Skye Central Complex, Hebridean Igneous Province
| Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey. |
Skye Central Complex
The Skye Central Complex comprises the Cuillin (oldest), Srath na Creitheach, Western Red Hills and Eastern Red Hills (youngest) centres (Bell and Harris, 1986; P914139/P914140).
Cuillin Centre
The Cuillin Centre consists of gabbroic and peridotitic ring-intrusions, some of which have well-developed mineral layering and lamination (P580475). These structures attracted the attention of early geologists, who attributed layering in the gabbros of Druim Hain to intrusion of a heterogeneous magma (Harker, 1904). The intrusive contacts between units are typically well exposed. These coarse-grained rocks are cut by numerous centrally inclined, fine-grained sheets of tholeiitic basalt and dolerite, and are also invaded by volcaniclastic breccia pipes. All of the units of the centre are cut by north-west-trending basaltic dykes. The margin of the centre is steeply inclined inwards, as are many of the internal contacts between various basic and ultrabasic intrusions, resulting in an overall nested disposition. In general terms, the intrusions are older around the margin of the centre, younging progressively inwards. The north-east quadrant of the centre is missing as a consequence of the emplacement of the younger granites of the Srath na Creitheach and Western Red Hills granite centres.
The country rocks are predominantly lavas, which have been thermally metamorphosed up to pyroxene hornfels facies (Almond, 1964). Along the south-east margin, the contact is with Torridonian sandstones and siltstones, which have been baked and, locally, partially melted. Small outcrops of Jurassic limestone at Camasunary, along the south-east margin of the centre, are altered to high-grade calc-silicate hornfels.
The constituent units of the Cuillin Centre are shown on P914139/P914140 and are described below in order of intrusion.
The Outer Gabbros (P914139/P914140, A1 to A3) consist of a rather heterogeneous group of intrusions comprising layered and unlayered gabbro and bytownite gabbro ('eucrite'), which crop out along the margin of the centre from Sgurr nan Gillian in the north to Loch Scavaig in the south. At their inner contact against younger intrusions, the Outer Gabbros are altered and thermally metamorphosed, and take on a dull, matt-black appearance (like the gabbro at Lochan an Aodainn, Ardnamurchan; see below). One particularly distinctive intrusion is the fine-grained gabbro (Gars-bheinn type; A3), which crops out in the southern part of the centre on and around the summit of Gars-bheinn. This intrusion has a distinctive green coloration and is particularly altered along its northern contact with younger intrusions.
The Outer Bytownite Troctolites have, at their outer margin, a fluxioned, xenolithic tholeiitic dolerite (P914139/P914140, B1). This contains xenoliths of bytownite gabbro and grades inwards into a massive coarse-grained, plagioclase-rich bytownite troctolite (B2) of striking pale coloration that is commonly referred to as the 'White Allivalite' (Hutchison, 1968). The bytownite troctolite immediately adjacent to the marginal tholeiitic dolerite has a distinctive, steeply inclined, inward dipping 'wispy banding'.
The layered bytownite troctolites (P914139/P914140, B3) accumulated against the massive bytownite troctolites (B2). The nature of the layering is quite variable, comprising both plagioclase-pyroxene and plagioclase-olivine cumulates. The inward dipping attitude of the layering suggests that the Outer Bytownite Troctolites were formed within a funnel-shaped chamber (Hutchison and Beavan, 1977).
Six units are identified in the Layered Peridotites (Claydon and Bell, 1992; P914139/P914140, P1 to 6). These comprise various layered olivine-rich cumulates, ranging from dunite through to feldspathic peridotite (Weedon, 1965). The structurally lowest unit (P1) is dominated by dunite and contains abundant layers rich in chrome-spinel (Bell and Claydon, 1992). Other units contain plagioclase, both cumulus and intercumulus, and have excellent modal layering. Xenoliths of various ultrabasic rock-types are common and typically accentuate the layering. Unit 4 (P4) is a heterogeneous breccia comprising plagioclase-dominated blocks in a matrix dominated by olivine, and olivine-dominated blocks in a matrix dominated by plagioclase. Block types vary from anorthosite through to dunite. Such breccias, which are similar to breccias in the Central Intrusion of the Rum Central Complex (see p. 117), are most likely to be of intrusive origin, and formed by the upward forceful intrusion of aluminous, ultrabasic magma into ultrabasic rocks, which were consequently disrupted and brecciated.
