Metamorphic rocks of Shetland: area west of Walls Boundary Fault

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Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.

Introduction

Geological sketch-map of Shetland. P915566.
The metamorphic rocks and plutonic complexes of North Roe and Northmaven. P915568.
The metamorphic rocks of the Walls Peninsula. P915569.

The metamorphic rocks west of the Walls Boundary Fault form a belt of variable width which extends from North Roe at the extreme northern end of Shetland Mainland southwards along the fault as far as the island of Papa Little [34 6l], (P915566) and thence westward for 15 km along the northern margin of the Walls Peninsula. There are also a number of outcrops separated from the main belt by the granite or diorite of the Northmaven Igneous Complex. These include the rocks of the Hillswick area and the ‘enclaves’ in the granite and diorite of Muckle Roe and the Busta Peninsula. Metamorphic rocks also form the Ve Skerries [10 65] and the north-eastern coastal strip of Foula.

North Roe and Northmaven

In North Roe and Northmaven the rocks have been divided into three major series, shown in P915568. Of these the Western Series comprises gneisses which may be of Lewisian age and are older than the other two series. While no direct correlation is possible, the rocks of the Ve Skerries are thought to belong to this basement group. The Fethaland Series and the Ollaberry Series have been definitely distinguished only in the eastern part of the area, but it is likely that the rocks of the Hillswick Peninsula and the inclusions in the Northmaven Igneous Complex can also be referred to these series. Tentative correlations are shown in P915568. The Fethaland and Ollaberry Series consist of sedimentary and volcanic rocks metamorphosed during the Caledonian Orogeny. They are separated from the Western Series by a shear zone, the Wester Keolka Shear Zone, which may be a major thrust comparable to the Moine Thrust of the Scottish mainland. The true relationship of the Fethaland to the Ollaberry series is likewise complicated by shearing along the contact, but it is suggested that the Ollaberry Series is the younger.

In the Walls Peninsula the metamorphic rocks have been divided into four lithostratigraphic groups whose distribution is shown in P915569. The metamorphic rocks of Foula have been likened to one of these groups, although no direct correlation is possible. No direct correlations have been made between the metamorphic rocks of the Walls Peninsula and the two eastern series of Northmaven, but the similarities in the metamorphic and tectonic history of the two rock-groups suggest that they share a common tectonic environment and are likely therefore to be of similar age. Although these rocks appear to have been deposited within the Caledonian Geosyncline they cannot be readily correlated with any part of the Dalradian or Moinian assemblages of the Scottish Highlands. Radiometric potassium-argon ages which average 415–5 m.y., obtained from the rocks of the Walls Peninsula, do not give any clue as to the age of deposition of the strata, merely representing the age when, after the last phase of metamorphism that affected the rocks, the temperature decreased to a point at which radiogenic argon was retained. They do, however, support the view that the formation of the schists as we now see them was the result of a Caledonian metamorphic episode.

Western Series

The Western Series (Summ. Prog. 1932, p.72)[1] crops out in the north-west corner of North Roe and consists of acid orthogneiss with sheets of basic and ultrabasic gneiss. Pringle (1970)[2] has subdivided this series into two groups, the Uyea Group in the west and the Wilgi Geos Group in the east. The former is composed of irregularly foliated gneiss made up mainly of orthoclase and oligoclase with minor amounts of quartz, hornblende, biotite and epidote. It contains some lenses of pyroxene-hornblende-granulite and is cut by many veins of foliated pegmatite. Large sheets of metagabbro are well seen at Fugla Ness and on the Isle of Uyea. The Wilgi Geos Group is separated from the Uyea Group by the Uyea Shear Zone and consists of pale augen-gneiss with thin sheets of fine-grained amphibolite. It dilfers from the gneiss of the Uyea Group in that large parts of it have a strongly developed schistose fabric which is parallel to the shear planes bounding the group. Locally the schistose gneiss is brecciated or has mylonitic bands parallel to the schistosity.

