Plutonic complexes of Shetland

From Earthwise
Jump to: navigation, search
Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.

Introduction

The plutonic complexes of Shetland Mainland. P915573.
The metamorphic rocks and plutonic complexes of North Roe and Northmaven. P915568.
Net vein complex of basic and granitic rock, Northmaven Complex. Wilson’s Noup, Northmaven, Shetland Mainland. P218878.
Cliffs of Ronas Hill Granite, Heads of Grocken, Shetland north Mainland, looking east towards Hillswick peninsula. P002770.
Minor intrusions of the west and north Mainland of Shetland facing. P915589.
Geological map of the Sandsting Complex. P915574.

The Shetland Islands contain two groups of Caledonian plutonic complexes (P915573). The earlier of these is intruded into the metamorphic rocks east of the Walls Boundary Fault and includes the Graven and Brae complexes of Delting, the Spiggie Complex of central and south Mainland and the very small areas of granite at Channerwick and Cunningsburgh in south-eastern Mainland. Some of these complexes are in places weakly foliated, indicating that they were emplaced before Caledonian tectonic activity had ceased. The second group crops out just west of the Walls Boundary Fault and forms a north—south trending series of possibly interconnected complexes which extend from North Roe to the southern side of the Walls Peninsula. A possible representative of this group on the east side of the Walls Boundary Fault is the granite that may lie just off the south-west shore of Fair Isle. These complexes are not foliated and are thus essentially post-tectonic intrusions. The Sandsting Plutonic Complex, however, cuts Middle Old Red Sandstone sediments, which were intensely folded during or after the intrusion. This suggests that the emplacement of at least some of the complexes may have been connected with the final localised movements of the Caledonian Orogeny. Swarms of acid and basic dykes are centred on some of the western complexes.


Eastern Late-Orogenic complexes

Graven Complex

This intrusive complex occupies an area which extends from the Walls Boundary Fault in Northmaven eastward via Sullom Voe, northern Delting and the adjoining islands in Yell Sound to the Nesting Fault. It reappears further south in North Nesting where it is exposed along the south shore of Dury Voe, having apparently been displaced dextrally for 14 to 16 km by the Nesting Fault (P915573).

The Graven Complex is not a single massive intrusion. It could be more accurately described as two superimposed vein complexes which cut the metamorphic country rocks and now occupy about half of the affected area. This means that within the complex there are now many large and small enclaves of country rock, most of which maintain their original position and orientation so that the original structure of the metamorphic rocks can still be determined. The first major phase of intrusion was the emplacement of a plexus of coarse granite-pegmatite veins, pods, dykes and irregular larger bodies which are now separated by screens of variably permeated country rock. This was followed by the intrusion of lamprophyre and porphyrite dykes, which extend beyond the limits of the complex. The second major phase was the intrusion of a network of medium- to fine-grained hybrid plutonic rocks which Flinn has collectively termed the Inclusion Granite. These consist of a continuously variable mixture of diorite, monzonite, granodiorite and granite and are characterised by the abundance of small rounded cognate xenoliths of almost pure hornblende rock. The most granitic portion of the complex forms the Stava Ness Granite. The Inclusion Granite is also full of orientated angular inclusions and larger enclaves of country rock. The effects of thermal metamorphism by the intrusive rocks of the complex are most marked in the xenoliths. These contain abundant sillimanite in the pelitic rocks and diopside in the calcareous ones.

Brae Complex

The Brae Complex is the most northerly and largest of a series of ultrabasic to acid plutonic complexes which extend as far south as Whitelaw Hill (HU 356 540), 2 km SE of Aith. These complexes have a family similarity and all have amphibole as their most characteristic mineral. Their emplacement may well represent an early phase in the igneous activity which culminated in the formation of the Graven Complex.

The Brae Complex itself (P915573) occupies the south-western peninsula of Delting which is bounded in the west by Busta Voe and in the south by Olna Firth. Unlike the Graven Complex it is a compact, composite stock-like intrusion. It is largely composed of a two-pyroxene-diorite with andesine, potash-feldspar and biotite, which has been extensively altered to a dioritic rock composed of the original andesine and new green secondary amphibole, biotite and quartz. Scattered throughout the complex are masses of ultrabasic rock, the largest of which lies in the south-west corner. These appear originally to have varied from peridotite to pyroxenite (with both orthopyroxene and clinopyroxene) and dunite, although most of the olivine is now serpentinised and the pyroxenes are commonly altered to amphibole. In good coastal exposures it is possible to see that these rocks are composed of variably sized and irregularly shaped fragments of pyroxenite and pyroxene-rich peridotite set in a sparse matrix of serpentinised dunite and olivine-rich peridotite.