A number of unusual textural varieties of peridotite, as well as structures involving various ultrabasic rocks, are preserved within the Layered Peridotites; similar features are described from the Rum layered intrusions (see p. 117). For example, within Unit P5 on the south side of An Garbh-choire, structures that are approximately hemispherical and range in size from 15 to 200 cm, comprise dendritic intergrowths of poikilitic intercumulus plagioclase enclosing orientated cumulus olivine. Such structures most likely formed within a crystal mush from a hydrous, aluminous, ultrabasic magma. Also within Unit P5, along the interface of peridotite and troctolite layers, 'fingers' of peridotite penetrate upwards into troctolite (P580476). These structures are interpreted as evidence that hot ultrabasic magma invaded pre-existing solid troctolite and eroded it by melting. From these and other observations it is now generally accepted that thick sequences of ultrabasic layered rocks, such as those which form the Cuillin Centre and the Rum layered intrusions, were formed through the combined operation of a number of processes including crystal settling, in-situ crystallisation, and the intrusion of sheets of magnesium-rich magma into pre-existing solid material.
The Outer Bytownite Gabbros (P914139/P914140, C) crop out around a significant sector of the centre. These gabbros are subdivided into three units, based upon obvious field characterisitics, such as layering (P580475), modal mineralogy and the presence of xenoliths. Where bytownite gabbros are in contact with the Layered Peridotites, they contain abundant xenoliths of peridotite (Weedon, 1961). Elsewhere, for example on the south side of Sgurr na Stri, the Outer Bytownite Gabbros have an unlayered marginal facies in steep, inward-dipping contact with country-rock lavas.
Interior to the crescent-shaped outcrop of the Outer Bytownite Gabbro is the Inner Cross-cutting Bytownite Gabbro of Druim nan Ramh (P914139/P914140, D). This unlayered, steep-sided ring-intrusion is about 200 m wide and might represent some form of marginal unit to the layered rocks of the Inner Bytownite Troctolites and the Inner Gabbros, although this interpretation has not been proved.
The Inner Bytownite Troctolites (P914139/P914140, E) form a thick sequence of layered rocks in an outcrop concentric with and inside that of Unit D. They consist of plagioclase-pyroxene and plagioclase-olivine cumulates, commonly with a fine mineral lamination. The layering dips typically at about 25° towards a point below Meall Dearg at the southern end of Glen Sligachan.
The Inner Gabbros (P914139/P914140, F) are interpreted as the youngest sequence of layered rocks preserved within the centre, and crop out on the ridge of Druim Hain, north-east of Loch Coruisk. These gabbros are separated from the Inner Bytownite Troctolites by a 50 m-wide vertical zone of intrusive breccias, consisting of shattered tuffaceous rocks invaded by numerous basalt and dolerite sheets and containing xenoliths of various gabbros and peridotites. The Inner Gabbros have an arcuate distribution, with modal layering inclined towards a point below Meall Dearg. Layering is poorly developed immediately adjacent to the bounding intrusive breccias, whereas close to the top of the preserved sequence the layering is well preserved and dips at up to 70° . Mineral lamination is well developed in places, with magnetite enrichment at the base of individual layered units, passing upwards into plagioclase-enriched cumulates. Xenoliths of basalt and dolerite are common throughout, their flattened shapes suggesting plastic deformation of hot material soon after incorporation into the cumulate pile.