Pringle (1970)[2] believes that both these groups were originally granitic complexes cut by gabbroic sheets and dykes. He also states that the complex which now forms the Uyea Group was emplaced first and had perhaps already undergone two periods of metamorphism before the Wilgi Geos igneous complex was emplaced. Because both groups were affected by several periods of metamorphism and deformation before the rocks of the two eastern series were laid down they are believed to be of Lewisian age. The Wilgi Geos Group was later involved in the Caledonian earth movements which produced major shear zones and pushed individual thrust slices or nappes westwards. The Uyea Group, on the other hand, appears to have remained a stable block throughout that period. Pringle has suggested that the Uyea Group may be part of the Lewisian foreland of the Caledonides and that the Uyea and Wester Keolka shear zones (P915568) may be branches of the northward continuation of the Moine Thrust.

The Ve Skerries (north-west of Papa Stour on P915566) consist largely of pale grey to pink strongly banded albite-gneiss with varying proportions of mica and hornblende. There are also some lenticular masses of foliated granite and two thick bands of partially granitised hornblende-schist and mica-schist. The Ve Skerries gneisses are unlike the metamorphic rocks of the Walls Peninsula but are comparable to the orthogneisses forming the Wilgi Geos Group of North Roe (Pringle 1970, p.164)[2]. It is thus likely that the Ve Skerries rocks form part of the Lewisian foreland, or, at least, a Lewisian tectonic enclave within the Caledonian belt.

Fethaland Series and Ollaberry Series

To the east of the Wester Keolka Shear Zone lie two series of banded metamorphosed sedimentary, basic volcanic and pyroclastic rocks separated from each other by a shear zone, named the Virdibreck Shear. These are, respectively, the Fethaland Series and the Ollaberry Series, whose distribution can be seen from P915568. The major subdivisions of these groups, as recognised by Phemister (1976, in Summ. Prog. 1930), are set out in the table below. The succession is best seen in the northern part of North Roe where Pringle (1970, figs. 5 and 6)[2] has established a more detailed lithological subdivision.

The metasediments of north-west Mainland
Components and Lithology Major subdivisions
Phemister's Series Groups in Pringle’s Eastern Series
East 6 Banded quartz- and muscovite-schist Ollaberry Series Queyfirth Group
5 Calcareous Group ranging from quartzose schist to siliceous limestone
4 Greenschist Group
3 Banded quartz- and muscovite-schist
2 Muscovite-schist with garnet and chloritoid in parts Fethaland Series Sand Voe Group
West 1 Banded hornblendic gneiss with granulitic gneiss and bands of impure quartzite


The Fethaland Series is characterised by the presence of thick hornblendic gneisses, but also contains thick bands of impure quartzite, some of which are locally pebbly. Near its top there are thick lenses of amphibolite and metagabbro which probably originated as sills or flows of basic igneous rock. The series is coarsely crystalline and garnetiferous over most of its outcrop. The Ollaberry Series is generally finer-grained and only locally garnetiferous. Two of its groups have a distinctive lithology. These are the Greenschist Group which consists of fine-grained green-coloured schists with albite, epidote, chlorite and, locally, small garnets, and the Calcareous Group which contains calcareous schists with limestones and some bands rich in calc-silicate minerals. In the Lunnister area the Ollaberry Series contains a strongly deformed conglomerate and at Clothister Hill there is a bed of graphite- schist associated with. a steeply inclined epigenetic deposit of magnetite enclosed in a skarn composed of garnet, hornblende, pyroxene and epidote. In Fethaland and Queyfirth the series contains intrusions of greenstone and serpentinite.

In the area around Colla Firth and Ollaberry the rocks of the Ollaberry Series are folded into a southward plunging synform. Farther south, at Lunnister, the structure of the rocks also seems to be a tight synform but, as the rocks become progressively more mylonitised and phyllonitised towards the south, their basic structural pattern becomes more difficult to unravel. Near the head of Busta Voe the belt of metamorphic rocks has decreased in width to a few hundred metres and it is here bounded and cut by a series of faults which splay out north—westwards from the Walls Boundary Fault.