In two places along the south coast there are small xenolithic areas. The xenoliths include altered metasedimentary and ultrabasic rocks and they are associated with two-pyroxene gabbro. These rocks are characterised by the presence of reddish-brown hornblende; they are considerably richer in iron than similar rocks in the rest of the complex, and Gill (1965)[1] considers them to be an early phase of emplacement. However, all the pyroxenites and pyroxene-rich peridotites almost invariably contain traces of the reddish-brown hornblende, suggesting that all the ultrabasic rocks probably belong to the early phase of emplacement.

The original pyroxene-rich rocks (cumulates?) appear to have been mobilised by brecciation and injection with olivine-rich peridotite and transported to a higher level in the crust. There, together with the iron-rich xenoliths and the gabbro, they developed reddish-brown hornblende. Large masses of these rocks were then transported as xenoliths in the two-pyroxene diorite to their present positions in the Brae Complex. In some of the complexes to the south of Brae the ultrabasic rocks appear to have remained in place at the intermediate level and were there transformed into hornblendites with few of the original minerals remaining.

The complex has been cut, first by sheets of granodiorite, tonalite and porphyrite, then by pegmatites like those of the Graven Complex, and finally by lamprophyres.

Spiggie Complex

The intrusive complex which has been termed the Spiggie Complex or Spiggie Granite occupies a very extensive area which is now largely below the sea. Its outcrop extends from the south-west shore of Aith Voe in central Mainland, where it is truncated by the Walls Boundary Fault, discontinuously southwards via the group of islands between the Walls Peninsula and Burra Isle to Hamnavoe on West Burra. It then reappears on the east side of the Nesting Fault between Spiggie and Quendale. Its most southerly outcrop is on Lady’s Holm to the west of Scatsness.

The complex consists mainly of granodiorite and porphyritic adamellite but monzonite, pyroxenite and serpentinite are also present. The order of intrusion is as follows: 1. Ultrabasic rocks, 2. Monzonite and related rocks, 3. Granodiorite and porphyritic adamellite.

Ultrabasic rocks

There are two areas of serpentinite within the complex: a band of serpentinite up to 350 m wide along the margin at Scousburgh (HU 38 18) and a sizeable outcrop around East Houlland (HU 345 535), 1.5 km north of Bixter. Biotite-pyroxenite occurs as a raft in monzonite south of Hamnavoe (HU 366 355) and forms the Inner Skerry (HU 363 340) off the coast of West Burra.

Monzonite and related rocks

Monzonite cut by granitic and pegmatitic veins is exposed in a roadside quarry 1 km E of Bixter (HU 342 521) and at Hamnavoe (HU 370 360). At both these localities it is a pyroxene-bearing rock with granulitised and recrystallised feldspars and a pronounced linear fabric. Rounded, partly digested, xenoliths of serpentinite occur at the former locality. In the Spiggie area monzonite crops out alongside the serpentinite at Scousburgh and around Bakkasetter where it is extensively decomposed. Strips of hornblende-rich rock occur at the edge of the complex at Noss (HU 358 166), Quendale (HU 372 132) and Fora Ness (HU 350 450). At Fora Ness the hornblendic rock is strongly foliated and cut by dykes of microgranite which are also foliated.

Granodiorite and porphyritic adamellite

These rocks are respectively the inner and marginal facies of a single major intrusion. Their relationship is best seen at Hamnavoe and on the islands to the north-west of West Burra. On the island of Papa (HU 365 375) the rock adjoining the eastern margin of the mass is packed with microcline phenocrysts about 1 cm long. Farther west the phenocrysts become progressively less abundant but individually larger and in the central part of the island they are well scattered and up to 4 cm long. At the west end of the island and on Oxna (HU 350 370) and Cheynies (HU 347 386) the rock is a non-porphyritic granodiorite. Both the porphyritic and non-porphyritic facies also occur in the Sandsound—Aith area and at Spiggie. Throughout most of the mass there is a weakly-developed foliation due to the presence of flattened quartz blebs. In thin section the rock shows the effects of incipient granulitisation and recrystallisation and contains abundant secondary epidote. Parts of the mass are cut by veins of pegmatite up to 1 m thick.