Late-stage intrusions of tholeiitic basalt and dolerite
The various coarse-grained basic and ultrabasic rocks of the Cuillin Centre are cut by myriad basic minor intrusions, which may be divided into two types:
i Multiple intrusions of tholeiitic basalt and dolerite, some xenolithic, which crop out in the western part of the centre. Inner and outer sets are recognised, with all of the components dipping towards a focal point in the middle of the Cuillin Centre (Hutchison, 1966). The inclination of the sheets generally increases inwards. Members of the inner set are further complicated by a volcaniclastic facies that includes welded breccias and xenolithic intrusions. Brecciation may have been caused by the release of volatiles during emplacement of individual sheets.
ii Cone-sheets, of tholeiitic basalt and dolerite, are most concentrated in the north-west and south-east quadrants of the centre, coinciding with the north-west-trending regional dyke swarm (P914136a). A number of features suggest that the final stages in the growth of the lava field overlapped in time with the formation of the central basaltic volcano that is now represented by the Cuillin Centre. These include the distinctive and near-identical composition of the cone-sheets, the dykes from the axial portion of the regional dyke swarm, and flows of the Talisker Lava Formation (Preshal More type) at the top of the lava succession in west-central Skye (Chapter 6).
Volcaniclastic pipes
More than forty pipe-like structures containing volcaniclastic breccia pierce various rocks of the Cuillin Centre; only the largest example is shown on P914139/P914140. They are typically less than 100 m across, circular or oval in plan, and are narrower at lower structural levels, suggesting an overall funnel shape. They are composed of blocks of coarse-grained basic or ultrabasic lithologies of local derivation set in a matrix of basalt, dolerite or tuffaceous material. Crush zones are common. The pipes formed when volatiles, expelled from upward-penetrating minor intrusions, caused initial brecciation of country-rock lithologies, and were also responsible for further fragmentation, rounding and sorting of the breccia clasts. Final emplacement of the breccias occurred when the fluidised mass collapsed as a consequence of insufficient pressure gradient to drive the system.
Coire Uaigneich Granite
This intrusion crops out around the south-east margin of the Cuillin Centre (P914139/P914140, CU), between Coire Uaigneich on the east side of Blà Bheinn, where it is dyke-like with fine-grained margins, and the Sgurr na Stri peninsula, where it appears to be a sill, emplaced between Torridonian sedimentary rocks and Paleocene lavas. It is a narrow, steeply inclined intrusion, and space for it appears to have been made largely by horizontal crustal dilation. The granite is heterogeneous, both fine and coarse grained, with partially digested xenoliths of sandstone (most likely Torridonian). The most conspicuous mineral is hypersthene, set in a pale granitic groundmass in which quartz paramorphs after tridymite have been recognised. The field characteristics, mineralogy and geochemistry indicate that it is the product, at least in part, of partial melting of Torridonian sedimentary rocks (Chapter 10). Melting most likely occurred at a shallow depth, the heat being supplied by the Cuillin Centre magmas. The granite is cut by cone-sheets associated with the Cuillin Centre, whereas the granites of the Srath na Creitheach and Western Red Hills centres, farther north and east, are not.
Red Hills granite centres
Granites form the Red Hills of Skye, which extend from Glen Sligachan to Strath, near Broadford. There are three distinct centres: from oldest to youngest they are the Srath na Creitheach, Western Red Hills and Eastern Red Hills centres (P914139/P914140). Each centre contains a number of discrete intrusions, varying from steep-sided bosses through to segments of ring-intrusions. Field relationships suggest that certain of the intrusions are true ring-dykes, where space was made available by central subsidence of a block of older country rocks (which may in part have comprised older granites, e.g. P914136b), but this style of intrusion is by no means ubiquitous. More commonly, the outcrop patterns of the granites suggest broad, curved, dyke-like masses. Emplacement of the granites does not appear to have resulted in appreciable disturbance of the earlier rocks. Stoping and brecciation of the country rocks is generally absent, although there may be peripheral folding of country rocks on the northern slopes of Glamaig and in the vicinity of the Beinn an Dubhaich Granite (pp. 108, 149).