Pringle has suggested that in North Roe the two series of metasediments were affected by two major phases of deformation. The first produced the Wester Keolka Shear Zone and was responsible for the pervading schistosity in the metasediments and the Wilgi Geos gneisses. It also produced some near- isoclinal intrafolial folds. The second deformation produced the other easterly dipping shear belts within the metasediments and gave rise to a series of folds. These folds are overturned to the west, suggesting that the rocks of the eastern series were being pushed westward over the ‘basement’. Later dislocations include the development of open folds plunging to the north-west and south-east and the formation of the north-east trending faults. In the central and southern parts of the outcrop Phemister has recognised two major phases of deformation. The first is thought to have produced a large eastward-closing recumbent fold with shear zones along and parallel to its axial plane. The individual thrust slices appear to have been pushed to the west. The second phase was responsible for the rotation of this near-horizontal structure into a vertical position to give rise to the complex sheared synformal structure which we can recognise today. The brittle dislocations along the Walls Boundary Fault and its branches are ascribed to a later phase of large-scale, possibly transcurrent, movement (see correlation).

The main episode of prograde metamorphism affecting both the Fethaland and Ollaberry series probably coincided with the first phase of folding in these rocks and produced a mineral assemblage in the lower amphibolite facies. Subsequent mineral changes were due mainly to mechanical deformation which reduced biotite to chlorite, but these were so slight that pyroxene is still preserved in some of the mylonitic rocks. Exceptions to this pattern are found in a few small areas as in the skarn of the Clothister Hill magnetite where the presence of garnet and pyroxene indicates the local development of relatively high temperatures (see Shetland, Iron ores). The effects of contact metamorphism by the Northmaven Complex are slight and very localised.

Metamorphic Rocks of the Ness of Hillswick

In the Hillswick area two series of metamorphic rocks are present. That occupying the southern part of the Ness of Hillswick consists largely of banded hornblende—gneiss and hornblende-schist with a band of silvery mica- schist and with sheared lenticles of serpentine and talc. This lithology is comparable to that of the Fethaland Series. The remainder of the outcrop contains a series of siliceous schists and granulites, muscovite-schists with garnet, kyanite and chloritoid and a band of hornblendic gneiss and granulite. Pringle has tentatively equated this series with his Queyfirth Group (i.e. Ollaberry Series) but because neither typical greenschists nor calcareous rocks are present this correlation is doubtful. The Hillswick rocks are strongly folded.

Metamorphic rocks within the Northmaven Plutonic Complex

There are three large and several smaller outcrops of metamorphic rocks partly or completely surrounded by the plutonic rocks which form most of the Busta Peninsula, the island of Muckle Roe and the adjacent smaller islands (P915568). The metamorphic rocks form part of a banded group of hornblende-rich schists in which pelitic mica-schists, locally garnetiferous, are important and quartzose granulites are minor constituents. Most of the rocks are similar to those found in the Fethaland Series. It is possible that some of the smaller masses of metamorphic rock are true inclusions or xenoliths within the igneous complex, but other masses, such as those at Egilsay and Black Skerry, are probably roof pendants. The metamorphic rocks of Muckle Roe and the Busta Peninsula could be either parts of the roof or floor of the adjoining igneous complex.

The rocks were regionally metamorphosed in the amphibolite facies and experienced subsequent shearing and fracturing at low temperature prior to being intruded by the igneous complex. Later thermal metamorphism adjacent to basic rocks of the complex locally reached pyroxene-hornfels facies, and andalusite and cordierite were extensively developed in the pelites. The thermal alteration was followed by low-temperature hydrothermal changes which led to the alteration of cordierite and andalusite to pinite. Still later the rocks were subjected to local potassium metasomatism associated with the intrusion of the granite, and finally the masses close to the Walls Boundary Fault were both shattered and intensively scapolitised.