In south Mainland the Spiggie Granite is unconformably overlain by Old Red Sandstone sediments. Where it is in contact with the metamorphic rocks it has generally produced only a narrow thermal aureole, but at its contact with the Dunrossness Phyllitic Group the thermal effects are extensive and spectacular. At Scousburgh, for instance, the newly formed minerals include: garnet, andalusite, sillimanite, kyanite, chloritoid and staurolite. The distribution and textural relations of these minerals suggest that the thermal metamorphism took place in at least two stages separated by a period of deformation during which a strain-slip cleavage developed (see Flinn 1967a, pp. 270-73)[2].

Channerwick and Cunningsburgh granites

The Channerwick Granite consists of a number of interconnected dyke-like bodies up to 25 m thick with sill-like offshoots, which are exposed close to the main road at Channerwick, 18 km SSW of Lerwick. It consists of albite, muscovite and quartz and is associated with sheets of quartz-porphyry. This small outcrop is remarkable for the extent of its thermal aureole which has a diameter of about 900 m. The aureole is conspicuous because of the presence of white and dark spots, consisting respectively of shimmer aggregate and chlorite, in the surrounding Clift Hills Phyllitic Group. Of even greater interest is the presence of newly formed chloritoid in Dunrossness phyllite within the outer zone of the aureole, more than 500 m from the granite outcrop.

The outcrop of the Cunningsburgh Granite is even smaller, but as it is intruded into Dunrossness phyllites which are highly sensitive to thermal metamorphism, its aureole is extensive and consists of an inner zone containing porphyroblasts of staurolite, a middle zone in which chloritoid has partially replaced staurolite and an outer zone of completely recrystallised chloritoid.


Western Post-orogenic complexes

Northmaven Complex

The outcrop of the Northmaven Complex (P915568) extends from the Beorgs of Uyea, close to the northern coast of Shetland Mainland, southwards via the great mass of Ronas Hill and the rugged terrain which forms most of Northmaven, to the island of Muckle Roe and beyond into Vementry island. The complex includes various types of granite, granophyre, diorite and gabbro, and a few small outcrops of ultrabasic rock as well as some altered basaltic rocks.

The emplacement of the plutonic rocks may have taken place in the following three stages: (1) Intrusion of basic magma: (2) Intrusion of acid magma and hybridisation: (3) Late intrusion of granophyre. The plutonic activity was preceded by the formation of early minor hypabyssal intrusions and perhaps also lavas. It was followed by the intrusion of great swarms of acid, intermediate and basic dykes. At a late stage the rocks were affected by hydrothermal alteration which uralitised the basic plutonic and hypabyssal rocks. Even later there was a period of localised mineralisation leading to the formation of scapolite and zeolite. Scapolite is seen in veins along the eastern margin of the complex at Mavis Grind and further north.

Early hypabyssal intrusions and lavas

Intrusive rocks which are older than the plutonic rocks have been identified within the Ronas Hill Granite and on the Isle of Egilsay at the mouth of Mangaster Voe, where metamorphic rock out by a dyke of porphyrite is enclosed in and thermally altered by diorite. Small masses of dolerite and basalt altered by the surrounding diorite or gabbro are also found in the Busta Peninsula and in the eastern coastal area of Muckle Roe. The coastal stretch between Gunnister Voe and Mangaster Voe, particularly around Wilson’s Noup, contains many irregular xenoliths of altered fine-grained basic igneous rock enclosed and partly assimilated by a network of veins of diorite and granodiorite (P218878). These xenoliths may be the remnants of a series of early lava flows whose composition may perhaps be representative of the undifferentiated parent magma of the complex.