Evidence for volcanic activity, possibly from calderas that developed as a consequence of central subsidence, is at best circumstantial. Silicic pyroclastic rocks are preserved locally, for example within the sequences of volcaniclastic deposits in the Kilchrist area of the Strath district (see below). In addition, within the offshore sedimentary basins of the North Sea and west of the Shetland Isles, thin beds of silicic pyroclastic rocks are intercalated with marine sedimentary rocks of Paleocene age, and are considered to have been derived from the volcanic centres of either the Hebridean Igneous Province or even East Greenland (e.g. Knox and Morton, 1988).
Srath na Creitheach Centre
This centre is located at the southern end of Glen Sligachan and comprises three granitic intrusions: Meall Dearg (S1), Ruadh Stac (S2) and Blaven (S3) (Jassim and Gass, 1970; P914139/P914140). Its western and eastern sides intrude gabbros of the Cuillin Centre. Apophyses of the Meall Dearg Granite, in the form of metre-wide dykes of spherulitic rhyolite, invade the Inner Gabbros on Druim Hain and attest to the younger age of the granite. Along the northern margin of the centre, the Srath na Creitheach granites are cut out by younger granites of the Western Red Hills Centre. To the south, the granites are in contact with the older Srath na Creitheach volcaniclastic rocks.
The white-weathering Meall Dearg Granite (P914139/P914140, S1) crops out on the upper slopes of Meall Dearg and Ruadh Stac and overlies the younger dome-shaped pale brown-weathering Ruadh Stac Granite (S2). Screens of gabbro are present along the contact. On Ruadh Stac, the Meall Dearg Granite comprises interleaved sheets of two distinct types: a fine-grained, hornblende-bearing granite and a younger, coarse-grained pyroxene (hedenbergite)-bearing granite with granophyric texture. The Ruadh Stac Granite contains an iron-rich, sodic amphibole that varies between arfvedsonite and ferrorichterite. The Blaven Granite (S3) crops out on the lower western slopes of Blà Bheinn where the light-coloured granite underlies and intrudes dark gabbros in the Outer Bytownite Gabbros of the Cuillin Centre. Its age relative to the two other granites is not known.
Western Red Hills Centre
The granites of this centre have an outcrop area of about 35 km2 in rounded, scree-mantled hills that contrast with the adjacent rugged Cuillin (Plate 26). Detailed mapping has identified ten granites, together with a composite ring-dyke (Marsco Hybrids), various masses of explosion breccia and a gabbro that was emplaced during the main granite intrusion events (Thompson, 1969). The margin of the centre is generally steeply inclined, against a variety of country rocks. Crush zones are recognised, in particular against hydrothermally altered Paleocene lavas and Torridonian strata.
The earliest event attributed to the Western Red Hills Centre is the formation of various masses of explosion breccia (P914139/P914140, Z), now preserved on Belig and Meall a' Mhaoil in the south-east and north-east parts of the centre, respectively. These comprise rounded to subangular blocks of pre-granite lithologies, such as Torridonian and Mesozoic sedimentary rocks, basalt and bytownite gabbro, together with blocks of silicic igneous rocks, including granite, felsite and quartz-porphyry. The breccias were formed by the release of volatiles during subvolcanic emplacement of the granitic magmas. Juvenile material is absent and, consequently, the breccias represent the action of gas streaming through, and fluidising of, the country rocks (J D Bell, 1966).
Three distinct structural groupings are identified in the Western Red Hills granites.
i A set of intrusions broadly elongated in a north-east—south-west direction, for example the Glamaig (P914139/P914140, W1), Beinn Dearg Mhor (W5) and Loch Ainort (W6) granites.
ii In the north, a set of intrusions elongated in an east—west direction, the Northern Porphyritic Felsite (NPF), and the Maol na Gainmhich (W7), Meall Buidhe (W8) and Eas Mor (W9) granites.
iii In the south, a set of intrusions also elongated east—west, the Southern Porphyritic (W3), Glen Sligachan (W2) and Marsco (W4) granites.
