Dykes, dyke swarms and volcanic plugs, Palaeogene volcanic districts of Scotland

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Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Introduction

Dilation axes of the Palaeogene dyke swarms in the Hebridean area. P914133
South Arran Dyke Swarm cutting Triassic sandstones, Kildonnan,Arran. P580470
Diagrammatic representation of the Mull Dyke Swarm. P914134

Numerous dykes of Palaeogene age are present throughout western Scotland and the Hebrides. The dykes commonly occur in parallel, north-west- to north-north-west-trending regional swarms, becoming more numerous and varied in direction near to and within the central complexes (Speight et al., 1982; P914133). Volcanic plugs are much less common and occur principally within or close to the lava fields and central complexes.

Although most of the dykes are a metre or less in thickness, intrusions between 2 and 10 m in width are not uncommon, and thicker examples are known. On Skye, there is evidence that larger dykes may be built up incrementally by numerous small injections of magma (Platten, 2000). The pronounced Minch Magnetic Anomaly to the east of the Isle of Lewis (P914133) has been attributed to a dyke that averages about 1.1 km in thickness (Ofoegbu and Bott, 1985; Fyfe et al., 1993) but no outcrop occurs onshore. A notable example of an unusually thick dyke, up to 100 m in width, crosses Muck in several en échelon outcrops. This dyke consists of olivine-gabbro and olivine-dolerite, and the gabbroic facies has developed small-scale mineral layering, similar to layering in the sills of north Skye (p. 87). Paleocene lavas and sedimentary rocks of Mid Jurassic age intruded by the dyke are altered to high-temperature, sanidinite-facies hornfelses (Emeleus, 1997). The Muck dyke probably acted as a feeder for lava flows; prolonged passage of magma, maintaining high temperatures, is indicated by the intense thermal alteration of the host rocks. In general, however, the dykes produced only slight induration and thermal metamorphism of the immediately adjoining host rocks.

The dykes in and near the central complexes are generally less than 2 m in thickness. However, injection of dykes side by side into the same fissure can result in thick multiple intrusions, which are most common close to the central complexes. On the Strathaird peninsula to the south-east of the Skye Central Complex, 60 per cent of the dykes in the Skye Dyke Swarm are multiple; large numbers of multiple dykes also occur close to the Mull Central Complex and multiple dykes are present along the south coast of Arran. Composite dykes, in which magmas of contrasting composition have been intruded in quick succession, generally without internal chilling, are less common and are also usually restricted to the vicinity of central complexes. On Arran, good examples of composite dykes crop out on the Tormore shore section, north of the composite Drumadoon Sill (p. 90). At these localities, initial intrusion of quartz-dolerite has been followed in rapid succession by quartz-porphyry or felsite injected into the centre of the dyke.

Dykes generally have chilled, fine-grained margins against earlier rocks. Glassy selvedges are rare in basaltic dykes, but at Camas na Cairdh, Muck, dark, vitreous tachylyte occurs on the margins of several dolerite dykes and flow-structures in the glass indicate that locally the magma moved nearly horizontally through the dyke fissure. Wholly or largely glassy silicic dykes are, by contrast, comparatively common. Classic examples occur at Tormore on the west coast of Arran, where dark green, flow-banded, glassy pitchstones have intruded Triassic sandstones. The commonest internal structures in basaltic dykes are planar zones of amygdales parallel to the dyke margins; rarely, there is also internal textural variation parallel to the dyke walls (e.g. Drever in Brown, 1969). The dyke walls are typically near-planar, but where dykes have intruded relatively incompetent surroundings the walls may be highly irregular and are bounded by well-defined chilled margins. This is seen, for example, in several thin, lobate, apparently discontinuous basaltic dykes intruded into Cambro-Ordovician dolostones at Camas Malag, Skye (Nicholson, 1985).

Dykes commonly form upstanding, wall-like features, especially where intruded into relatively soft-weathering sedimentary rocks. Many examples are visible at low tide on the foreshore at Kildonnan, south Arran (P580470), and on the sea bed between Antrim and Kintyre wall-like dykes, up to 28 m in height have been recorded (Fyfe et al., 1993). In-weathered dykes are also common, for example at the Bay of Laig, Eigg, where basalt dykes occupy deep trenches between sharply defined, upstanding walls of indurated Mid Jurassic Valtos Sandstone Formation strata (Emeleus, 1997). Columnar jointing in dykes is generally developed perpendicular to the cooling surfaces provided by the country rocks. This helps to identify those dykes that have intruded basalt lavas or dolerite sills (with or without their own columnar jointing), for example at Dippin Head, south-east Arran.


