Sills and sill-complexes, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
- 1 Introduction
- 2 Little Minch Sill-complex
- 3 Loch Scridain Sill-complex, Mull
- 4 Holy Island and Dippin sills, Arran
- 5 Microgranitic and rhyolitic sills, south Arran
- 6 Composite sills of Arran and south Bute
- 7 Tighvein Intrusion-complex, Arran
- 8 Raasay Sill
- 9 Gars-bheinn Ultrabasic Sill, Skye
- 10 Other sills
- 11 References
Sills and sill-complexes are intimately associated with the lava fields and central complexes of the Hebridean Igneous Province. In general, the sills occur within the Mesozoic sedimentary sequences below the lavas, although on Mull the Loch Scridain Sill-complex also intrudes the overlying Paleocene lava field. Thus, the depth of emplacement of the upper parts of the sill-complexes is relatively shallow, most likely less than 1 km. Individual sills can vary from less than 1 m thick up to several tens of metres, and are exceptionally over 100 m. Bifurcations, transgressions and terminations are common. Locally, the country rocks may be disturbed and chilled margins are generally well developed, but thermal alteration of country rocks is typically slight. It is difficult to estimate the total thickness of each of the sill-complexes due to a lack of continuous vertical sections and the fact that many of them continue below sea level. However, it is likely that they are at least a few hundreds of metres thick.
Each sill-complex has its own distinctive compositional signature, unrelated to the typically older lava sequences. Alkali olivine basalt and tholeiitic basalt magmas are represented, together with their fractionation products (trachyte and rhyolite, respectively). Magma mixing between basaltic and silicic liquids can be recognised in some sills and may be a significant process in the evolution of some sill-complexes. Intra-sill variation in mineral proportions may be due to a variety of processes, including crystal settling or flotation and flow differentiation. Crustal contamination of the sill magmas may be deduced from whole-rock isotopic characteristics. Xenoliths are rare, except within the Loch Scridain Sill-complex.
The main sill-complexes are:
- the Little Minch Sill-complex, of alkali olivine basalt affinity on north Skye, Raasay and the Shiant Isles
- the Loch Scridain Sill-complex, of tholeiitic basalt affinity on south-west Mull
In south Arran there are several unrelated groups of sills of very different compositions. These include:
- the Holy Island and Dippin sills, of alkaline affinity
- microgranite/porphyritic rhyolite sills in southern Arran
Certain groups of sills show clear spatial and temporal relationships with the central complexes. These include composite sills (basaltic andesite–rhyolite) associated with the Mull Central Complex and the Eastern Red Hills Centre of Skye. Other groups of composite sills show no obvious link to central complexes, for example those in south Arran (Rogers and Gibson, 1977) and south Bute. The thick microgranite sill in southern Raasay is most likely related to the nearby granitic intrusions of the Western Red Hills Centre on Skye.
Ultrabasic sills are much less common and are generally associated with central complexes. Typically they are modally layered, with alternating olivine- and plagioclase-rich layers, as exemplified by the Gars-bheinn Sill on the southern margin of the Cuillin Centre of Skye (Weedon, 1960). Cryptic layering may also occur.
Little Minch Sill-complex
The Little Minch Sill-complex crops out on the Trotternish peninsula of north Skye, and it forms almost all of the Shiant Isles, with more restricted outcrops on the Waternish and Duirinish peninsulas (north Skye) and the west coast of Raasay (P914135). Significant submarine outcrops occur within the Little Minch, between Skye and Harris. The country rocks are typically Jurassic sandstones and mudstones. Studies by Drever and Johnston (1965) and Anderson and Dunham (1966) were concerned predominantly with the field relationships and petrography of the sills. These investigations identified their alkaline nature, ranging in composition from olivine-rich picrite and picrodolerite, through to more-evolved analcime-bearing olivine-dolerite (‘crinanite’). Simkin (1967) developed a model involving flow differentiation and gravity settling or flotation processes to explain the mineralogical variation and layering within certain of the sills. More recently, the sills on the Shiant Isles and Skye have been investigated in order to understand their mode of emplacement and petrogenesis (Gibson, 1990; Gibson and Jones, 1991; Foland et al., 2000; Henderson et al., 2000 and references therein).