Members of (i) were emplaced sequentially inwards, whereas members of (ii) and (iii) were emplaced sequentially outwards. All of the intrusions are approximately co-focal around a point east of Loch Ainort (P914139/P914140).
These field relationships may be explained in terms of a cauldron subsidence model, as follows. After the formation of the explosion breccias and the emplacement of the Marsco Summit Gabbro (MS), which forms a resistant cap on the summit of Marsco, the first major silicic intrusion to be emplaced was the Glamaig Granite (W1). This intrusion has a steep-sided outer western margin against lavas and a steep inner margin against the younger Beinn Dearg Mhor Granite (W5). The Beinn Dearg Mhor Granite was, in turn, intruded along its eastern margin by the Loch Ainort Granite (W6), resulting in three north-east-trending, nested plutons. Emplacement of the east—west sets of intrusions involved subsidence of the block of crust formed by the three earlier granites. Jamming of the block, possibly in the vicinity of Glen Sligachan, caused it to rotate about an east—west-trending axis. According to this model, each successive tilt of the block would lead to similar, though not necessarily identical, intrusions being emplaced to the north and south (Thompson, 1969).
The Glamaig Granite (P914139/P914140, W1) is characterised by abundant inclusions, apparently cognate in origin. Many small, rounded to subangular, basic inclusions (5 to 50 mm) typically constitute up to 5 per cent of the rock. The larger inclusions have relatively sharp edges, whereas smaller inclusions have the appearance of clots of mafic minerals. These inclusions may have been introduced into the granite during an early stage, involving the mixing and homogenisation of volumetrically dominant granitic magma and a basaltic magma (Chapter 10). Rounded inclusions, up to 50 cm across, of a microgranite are also present.
Further evidence of bimodal magma mixing is provided by the Marsco Hybrids (M). One of the classic field locations in central Skye is the gully on the north-west side of Marsco at the southern end of Glen Sligachan that is now referred to as Harker's Gully, in deference to Alfred Harker who first described the locality (P914141). Within the accessible lower reaches of the gully are rocks that provide unambiguous evidence for the mixing of two magmas, one a differentiated iron-enriched tholeiitic basalt (crystallising as ferrodiorite) and the other of rhyolitic composition (Chapter 10). In the upper, less accessible part of Harker's Gully, the centre of the Marsco Hybrids ring-dyke is occupied by ferrodiorite flanked by a homogeneous hybrid, 'marscoite', and a porphyritic felsite successively on each side; however, in the lower exposures the ring-dyke is apparently assymetrical since the south-western side has been removed by intrusion of the later Marsco Granite. The ferrodiorite encloses a few large xenoliths of partially melted Lewisian gneiss.
The course of the ring-dyke composed of Marsco Hybrids is visible on the eastern slopes of Marsco. It is not exposed on the floor of Glen Sligachan, but it crops out to the north, on the south-west flank of Glamaig and on the coast of Loch Ainort, north of Moll. On Glamaig, more complex magma mixing processes are recognised in the form of a considerably more heterogeneous hybrid, given the local name 'glamaigite' and consisting of dark, rounded cognate inclusions, generally less than 2 cm in diameter, set in a paler matrix, with xenocrysts of plagioclase in both components.
Eastern Red Hills Centre
This centre covers a larger area than either of the two other granite centres but, in contrast, has a significant proportion of country rocks preserved between the intrusions (P914139/P914140).
The oldest major intrusion of the Eastern Red Hills Centre is the Glas Bheinn Mhor Granite (P914139/P914140, E1), which cuts across the eastern margin of the Western Red Hills Centre and thus provides clear evidence of their relative ages. Within and to the east of the Glas Bheinn Mhor Granite is a granite comprising several discrete outcrops: Beinn na Cro (E2), Beinn an Dubhaich (E3), Creag Strollamus (E4) and Allt Fearna (also E4); these are collectively referred to as the Outer Granite. The most studied of these is the Beinn an Dubhaich Granite which, uniquely, was emplaced into Cambro-Ordovician (Durness Group) dolostones. Thermal metamorphism of the dolostones has produced a well-developed aureole, which can be subdivided into a number of zones based upon the appearance of distinctive index minerals (in terms of increasing temperature: talc, tremolite, diopside, olivine, periclase; Holness, 1992). Locally, magnetite-rich skarns have formed at the granite contacts. The altered dolostones form a commercially exploited marble (pp. 174—5).