Dyke swarms

The presence of linear dyke swarms of Palaeogene age has long been recognised in north-west Britain and north-east Ireland (P914133). Several regional dyke swarms and localised subswarms have been identified in the Hebridean Igneous Province (Speight et al., 1982) of which the Skye and Mull dyke swarms are the most extensive. The principal features of these dyke swarms, including their dimensions (Table 16), were summarised by J E Richey in earlier editions of this book, and more recently by Speight et al. (1982). The swarms are generally densest in the vicinity of the central complexes, where smaller subswarms of varying trends also occur, and where the dykes commonly exhibit considerable compositional diversity. Intrusion of the dyke swarms has caused pronounced local crustal dilation, the greatest amount estimated for any swarm being 25 per cent to the south-east of the Skye Central Complex (Table 16). In northern Rum, the combination of dykes (and plugs) belonging to the north-west-trending Muck Dyke Swarm and the subsidiary north- to north-east-trending Rum Subswarm simulates a radial dyke swarm focussed on the central complex (Harker, 1908); it is, however, difficult to prove the presence of true radial dyke swarms in the province.

The distribution of the dyke swarms was controlled by a regional north-east–south-west extensional stress field, and the over-riding control on the orientation of the swarms is considered to have been the orientation of lower crustal intrusions that fed the dykes. Locally, subsidiary swarms are orientated approximately north–south, for example in the Outer Hebrides, and between south Skye and Morvern where the subsidiary swarm forms an en échelon link between the Skye and Mull swarms (P914133). England (1988) suggested that a minor component of dextral shear within the regional stress field controlled emplacement of the north–south dykes on Lewis which cross-cut the pronounced regional ‘grain’ of the gneisses. Elsewhere, as in Morar, Moidart and Morvern, and on Arran and Ailsa Craig, upper crustal structures may have influenced the orientation of the swarms.

The structure and compositional character of the Mull Dyke Swarm changes as it is traced towards the south-east, away from the central complex. Jolly and Sanderson (1995) examined well-exposed shore sections between Mull and Loch Fyne and found that the multiplicity of thin dykes on and near Mull gives way south-eastwards to more widely spaced, thicker and better ordered dykes (P914134). Farther to the east-south-east, from Ayrshire to the extremity of the swarm in the north-east of England, individual thick dykes may be traced over considerable distances. Echelons of the Cleveland Dyke are up to 25m thick in County Durham and north Yorkshire, some 350 to 400 km from Mull. It has been proposed that the Cleveland Dyke, which is closely similar in composition to tholeiitic basaltic rocks found in the Mull Central Complex, was fed laterally, in a matter of days, by a single pulse of magma that originated in the Mull Volcano (Macdonald et al., 1988). However, Speight et al. (1982) had contended that lateral flow of magma in dyke fissures could only have been very limited and they postulated that the swarms were fed vertically from ridge-like basaltic magma chambers situated at depths of 20 to 50 km. They also suggested that the central complexes developed where elongate magma chambers intersected major lines of crustal weakness such as faults. While the extent of lateral flow remains contentious, recent geophysical observations during active volcanism in Hawaii support the concept (e.g. Walker, 1993a).

The close relationships between the dyke swarms and the central complexes (see above) are evident from P914133. The abundance of dykes in the country rocks, including the lava fields, contrasts with their scarcity within many members of the central complexes, although there is much detailed evidence of substantial overlap between emplacement of dyke swarms and the intrusion of successive members of the central complexes. In Skye, many north-west-trending basic dykes intrude both the lavas and the gabbros of the Cuillin Centre from Blà Bheinn in the east to the northern part of the Cuillin near Sligachan; the dykes both cut and are cut by cone-sheets (Chapter 9). By comparison, far fewer north-west-trending dykes intrude the younger Red Hills granites. On Rum, Eigg and Muck, many dykes belonging to both the regional Muck Swarm and the Rum Subswarm intrude earlier members of the Rum Central Complex (Phase 1, see Chapter 9) but the later layered intrusions (Phase 2) postdate the majority of the dykes. On Mull, north-west­trending basic dykes intrude most members of the Mull Central Complex, but appear to be less numerous than in the surrounding lavas. This view is supported by evidence from the shores of Loch na Keal, where up to one-third of the dykes are free from the pervasive pneumatolytic alteration that affects their host lavas around the Mull Central Complex. On Ardnamurchan, only a few dykes cut the intrusions of Centres 2 and 3, although they are common in the country rocks, and a number cut pre-Centre 2 cone-sheets. Members of the north-north-west-trending Arran Dyke Swarm intrude the North Arran Granite Pluton, but in fewer numbers than are seen in the country rocks. Few dykes are found in the Central Arran Ring-complex.