The Little Minch Sill-complex postdates the Skye Lava Group and has a shallow dip towards the west. On the Trotternish peninsula, where the columnar-jointed sills are responsible for much of the impressive coastal scenery (P580472), the sill-complex is transgressive, invading progressively younger Jurassic strata towards the north. The sill-complex has an aggregate thickness of at least 250 m, with individual sills from 10 to over 100 m in thickness. The sills can either be simple (one lithology), multiple, or composite involving some combination of picrite, picrodolerite and alkali olivine-dolerite. Where sills are composite, the lower unit tends to be the least evolved. The internal contacts between different units can show evidence of their relative age, based on the development of chilled margins. Isolated rafts of country rock may also occur at internal contacts in the multiple sills. This material is thermally altered, ranging from mild baking which involved recrystallisation and reduction in porosity, through to the formation of very fine-grained or glassy buchites where melting has occurred; the buchites provided material for Mesolithic stone tools at Staffin.
Layering is common in sills of basaltic compositions; within the Little Minch Sill-complex good examples are seen at Duntulm Castle (P580471), Eilean Flodigarry, Meall Tuath and the Ascrib Islands, all on or just offshore of the north of the Trotternish peninsula. Layering tends to occur mostly within picritic units and is most commonly found adjacent to contacts with the country rocks. It is typically on a centimetre scale, involving variations in texture and in the proportions of olivine, clinopyroxene and plagioclase. However, such layering can also occur within a sill adjacent to rafts of sedimentary rock, implying that heat loss from the magma was important in the development of layering. Furthermore, there is no field or petrographical evidence to suggest that crystal settling has taken place to any significant extent. Thus, an in-situ crystallisation model is preferred to account for the layering (Gibson and Jones, 1991).
Gibson and Jones (1991) deduced that the parental magma was of alkali olivine basalt composition, with about 10% MgO. Fractionation occurred at relatively shallow depths, and batches of the three main magma compositions (picrite, picrodolerite and alkali olivine-dolerite) were then tapped from the processing chambers and emplaced at shallow levels as sills. The most primitive melt, which formed the picrites, carried chrome-spinel, olivine and plagioclase prior to emplacement. During emplacement, flow differentiation was a significant process close to the margins of the sills, whereas relatively late-stage filter pressing yielded various pegmatite sheets. The chilled margins to the sills are typically contaminated by country-rock mudstone and sandstone.
Loch Scridain Sill-complex, Mull
The Loch Scridain Sill-complex intrudes the Moine basement, the Mesozoic sedimentary sequence and the overlying Paleocene lavas in south-west Mull, particularly on the Ross of Mull and on the north side of Loch Scridain. The sills are typically betwen 0.5 and 6 m in thickness but exceptionally exceed 10 m, and commonly have well developed chilled margins. They are of tholeiitic affinity, ranging in composition from tholeiitic basalt, through andesite and dacite, to rhyolite (including glassy variants). Basaltic compositions are the most common, with about 20 per cent of the intrusions being of intermediate composition, and a relatively small number of rhyolitic composition. Many of the sills are xenolithic, with cognate and accidental (upper crustal) material represented; two of the best examples are to be found at Killunaig and Kilfinichen Bay, on the south and north sides of Loch Scridain, respectively (P580473). Composite sills occur, commonly involving crystalline and glassy rock types of significantly different compositions as in, for example, the classic xenolithic sill at Rudh’ a’ Chromain on the south side of the Ross of Mull. The sills may be related to the initial stage of the development of the Mull Central Complex (Centre 1 or Glen More Centre, see p. 126; Dagley et al., 1987).
The sills were described in detail by Bailey et al. (1924). These early studies were concerned mainly with the complex high-temperature mineral assemblages that developed within crustal xenoliths prior to and/or during sill emplacement. More recently, Brearley (1986) described the melting reactions of the Moine basement materials, and Kille et al. (1986) inferred that the sill magmas were actively convecting during emplacement, maintaining high temperatures at the contacts, which resulted in localised thermal erosion of the pelitic Moine wall-rocks.