Ultrabasic rocks

Blocks of ultrabasic rock ranging from harrisite to lherzolite have been recorded at two localities just west of Clothister Hill within the dioritic portion of the complex (P915568). These rocks are composed mainly of orthopyroxene, clinopyroxene, calcic plagioclase and olivine. It is not known if they are derived from small discrete bosses or from detached enclaves within the diorite. A different type of ultrabasic rock, hornblende- hypersthenite, crops out near the head of Ronas Voe, where it forms a small body entirely enclosed in metamorphic rock. The nature of its contact with the surrounding rock is, however, not seen.

Diorite and gabbro

Plutonic rocks of intermediate and basic composition occupy three main areas within the complex (P915568). The two smaller outcrops are in North Roe, where they are largely enclosed in the Ronas Hill Granite; the largest extends from the shores of Ronas Voe southwards to the east coast of Muckle Roe. Diorites are by far the most abundant rock types in these outcrops and in many areas they are riddled by granite and granophyre veins, giving rise to a very large net-vein complex. Gabbro occurs in small irregular areas throughout the diorite, and in these the rocks of different composition are closely intermingled, with gabbros passing laterally within a short distance into dioritic or more acid types. The greatest concentrations of gabbro occur in the southern part of the area, around Mangaster Voe and Mavis Grind and in the Busta Peninsula, but there is also one large outcrop along the north shore of Clubbi Shuns in North Roe.

The gabbros, though variable in grain-size and composition, are generally medium- to coarse-grained. They grade from true gabbro into hornblende-biotite-diorite with minor quartz and potash-feldspar. Both the augite and hornblende of these rocks are completely or peripherally altered to uralitic amphibole, which also occupies larger pockets resembling vesicles. Some of the fine-grained basic rocks are uralitised dolerites or basalts. These are most abundant near Mavis Grind and on the east shore of Muckle Roe.

The diorites are medium-grained black and white speckled rocks with a wide range in texture, grain-size and composition. They range from granodiorite to fine-grained hornblendic meladiorite. These variations do not have any relation to their position in the complex. The diorite outcrops also contain many small acid bodies with irregular outcrops and in many areas the latter form a network of veins. The junctions between acid and more basic material may either be sharp and angular, suggesting that the latter had consolidated before veining took place, or highly undulating, which might point to the existence of adjoining fluid magmas. Other contacts again are diffuse due to metasomatic feldspathisation and coarsening within the basic xenolith.

Granite and granophyre

The granitic components of the Northmaven Plutonic Complex belong to a number of separate bodies which may have been emplaced during distinct phases in the development of the complex.

In the north the earliest granitic intrusion is a near-vertical dyke-like body (?ring-dyke) of xenolithic granite which can be traced for a distance of 5.5 km along the south-eastern border of the Ronas Hill Granite (P218878). It is 60 m wide north of the Brig of Colla Firth, but widens south-west of the Brig to form a zone of xenolithic granite and schist about 250 m wide. The xenoliths are angular and consist of siliceous and micaceous schist with occasional fragments of hornblende-schist. Along a considerable part of its outcrop a screen of metamorphic rock intervenes between the dyke and the granite to the west.

The main granitic body in North Roe is the Ronas Hill Granite (P002770). In the west it is separated from the volcanic rocks of Esha Ness by the probable northward continuation of the Melby Fault; in the north and east it has a near-vertical contact with the metamorphic country rock and the diorite. The ‘granite’ is in fact a deep red leucocratic granophyre. It contains cavities (druses) which are generally about 1 mm in diameter, and which often contain stilpnomelane and are in places lined with crystals of milky quartz.

A long narrow dyke-like outcrop of red leucocratic granite 150 m to 1.6 km wide, called the Eastern Granite, extends along the eastern margin of the complex from the head of Ronas Voe southwards for 14.5 km past Mavis Grind to Busta Voe. The eastern margin of this mass is near-vertical and appears to be controlled by the existence of an early dislocation along the line of the Walls Boundary Fault zone. On its western margin, however, the contact with the gabbro-diorite takes the form of a plexus of large and small sheets, dykes and veins of granite in the diorite. These relationships indicate that the Eastern Granite was intruded under conditions which permitted easy penetration of the diorite.