The dolostones are folded into large, open, annular structures, the largest of which is the Broadford Anticline (p. 150). In addition, many of the dykes of the regional swarm intruded into the dolostones preserve evidence of deformation during emplacement. However, these deformation events occurred prior to the emplacement of the granite, which appears to have utilised, but not caused, the main fold structures. Similar metamorphic and deformation events are identified within the Cambro-Ordovician dolostones farther north, in the area west of Broadford, but here the older Broadford Gabbro may be responsible for the present-day mineralogy and structure of the dolostones.
The Broadford Gabbro (P914139/P914140, BR) has an outcrop area of about 3 km2. It has both faulted and intrusive margins. On Creag Strollamus, the gabbro is in sharp contact with bedded pyroclastic rocks, which in turn overlie a weathered surface of the Allt Fearna Granite. The granite is thought to predate the gabbro (Johnston, 1996). The shape of the intrusion is not readily deduced from its field relationships, although its north-west extension is clearly dyke-like, with a width of about 200 m. Unlike the gabbros of the Cuillin Centre, it is unlayered and devoid of olivine. In general, it has been subjected to hydrothermal alteration, with the resultant development of secondary amphibole, chlorite and epidote.
The geographical middle of the Eastern Red Hills Centre is occupied by the Beinn na Caillich (or Inner) Granite (P914139/P914140, E5). This boss-shaped intrusion has a near-perfect circular outcrop pattern and, locally, has well-developed marginal facies composed of fayalite- and ferrohedenbergite-bearing porphyritic felsite. The marginal rocks are well exposed in the gorge of the Allt Slapin, on the south-west side of the intrusion, where there is also evidence of severe, local deformation of the Mesozoic country rocks.
Also associated with, and generally to the east of, the centre is a suite of composite sills (P914139/P914140, CS) and dykes, with a basaltic andesite marginal facies and a central felsic facies. The sills and dykes crop out over a broad arc exterior to the granites and co-focal with the centre of the Beinn na Caillach Granite (P914139/P914140). The sills dip inwards at about 15° towards the Beinn na Caillach Granite, and are typically emplaced into Mesozoic strata. The upper and lower basaltic andesite portions are typically 0.5 to 2.5 m thick, whereas the central felsites are considerably thicker, up to 50 m. The dykes of the suite may have acted as feeder conduits to the sills. Many of the intrusions are symmetrical and the field evidence suggests that the basaltic andesite portions were emplaced first, since there are fragments of basaltic rock within the central felsites, and veins of felsite in the marginal basaltic andesites. However, the presence of gradational contacts between basaltic andesite and felsic rocks, as found for example at Rubh' an Eireannaich, north of Broadford, suggests that the two magmas were available (i.e. liquid) at the same time (Harker, 1904). Evidence for mixing of basaltic and rhyolitic magmas also comes from a detailed analysis of the petrography and geochemistry of the intrusions (Bell and Pankhurst, 1993; Chapter 10). The composite sheets of the Broadford area have many features in common with the composite sheets of Arran (p. 90).
Volcaniclastic rocks of the Central Complex
Two large areas of volcaniclastic rocks occur within the Skye Central Complex, in Srath na Creitheach, at the northern end of Loch na Creitheach, and near Kilchrist, in the district of Strath. Unlike the other volcaniclastic breccias described above, these two breccias do not have any obvious association with the main intrusive events.