The Loch Scridain sills may be divided into three distinct geochemical groups (Preston et al., 1998a; Chapter 10). The basic sills (Group 1) are markedly xenolithic, containing both cognate and crustal types (P580473). The cognate xenoliths are of ultrabasic and basic composition, and are most likely of cumulate origin (Preston and Bell, 1997). There are two broad groupings of the crustal xenoliths (Preston et al., 1999):
- siliceous xenoliths derived from psammites of the Moine Supergroup or, less commonly, as in the Rudh’ a’ Chromain Sill, sandstone and conglomerate xenoliths from the local Mesozoic country rocks
- aluminous xenoliths, of which there are three types, each dominated by glass produced by melting of the xenolith, hence the term buchite
The aluminous xenoliths include:
- mullite buchites — a mass of mullite needles (3Al203. 2Si02) set in clear glass and pale lilac in hand-specimen
- cordierite buchites — small crystals of cordierite and mullite needles set in clear glass and virtually black in hand specimen
- plagioclase-rimmed mullite buchites — a core of mullite-rich glass surrounded by a thick rim of white-weathering, coarse-grained plagioclase. (An87–60)
Clear blue corundum (sapphire) also occurs. Pockets of quenched, isotopically contaminated basic glass with skeletal plagioclase and clinopyroxene occur trapped between the plagioclase crystals. The highly aluminous composition of the buchite glasses is consistent with their derivation from a clay-rich sediment or its metamorphic equivalent, most likely the pelites of the Moine Supergroup (Dempster et al., 1999).
Holy Island and Dippin sills, Arran
Holy Island, on the east side of Arran, is dominated by a sheet of sanidine-microphyric trachyte about 250 m thick, emplaced into Permian sandstones. This peralkaline intrusion is riebeckite-bearing, similar to the microgranite intrusion that forms Ailsa Craig (p. 138). Other related intrusions are sills or transgressive sheets of analcime-bearing olivine-dolerite which crop out around Lamlash Bay. These inclined sheets may have a focal point at depth in the vicinity of Lamlash Bay (Tomkeieff, 1961), akin to the focal point of cone-sheets associated with the central complexes (P914150).
The Dippin Sill is 40 m thick at Dippin Head and forms a transgressive sheet emplaced into Triassic mudstones and siltstones (Gibb and Henderson, 1978). It is a composite sill, with a thick central unit of analcime-bearing olivine-dolerite (‘crinanite’) bounded top and bottom by analcimedolerite (‘teschenite’). Pegmatitic facies occur sporadically throughout the sill. The parental magma was of alkali olivine basalt type that fractionated within a magma chamber at a deeper level than the sill. The first phase of intrusion involved relatively evolved magmas from the upper part of the chamber, followed by intrusion into the central part of the sill of less evolved magmas carrying suspended olivine, with or without plagioclase crystals from the lower part of the chamber. As with the Little Minch sills, flow differentiation of the magmas occurred during emplacement, together with assimilation of material from the conduit walls. The numerous pegmatitic segregations are the product of late-stage filter pressing within the crystallising body. A number of subsolidus processes are also recognised, including decomposition of magmatic nepheline (± alkali feldspar) to produce analcime. Circulation of hydrothermal (meteoric) fluids caused the formation of secondary serpentine, chlorite, iron oxides, and natrolite and other zeolites (Dickin et al., 1984b).
Microgranitic and rhyolitic sills, south Arran
These thick silicic sills form prominent crags at Brown Head south of Drumadoon Bay and inland for some distance to the east (Tyrrell, 1928; P914150). They are most likely linked to dykes of similar composition, for example those that crop out south of Corrygills Point on the east coast. In some respects these sills are gradational with the composite sills described below.