Metamorphic rocks of the Walls Peninsula and Foula

Walls Peninsula

Metamorphic rocks form the northern coastal strip of the Walls Peninsula and parts of the islands of Vementry (P915569) and Papa Little. In these the following four lithological units, listed from north to south, have been recognised:

1. Vementry Group. Hornblende-schist and amphibolite with bands of semi-pelite and some quartz-granulite.
2. Neeans Group. Platy feldspathic muscovite-schist with lenticular masses of coarse hornblende-schist and some thin bands of limestone and epidote- clinozoisite rock.
3. West Burra Firth Group. Tremolite- and phlogopite-schist with calc-schist and limestone.
4. Snarra Ness Group. Mainly hornblende- and mica–schist with bands and lenses of amphibolite.

The outcrops of these groups do not form continuous bands and some of the boundaries between the groups appear to be shear planes. A high proportion of these rocks were originally siltstones, shales and sandstones. Calcareous mudstones and thick limestones probably formed an appreciable part of the West Burra Firth Group, while basic lavas and/or pyroclastic deposits were abundant in the two hornblendic groups. The thick masses of hornblende-schist and amphibolite may have been basic sheets or sills. The rocks now contain many veins of granite and pegmatite as well as lits and porphyroblasts of feldspar. The granitic material is particularly abundant in the hornblendic rocks and forms up to 20 per cent of the total volume of the Snarra Ness Group.

The tectonic history of this belt of metasediments is similar to that of the North Mainland belt. During the earliest recognisable phase of deformation the banding of the rock was folded into a series of tight minor folds with axial planes parallel to the regional foliation. This was the main period of folding and it produced a pronounced lineation parallel to the axes of the folds. This phase was followed by a local, relatively minor, episode of folding during which a weak axial-plane strain–slip cleavage was formed in some parts of the Snarra Ness Group. Meanwhile, or perhaps somewhat later, the Vementry Group, which now forms the northern part of the outcrop, was subjected to a phase of intense shearing with some mylonitisation, indicating that a major shear zone was probably being formed just north of the present outcrop. Later dislocations include the development of belts of conjugate kink-folds and more widespread kink-bands, all of which have an east–south-easterly axial trend. As the overall dip of the foliation of the metamorphic rocks is roughly parallel to the bedding of the overlying Old Red Sandstone strata, it follows that the foliation in the former must have been more or less horizontal when these sediments were laid down. The present southerly dip of the foliation resulted from the late Middle or Upper Devonian earth movements, described in Walls Formation.

The peak of metamorphism appears to have been attained during or just after the main period of folding. Over large parts of the area this took place under conditions transitional between the greenschist and amphibolite facies. Common minerals developed at this stage include garnet, which is abundant throughout the series, oligoclase, hornblende, tremolite, phlogopite, epidote and clinozoisite. There are a number of areas where diopside occurs in limestones and in calc-silicate rocks, indicating that, locally, higher temperatures may have prevailed. These thermal ‘highs’ occur in areas of intense granite veining and may be associated with the emplacement of these veins. Retrograde metamorphism occurred both during the period of shearing in the Vementry Group and, to a lesser extent, during the phase of conjugate folding. The shearing produced extensive granulitisation and local mylonitisation of the rock-fabric and led to the breakdown of garnet and biotite to chlorite.

Foula

The metamorphic rocks forming the fault–bounded strip along the east coast of the island of Foula (P915577) are made up of bands of garnetiferous psammitic granulite and garnetiferous mica-schist. They contain lenses of amphibolite and epidote-rock as well as two thin bands of crystalline limestone. All the metamorphic rocks are cut by granite veins and contain lits and porphyroblasts of feldspar. There is a marked similarity both in the lithology and in the structural and metamorphic history of these rocks and the Neeans Group of the Walls Peninsula.

Bibliography

Full bibliography list
  1. SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1933 (for 1932). Mem. geol. Surv. Gt Br., 77–80, 95–7.
  2. 2.0 2.1 2.2 2.3 PRINGLE, I R. 1970. The structural geology of the North Roe area of Shetland. Geol. Jnl, 7, 147–70.