the subsequent Laxfordian event are least. The group consists of an older set of microdiorites, monzodiorites and diorites, and a younger set of potash-rich granites, monzonites and pegmatites. The two sets are separated by a phase of deformation; subsequent deformation of late-Scourian age also affects the younger group of intrusions. These two deformational phases may be broadly equivalent to the Inverian of the mainland.
The Inverian or its equivalent is also marked in the Outer Hebrides by the development of regional scale shear zones with a dominant NW–SE trend (P915462A). During the waning stages of this event the gneisses were intruded by a suite of basic dykes collectively termed the Younger Basics. These are believed to be broadly equivalent to the Scourie dykes of the mainland, although some workers (Taft, 1978; Hopgood and Bowes, 1972) have argued that the Younger Basics represent a series of intrusive events of different ages. The suite consists of picrites, norites and dolerites, the dolerites being by far the most abundant. Where they have not been extensively deformed by later events the dykes have a generalised W–NNW trend. In common with their mainland equivalents, the dykes show evidence of having been intruded into a hot and tectonically active crust, although the shear zones appear to have been less active and to have exerted less control on the style of intrusion than on the mainland. The dykes exhibit a range of metamorphic textures and mineralogies, ranging from subophitic and equigranular assemblages of the granulite facies to highly schistose lower amphibolite assemblages. The high-grade assemblages, generally now found in the cores of the larger dykes, relate to recrystallisation associated with intrusion into hot crust, their higher grade (relative to the mainland dykes) indicating deeper levels of intrusion. Although the dyke assemblages are consistent with granulite-facies conditions it is believed that the ambient pressures and temperatures of the host rocks were only typical of the upper amphibolite facies, and that the dykes recrystallised in relatively dry conditions.
The South Harris Igneous Complex (P915463) is roughly the same age (c.2200 Ma, Cliff and others, 1983) as the Younger Basics. It consists of a number of plutonic masses and related dykes which are, in order of intrusion from oldest to youngest, of gabbro, anorthosite, norite, and diorite. The mineralogy and textures are now almost wholly metamorphic, although the chemistry still defines igneous trends. The suite is thought to have resulted from progressively deeper magma generation from one or more olivine tholeiite parent magmas. The early gabbros and anorthosite represent gravity-layered rocks with tholeiitic trends, whereas the diorite and norite plutons have calc-alkaline trends. The final product of the Complex was the intrusion of minor, thin (2–50 cm) shoshonite dykes. The rocks show assemblages consistent with the granulite facies, which are believed to have resulted from recrystallisation following intrusion into the hot lower crust.
The eastern half of South Uist is composed of a massive meta-igneous body termed the Corodale Gneiss (Coward, 1972) which is everywhere in thrust contact with the other gneisses. The gneiss is dioritic in overall composition but shows evidence of igneous layering, the more mafic bands containing pyroxene- and garnet-bearing assemblages. The body has been regarded as broadly equivalent to the South Harris Igneous Complex (Coward, 1972) but is cut by a number of basic dykes, which have been correlated with the Younger Basic suite. If this latter correlation is correct then the gneiss may be older than the South Harris Complex.
The gneisses and intrusive rocks of the Outer Hebrides were variably deformed during the polyphasal deformation and metamorphism of the Laxfordian, which attained its maximum development in the period 1700– 1800 Ma. The major folds of the Laxfordian have determined the present regional trends of the gneiss foliation (P915462A). The regional foliation in central Lewis is relatively flat, relating to major folds with subhorizontal axial planes, whereas in the southern isles and South Harris the dominant folds belong to a later set which have nearly vertical axial planes trending NW–SE. The orientation of the Laxfordian folds is believed to have been partly controlled by the pre-existing Inverian and Scourian structural and metamorphic pattern. The degree of Laxfordian deformation or reworking of the gneisses varies considerably and on all scales throughout the islands, from areas of low reworking (marked by cross-cutting Younger Basics with high-grade pyroxenegarnet- bearing assemblages) to areas of high reworking (with a markedly planar foliation and concordant, thoroughly recrystallised Younger Basics). The metamorphic grade during the various phases of the Laxfordian appears to have been entirely within the amphibolite facies. Locally remobilisation and recrystallisation of the gneiss occurs, and the rock loses its gneissose foliation.
The end of the Laxfordian was marked by the intrusion of a suite of thick granitic veins, lenses and sheets at c.1700 Ma. Although found at a number of localities, by far the greatest abundance of granite is in western Harris and Lewis where it constitutes the Uig Hills Complex. In this area the granite forms a complex network of veins and sheets ranging in thickness from centimetres to hundreds of metres. The centre of the complex is marked by the development of porphyritic granite, whereas the marginal zone is characterised by the relative abundance of leucogranite. Although the complex is associated in part with highly recrystallised and remobilised gneiss, the granites are not believed to be partial melts from the local gneiss. The granites show evidence of late-stage cataclasis, and several veins have highly mylonitic centres. These and other features are though to indicate the inception of regional thrusting (see below) before the later granite veins were fully consolidated (Fettes and others, in press).
The host gneisses to the granite also characteristically show evidence of widespread brittle structures and mylonite development, although to a lesser degree of intensity than the granites. Farther east deformation increases with typically ultramylonite and then pseudotachylite being found. The increasing deformation culminates in a major shallow ESE-dipping thrust zone running down the east seaboard — The Outer Isles (or Hebrides) Thrust. In Lewis, this thrust is marked by a wide zone of cataclastic gneiss and small thrusts with pseudotachylite, but further south it develops as a more discrete thrust plane. In the thrust zone the pseudotachylite development is overprinted by a later phase of lower greenschistfacies mylonite production and associated hydrous retrogression of the gneiss. Fettes and others (in press) have argued that the pseudotachylite development related to late Laxfordian movements, the greenschist-grade mylonites marking reactivation of the thrust during the Caledonian. Some workers (e.g. Sibson, 1977) however, have argued that the development of mylonite and pseudotachylite are both essentially Caledonian. They also postulate that any pseudotachylite production of late-Laxfordian age is relatively minor, and equivalent to the pseudotachylite belts of the mainland (p.19).