Most of the sills are conspicuously porphyritic, with phenocrysts of quartz and alkali feldspar, up to 1 cm and 1.5 cm across, respectively, set in a fine-grained groundmass. Sparse phenocrysts of sodic plagioclase are also present. Commonly, the phenocrysts are anhedral, suggestive of some form of magmatic corrosion by the groundmass ‘liquid’, akin to the types of reaction envisaged for the composite intrusions (see below).
Composite sills of Arran and south Bute
These composite sills are up to 80 m thick, and form some of the higher ground in south-east Arran, and the prominent cliffs at Bennan Head and at Drumadoon on the west coast (King, 1982) (P580474). Typically, they comprise a volumetrically dominant, central silicic unit flanked by relatively thin basaltic or doleritic units, but some consist of a central doleritic unit, flanked by marginal felsites (Rogers and Gibson, 1977; Macgregor, 1983). The Arran composite sills and dykes generally postdate the emplacement of the Holy Island and Dippin alkaline sills.
The internal contacts between the basic and the silicic units are typically gradational over a few tens of centimetres and ‘xenoliths’ or pillow-like masses of basic material may occur within the silicic unit. Within the xenoliths are xenocrysts of feldspar and/or quartz of the type seen in the silicic unit. In places, marginal felsites vein the central dolerite. These intimate relationships are suggestive of mixing processes, initially prior to intrusion, explaining the xenocrysts in the basic units, and then during intrusion, explaining the gradational contacts and the xenoliths of basic material in the silicic material (Rogers and Gibson, 1977).
Tighvein Intrusion-complex, Arran
Augite diorite in gradational contact with underlying xenolithic quartz-dolerite is intruded conformably into Triassic strata at Tighvein, in south Arran (P914150). Intrusions of microgranite, some granophyric, cut the augite diorite, which is also extensively veined by microgranite. The quartz-dolerite and augite diorite form a sill, but the microgranitic intrusions are steep-sided and may be parts of a ring-dyke (Herriot, 1975). Other sheets and dykes of felsite and porphyritic rhyolite cut the whole ‘complex’, together with dykes of pitchstone and tholeiitic basalt. The xenoliths in the quartz-dolerite are a fine-grained basic rock in various stages of digestion.
The Raasay Sill is of granitic composition, crops out over an area of about 12 km2 in southern Raasay, and is emplaced transgressively into Triassic and Lower Jurassic sedimentary rocks. Despite the sill being at least 30 m thick, its thermal effects on the adjacent country rocks are minor. Texturally, it varies from coarse to fine grained, with granophyric and porphyritic facies, and it contains alkali amphibole. The sill also occurs on the nearby small peninsula of An Aird on Skye, suggesting a link with the granites of the Western Red Hills Centre. One possible correlation is with the Maol nam Gainmhich Granite which, like the Raasay Sill, is distinctly alkaline in composition and contains riebeckite.
Gars-bheinn Ultrabasic Sill, Skye
On the southern slopes of Gars-bheinn, adjacent to the Cuillin Centre of Skye, a layered ultrabasic sill, 80 m thick, has been emplaced into hornfelsed lavas. It is composed of feldspathic peridotite, the uppermost 15 m of which comprises alternating plagioclase-rich and olivine-rich layers. Pegmatitic textures are common within the plagioclase-rich layers. The base of the sill appears to be connected with a dyke of similar composition, and material from the chilled margin of this dyke suggests a parental magma of picritic composition. The source of these intrusions has been identified as the marginal gabbro of the adjacent Cuillin Centre. Weedon (1960) interpreted the layering as a consequence of crystal–liquid fractionation, with settling out of the olivine crystals. By contrast, Bevan and Hutchison (1984) concluded that the plagioclase-rich layers arose from the injection of ‘sills’ into already-crystallised olivine-rich material.
Other dolerite sills of probable Paleocene age occur at a number of localities on the periphery of the Hebridean Igneous Province. Most notable are a massive, easterly dipping sill, which intrudes gneisses at Lochmaddy, North Uist (Mackinnon, 1974), and the Prestwick–Mauchline Sill-complex north of Ayr, which is petrographically similar to analcime-bearing olivine-dolerite sills on Arran, and has yielded a K-Ar mineral age of 58 Ma (De Souza, 1979).