Srath na Creitheach
The volcaniclastic breccias and associated pyroclastic deposits that crop out in Srath na Creitheach have an outcrop area of about 2 km2 (P914139/P914140, SZ) The deposits were formed after the emplacement of the Cuillin Centre, since they contain large slabs of bytownite troctolite, and prior to emplacement of the granites of the Srath na Creitheach Centre, since the northern margin is intruded by the Ruadh Stac Granite. The southern margin of the volcaniclastic breccias is an arcuate ring-fault. The dominant lithology is an unsorted breccia consisting of abundant subangular fragments, mainly of basalt, dolerite and gabbro, set in a tuffaceous matrix. Little, if any, juvenile magmatic material is recognised. Within these deposits are at least twelve large slabs of bytownite troctolite, ranging in size from 40 to 900 m across, which were derived from the Cuillin Centre. Layers of fine-grained, bedded tuff and tuffaceous sandstone help to define the overall structure and stratigraphy. The formation of all these deposits most likely involved the opening of a volcanic conduit to the surface. Fragmentation and incorporation of material derived from the vent walls produced the main breccia mass, whereas some form of reworking process, considered to be subaerial, produced the bedded tuffs and tuffaceous sandstones (Jassim and Gass, 1970).
Kilchrist
The volcaniclastic breccias and associated rocks preserved within the Kilchrist area, west of Loch Cill Chriosd, are extremely heterogeneous and comprise a wide variety of rock types (Bell, 1985; Bell and Harris, 1986; P914139/P914140, KZ).
The main mass of volcaniclastic breccia crops out over an area of about 2 km2 and is surrounded and invaded by mixed-magma intrusions, the Kilchrist Hybrids (P914139/P914140, K). These hybrid rocks, comprising five separate intrusions, contain dark irregular enclaves of basaltic material up to 3 cm across, commonly with diffuse edges, set in a paler granitic matrix. The basaltic rocks contain xenocrysts of quartz and alkali feldspar, which provide further evidence of magma mixing. Three of the hybrid intrusions have steeply inclined outer margins against Cambro-Ordovician (Durness Group) dolostones. These field relationships are consistent with some form of cauldron subsidence, with the volcaniclastic breccias subsiding to create space for the hybrid ring-dyke. Intruded into the volcaniclastic breccias are small stocks and sheets of brecciated porphyritic rhyolite.
The volcaniclastic breccias are composed of subangular to rounded blocks of various pre-Paleocene rocks (Torridonian sandstone and siltstone, Cambro-Ordovician dolostone and quartzite and Jurassic sandstone, limestone and mudstone) together with fragments of basalt, dolerite, gabbro, rhyolite, tuff (including ignimbrite), granite, pitchstone, and blocks of older volcaniclastic breccia. The breccias are unstratified, very poorly sorted and may be either block-supported or matrix-supported. Intercalated with the volcaniclastic breccias are numerous thin basaltic volcaniclastic sandstones with eroded tops, and rhyolitic tuffs less than 2.5 m thick. Three thin ignimbrites are also intercalated with the volcanic breccias and two large masses of ignimbrite are preserved (Bell, 1985; Bell and Harris, 1986). A layer of lateritised breccia, about 0.5 m thick, which crops out north of Loch Cill Chriosd in the Allt Coire Forsaidh, and a thin hyaloclastite layer north of Cnoc nam Fitheach are further evidence of surface processes.
It is evident that the very heterogeneous assemblage of volcanic and subvolcanic magmatic and volcaniclastic rocks preserved at Kilchrist is the product of a variety of surface and near-surface processes. The ignimbrites and other silicic tuffs appear to be in situ extrusive deposits intercalated within the dominant volcaniclastic breccias. The heterogeneous nature of the volcaniclastic breccias, with little or no stratification and very poor sorting, suggests they were formed within the upper part of a vent system, exposed from time-to-time to the atmosphere (giving rise to lateritised material) and invaded by water (resulting in hyaloclastite deposition). The stocks and sheets of brecciated rhyolite attest to the important role played by silicic magmas and may be related to the ignimbrites and other silicic tuffs.