With few exceptions (e.g. the Canna Lava Formation in north-west Rum and on Canna and Sanday), the majority of the dykes in the regional swarms and subswarms appear to postdate the Paleocene lava fields surrounding the central complexes. While it is generally assumed that many of the lavas were fed from the dykes, or from plugs that developed on dykes, it is extremely rare to find an example of a dyke feeding a flow. It must be concluded that dyke intrusion was continuous, albeit with diminishing intensity, throughout the growth of the lava fields and the central complexes. In general, the dyke swarms appear to be intimately linked to specific central complexes, although the association of the Islay and Kintyre–Jura dyke swarms with the submarine Blackstones Bank Central Complex cannot be proved (Walker, 1960). Emplacement of a dyke swarm may, therefore, have taken place over a million years or more and the individual swarms may have been separated by even more substantial periods of time. As with the lavas, there was no single period of dyke emplacement in the Hebridean Igneous Province during the Palaeogene.

Dyke composition

The majority of the dykes in the linear regional swarms are of basaltic or slightly more evolved composition. In and around the central complexes there are, additionally, dykes of silicic, intermediate and, less commonly, ultrabasic composition (Gibb, 1968, 1969). The basaltic dykes of the regional swarms are predominantly of tholeiitic basalt or tholeiitic olivine basalt, or of mildly alkaline olivine basalt. Dyke compositions may remain fairly uniform over considerable distances, as has been well demonstrated in regional dykes that extend across the Southern Uplands and into north Yorkshire (Macdonald et al., 1988).

Tholeiitic basalts and related, more evolved, lithologies predominate in the Mull Dyke Swarm. However, alkali olivine-dolerites, locally termed ‘crinanites’, are common as far south­east as Loch Fyne, and the swarm contains silicic and intermediate dykes in the vicinity of Oban and on Mull. The dykes intruding the Skye Central Complex and its immediate surroundings have a wide compositional range (Chapter 10). Sporadic north-west-trending dykes of silicic pitchstone and alkali olivine-dolerite intrude the granites of the Red Hills centres, and abundant earlier dykes of tholeiitic basalt and alkali olivine basalt overlap emplacement of the gabbros and the Coire Uaigneich Granite of the Cuillin Centre. Ultrabasic dykes are principally found close to the central complex, although a few ultrabasic dykes, rich in calcic plagioclase, intrude Paleocene lavas near Bracadale in north Skye (Donaldson, 1977). North-west-trending trachyte dykes occur along the axis of the Skye Dyke Swarm, for example in the vicinity of Drynoch, on Loch Harport.

The dykes on Rum are generally composed of transitional olivine basalt, less commonly of tholeiitic basalt and basaltic andesite, and very rarely of silicic pitchstone (Forster, 1980). Additionally, the Rum layered intrusions are cut by rare picritic basalt dykes, which most likely represent quenched ultrabasic liquids (Upton et al., 2002; see p. 143). The dykes of Eigg, which belong to the Rum Subswarm, are principally transitional to mildly alkaline olivine basalts with a few more evolved hawaiites and mugearites, and rare feldspar-phyric dolerites. The dykes of the Muck Dyke Swarm on Muck are generally less evolved than those on Eigg and Rum and include a high proportion of mildly alkaline olivine-dolerites. On Rum, Canna and Sanday, a few north­west- to north-east-trending dykes of olivine basalt and dolerite intrude the flows of the Canna Lava Formation. They are either late members of the Rum Subswarm or, more likely, related to younger activity on Skye. No trachytic dykes are known from the Small Isles.

The Islay and Jura–Kintyre dyke swarms consist of mildly alkaline olivine-dolerites and evolved variants, including hawaiite; on Jura, a small subswarm is of a primitive picrobasalt composition. Along the south coast of Arran, members of the Arran Dyke Swarm exposed in the classic Kildonnan shore section range in composition from alkali olivine-dolerite (‘crinanite’) to tholeiitic olivine basalt, tholeiitic basalt and quartz-dolerite. The alkali olivine-dolerite dykes of Arran may be earlier than the tholeiitic basalt dykes, since the latter intrude the Central Arran Ring-complex and North Arran Granite Pluton, whereas the former do not. Silicic pitchstones occur in some abundance on Arran where they intrude the North Arran Granite Pluton. There is also a number of composite dykes with quartz-dolerite or tholeiitic basalt margins (commonly containing xenocrysts of quartz and rare alkali feldspar) and quartz-feldspar porphyry or contaminated porphyritic central parts. They are of similar composition to the composite sills (Chapters 8 and 10).