The most southerly outcrops of granite within the Northmaven Plutonic Complex occur near Scarfataing in the south-east of Muckle Roe and in the north-eastern part of the island of Vementry (P915573). The Scarfataing Granite contains large enclaves of gneiss, which are cut by dykes of ophitic dolerite but not by diorite, suggesting that it was an early intrusion which may even have preceded the emplacement of the diorite. It appears to have been emplaced as a series of sheets which are now vertical. In many exposures both the granite and the included gneiss are sheared. The Vementry Granite consists of two lithological types, an outer coarse-grained pink leucocratic quartz-rich granite and an inner granite which consists of phenocrysts of feldspar and quartz set in a darker, relatively fine-grained matrix. The junction with the adjoining metamorphic rock is straight and sharp and where seen it is inclined at 40° to 50° away from the granite. This granite is of particular interest in that it forms the focus of a large number of acid dykes which appear to radiate from its centre (P915589).

The youngest rock in this southern part of the Northmaven Plutonic Complex is the Muckle Roe Granophyre which forms most of Muckle Roe and appears to have originated as a roughly circular stock. The granophyre is closely jointed and gives rise to rugged topography with extensive inland scree slopes and impressive coastal cliffs. It is pink in colour, locally tinged with yellow, and consists of blebs of quartz set in a stony base in which feldspar cleavage faces are sometimes seen. Drusy cavities are common and these may be lined with well-shaped quartz crystals.

Intrusive history and shape of complex

The Northmaven Plutonic Complex appears to have been emplaced during a fairly prolonged period of magmatic activity in the Devonian Period. The complex may be younger than the volcanic rocks of Esha Ness, and as is suggested by the radiometric (Rb:Sr) date of 358 ± m.y. for the Ronas Hill Granite (Miller and Flinn 1966)[3] its period of emplacement may span the Middle—Upper Devonian boundary. The main stages in the evolution of the complex are as follows:

1. Emplacement of early minor intrusions and ? lavas.
2. Emplacement of members of the plutonic complex in the following order:
(a) Early granite ring-dykes (i.e. Colla Firth, Scarfataing and ? Vementry granites)
(b) Diorites and gabbros
(c) Eastern Granite (in part contemporaneous with (b))
(d) Circular or oval granophyric stocks (i.e. Ronas Hill Granite and Muckle Roe Granophyre).
3. Emplacement of swarms of late minor intrusions.

Phemister (1950, p. 360)[4] has shown that the junction of the Ronas Hill Granite with the adjoining gneiss is everywhere sharp and steep, and he believes that the intrusion of the granite was preceded by the formation of an arcuate zone of fracture and brecciation, along part of which an early ring-dyke was intruded. The main body of magma then punched out a clean plug from the loosened country rock within this zone.

The diorite-gabbro component of the complex is believed to have been derived from a basic magma which was rich in water and whose emplacement was slow and tranquil, taking place virtually under hydrostatic conditions. During its emplacement there was a continuous acidification of the gabbroic rock by reaction, in part with the hydrous and quartzo-feldspathic fluids derived from the differentiation of the basic magma and in part with granitic magma derived from the limited accession of material from adjacent granitic magma reservoirs. Granitic magma was subsequently intruded, first quiescently as dykes and sheets within the diorite and as the irregular dyke-like Eastern Granite, and later forcefully by pushing its way upward to produce the stocks which now form the Ronas Hill Granite and Muckle Roe Granophyre.

Sandsting Complex

The Sandsting Complex forms the south-eastern part of the Walls Peninsula and is intruded into the sedimentary rocks of the Middle Old Red Sandstone Walls Formation (P915574). As in the Northmaven Plutonic Complex the rocks range from granitic to ultrabasic, but here gabbro is a very minor constituent and dioritic rocks form a smaller proportion of the total outcrop. As some of the outcrop of this complex may now be below sea level and only a small part of its margin is exposed, it is difficult to ascertain the shape and structure of the complex. In the western part of the area the junction with the sediments is inclined at 40° to 70° to the north, but in the east granite forms a series of near-vertical north-north-west trending sills which thin out northwards. The contact is in all areas roughly sub-parallel to the bedding of the sediments.

As in Northmaven a number of phases in the evolution of the complex can be recognised. These are as follows:

1. Intrusion of early basic and sub-basic dykes.
2. Emplacement of plutonic components in the following sequence:
(a) Diorite and gabbro (i.e. basic magma)
(b) Several pulses of acid magma producing in turn granodiorite, biotite-granite, granophyre and porphyritic microadamellite.
3. Formation of acid and sub-basic minor intrusions.
4. Scapolitisation along active shear belts.

Diorite, gabbro and ultrabasic rock

Dioritic rocks form a number of irregular outcrops between the mouth of Gruting Voe and Garderhouse. These are almost completely surrounded by granitic rocks. Two of the outcrops, at Culswick and at Wester Skeld, contain large and small enclaves of hornfelsed sediment, the largest of which is over 800 m long. The dioritic rocks range in composition from melamicrodiorite, through hornblende-diorite to quartz-biotite-diorite and granodiorite. On Hestinsetter Hill there is an outcrop of vertically banded diorite full of minute ovoids which consist of pyroxene-monzonite with conspicuous euhedral crystals of sphene.

Gabbro forms only two small outcrops on the east shore of Skelda Voe and near Garderhouse. As in Northmaven it has primary hornblende as well as augite and is strongly uralitised and saussuritised. The presence of a small outcrop of ultrabasic rock is inferred from a concentration of boulders near Stump Farm, just east of Gossa Water.

The diorite is in places veined by leucocratic quartz-diorite and pegmatite with abundant epidote. All these veins are considered to be the products of differentiation of the dioritic magma. In addition there are areas where the vein material is derived from the invading granitic magma. In these areas the margins of the xenoliths can be either angular or highly undulating, indicating that the diorite had reached various stages of consolidation when the granitic magma was intruded.

Granitic rocks

In the Culswick area lenticular east—west trending belts of granodiorite, porphyritic microgranite and porphyritic microadamellite adjoin the diorite (P915574). Farther east the granitic outcrop is formed by leucocratic biotite-granite, which completely encloses the central and eastern diorite outcrops and also forms the granite sills in the north-east of the complex. It passes into a coarse graphic granite in the eastern sills and this becomes progressively more fine-grained and granophyric in texture towards Bixter Voe.

Scapolitised shear belts

In the area around Wester Wick and in the Skelda Ness Peninsula the granite contains a number of lenticular, near-vertical north-north-west trending belts of intensely sheared and locally mylonitised rock. Sodic scapolite occurs both as a replacement product and as a vein-filling mineral in these belts; it also forms narrow veins in basic minor intrusions within the complex and in the sediments adjoining the granite. The scapolite was probably introduced by late hydrothermal solutions along active shear belts, joints, and other lines of weakness at some time after the emplacement of the granite. These scapolitised shear belts do not appear to be connected with the Walls Boundary Fault.

Intrusive history and shape of complex

No part of the Sandsting Complex has either the shape or the marginal relationships which could suggest that it forms part of a vertical or steep-sided stock at the present level of erosion. It is more likely that the complex originated as a sheet which was emplaced in several pulses of magmatic activity. After an early hypabyssal phase, when a few basic dykes were formed, the first major intrusion appears to have been by basic or sub-basic magma. This gave rise to a sill-like sheet of diorite which enclosed several masses of sediment. Before its consolidation was complete granitic magma was intruded in several pulses along or close to both the upper and lower margins of the diorite and also beyond its limits. Locally the diorite sheet was cut and transgressed by the granite. During this phase a number of lenticular granodioritic and granitic sheets were formed, the latter being the most extensive.

Though in the eastern part of the complex both the top and base of the granite sheet are seen, west of Skelda Voe there is no exposure of the base. The possibility that the western part of the outcrop is part of the upper portion of a stock cannot therefore be ruled out.

Bibliography

Full bibliography list
  1. GILL, K. R. 1965. The petrology of the Brae Complex, Delting, Shetland. Ph.D. Thesis, University of Cambridge (unpublished).
  2. FLINN, D. 1967a. The metamorphic rocks of the southern part of the Mainland of Shetland. Geol. Jnl, 5, 251-90.
  3. MILLER, J. A. and FLINN, D. 1966. A Survey of the Age Relations of Shetland Rocks. Geol. Jnl, 5, 95-116.
  4. PHEMISTER, J., SABINE, P. A. and HARVEY, C. O. 1950. The riebeckite-bearing dikes of Shetland. Mineralog. Mag., 29, 359-73.