A monchiquite dyke containing xenoliths of mantle and deep-crustal origin, and a variety of megacrysts, crops out at Loch Roag on the Isle of Lewis (Menzies et al., 1987). This dyke is quite unlike any other dyke recorded from the Hebridean Igneous Province (p. 144).


Volcanic plugs

Volcanic plugs are of widespread occurrence in the Hebridean Igneous Province, but are most common in and around the central complexes and in the lava fields. There is a very large concentration on Rum, where more than forty plugs of gabbro, dolerite and peridotite intrude the Torridonian and Triassic sedimentary rocks and most members of the central complex. The gabbro plugs may have fed surface flows but no compositional match can be made with any of the lavas preserved on Rum. The ultrabasic plugs of Rum contain magnesium-rich olivine and calcic plagioclase compositionally similar to the minerals in the Rum layered intrusions (Wadsworth, 1994).

The plugs range from a few tens of metres to over one kilometre in diameter. Their plan view may be nearly circular, but is more commonly oval or elongate. Where elongate, the long axes of the plugs are generally aligned in the direction of the regional dyke swarms as, for example, at ‘S Airde Beínn and other localities in northern Mull. However, on Rum, elongate plugs appear to radiate from the central complex. On Islay, a north-north-west-elongated olivine-dolerite plug, or boss, at Cnoc Rhaonstil is considered to be the distended head of a dyke in the Islay Dyke Swarm (Hole and Morrison, 1992). Several of the thicker dykes in the Hebridean Igneous Province are of limited lateral extent and may actually be elongate plugs, possibly related to volcanic necks, as in Antrim. Examples include a dolerite mass that intrudes granite on the north-east shoulder of Beinn na Caillich, Skye, and the olivine-dolerite of Gualainn na Sgurra that is intruded into lavas near the east end of the Sgurr of Eigg.

The majority of the plugs are basaltic in composition, but a few of trachyte composition have been recorded. On Mull, a large trachyte plug intrudes lavas and is, itself, intruded by north-west-trending dykes at Druim Buidhe, south-east of Tobermory. On Ardnamurchan, a plug of biotite trachyte intrudes psammites of the Moine Supergroup east of Ben Hiant. The dolerite plug at Cnoc Rhaonstil, Islay, is unusual in exhibiting considerable internal variation in composition and texture, with marked alkali enrichment (Hole and Morrison, 1992; Preston et al., 1998b; Chapter 10).

Basalt or dolerite plugs intruded into the lava fields may easily escape detection unless they form distinct topographical features. This may account for their apparent absence from the Skye Lava Field. However, they may not occur there as they have not been identified among the adjacent and relatively well exposed Mesozoic sedimentary rocks. Several plugs intrude the lavas of northern Mull (e.g. Kerr, 1997). They include the olivine-dolerite at ’S Airde Beínn which has produced a pronounced thermal aureole in the surrounding lavas and was almost certainly a feeder for lava flows (Cann, 1965), as were a number of the other plugs on Mull. Several plugs of fairly Mg-rich tholeiitic dolerite have, however, no compositional equivalents among the lavas and probably acted as feeders for flows subsequently removed by erosion. The lavas of Eigg and Muck are cut by rare dolerite plugs; on Muck the only plug is an alkali olivine­dolerite.

Sedimentary and metasedimentary rocks intruded by plugs are commonly thermally metamorphosed. On Rum, high-grade thermal alteration of the Torridonian feldspathic sandstones surrounding the gabbroic plugs involved partial melting and the formation of tridymite around larger quartz grains; limited fusion of sandstone, to form buchite, is also present. The mineralogy of the altered sandstone adjoining a gabbroic plug in Kinloch Glen indicates that the thermal alteration occurred when the sedimentary rocks were buried under as little as 500 m cover of strata (Holness, 1999). By contrast, alteration of sandstone around the peridotite plugs of Rum is generally restricted to induration and discoloration of the sedimentary rocks with evidence of only limited melting; since the peridotites were emplaced carrying large amounts of suspended olivine crystals, their heat capacity and ability to affect their surroundings was apparently much reduced. Several olivine-dolerite plugs intrude Moine metasedimentary rocks east of the Ardnamurchan Central Complex where, in Glenmore, a plug has caused a limited amount of fusion of the feldspathic country rocks to form buchite. Alteration of the Dalradian metamudstone around the Cnoc Rhaonstil plug on Islay is slight; however, the Dalradian metasiltstone surrounding a plug of coarse olivine-dolerite at Sithean Sluaigh, south of Strachur on Loch Fyne, is intensely altered and its foliation has been deformed to parallel the margins of the plug. Here, partial melting has resulted in granophyric veins and refractory residual material containing mullite, magnetite, corundum and spinel (Smith, 1969).


References

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