The Cuillin Complex, Skye

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From: Bell, B.R. and Harris, J.W. An excursion guide to the geology of the Isle of Skye : Geological Society of Glasgow, 1986. © 1986 B.R. Bell & J.W. Harris. All rights reserved.
Figure 4 Geological sketch-map of the Cuillin Complex

Chapter 4 The Cuillin Complex[edit]

(A) Introduction[edit]

The oldest major plutonic (subvolcanic) group of rocks of Lower Tertiary age preserved on Skye is referred to as the Cuillin Complex (Figure 4) and is dominated by coarse-grained basic and ultrabasic rock-types which, in part, exhibit igneous layering. The complex is approximately 8km in diameter, with the layering dipping towards a focal point below Meall Dearg, at the southern end of Glen Sligachan. Dips of 10–20° at the margins increase to 60–70° towards the centre. The NE portion of the complex has been destroyed by the subsequent intrusion of the granites of the Srath na Creitheach and Western Red Hills Centres. Elsewhere, the complex is seen to be intruded into plateau lavas (see Section (3D) of Chapter 3).

Geographically, the complex has two distinct parts. The most prominent is the main Cuillin ridge, running from Sgurr nan Gillean in the north, to Gars-bheinn in the south, forming an arc which is concave to the east. The numerous peaks which constitute the main ridge are all just under 1000m O.D. and give rise to a very irregular topography, due to the inweathering and outweathering of minor intrusions which cut the complex. Two large drainage areas have developed within the amphitheatre east of the ridge: Harta Corrie in the north and Coir'-uisg in the south. These two areas are separated by the ridge of Druim nan Ramh, which lies to the north of Loch Coruisk.

The other part of the complex forms the Garbh-bheinn and Blaven ridge, which lies to the east of Loch na Creitheach. Detailed mapping indicates that the main rock-units which have been identified within the Cuillin Hills (s.s.) can be traced into this eastern sector.

Harker (1904) discussed in detail the various rock-types present within the complex and concluded that they developed in response to the injection of several pulses of heterogeneous magma. Subsequently, it was suggested by Stewart and Wager (1947) that the layering which is present within these rocks developed in response to gravity stratification and crystal settling mechanisms. Work by Carr (1952, 1954), Weedon (1956, 1961, 1965), Zinovieff (1958), J.D. Bell (1959, 1966), Hutchison (1964, 1966b, 1968), Jassim (1970), and Hutchison and Bevan (1977) has further added to the understanding of these rocks and forms the basis of the descriptions given below. A useful evolutionary stratigraphy is as follows (modified from J.D. Bell 1976):


INTRUSIVE EVENTS PRINCIPAL ROCK-TYPE
(YOUNGEST) dykes dolerite)
cone-sheets dolerite)
Coire Uaigneich (and other) Granites granophyre-granite-felsite
intrusive tholeiites:
Outer and Main Ridge Complexes amygdaloidal olivine dolerite)
Inner Layered Series: allivalites, eucrites, gabbros
Inner Layered Gabbro Series plagioclase-augite-(olivine)-(magnetite)-(apatite) orthocumulates
Inner Layered Eucrite Series plagioclase-olivinepyroxene-(magnetite) orthocumulates/mesocumulates
Inner Layered Allivalite Series plagioclase-(olivine) adcumulates/ heteradcumulates
Druim nan Ramh Eucrite
agglomerates and explosion breccias various pyroclastic rocks)
dykes dolerite)
?Gars-bheinn
Ultrabasic Sill olivine-plagioclase cumulates
Outer Layered Series: peridotites, allivalites, eucrites, gabbros
Outer Layered Gabbro Series plagioclase-augite-(olivine)-(magnetite)-(apatite) orthocumulates
Outer Layered Eucrite Series plagioclase-olivinepyroxene-(magnetite) orthocumulates/mesocumulates
Outer Layered Allivalite Series plagioclase-(olivine) adcumulates/ heteradcumulates
Layered Peridotite Series olivine-(spinel) adcumulates, olivine-plagioclase heteradcumulates
Border Group gabbroic/eucritic/ultrabasic rocks
including the White Allivalite) including cumulates
cone-sheets dolerite
dykes dolerite
Outer Marginal Gabbros and Eucrites
?Early Granites
(OLDEST)

Although various structural complexities are present within the Inner and Outer Layered Series, in general, the oldest members crop out around the margins of the complex, whilst the younger members are found towards the centre. It is likely that several periods of internal uplift and subsidence have modified any simple stratigraphy which may have existed. Nevertheless, it is possible to identify a 'stratigraphic column', as follows (modified from Wadsworth 1982):


UNIT THICKNESS(m)
Inner Layered Gabbro Series 750 Inner Layered Series (ILS)
Inner Layered Eucrite Series 450
Inner Layered Allivalite Series 500
Druim nan Ramh Eucrite
Outer Layered Gabbro Series ?

Outer Layered Series (OLS)

Outer Layered Eucrite Series 1600
Outer Layered Allivalite Series 1800
Layered Peridotite Series 500
Border Group (including the White Allivalite)
Outer Marginal Gabbros & Eucrites

In this scheme, the Druim nan Ramh Eucrite is considered to be a significant intrusion, separating the Outer Layered Series (OLS) and the Inner Layered Series (ILS). Zinovieff (1958) suggests that the layered allivalites of the OLS and the ILS are, in fact, members of the same unit, and that large-scale faulting has brought about the apparent repetition in the sequence. Alternatively, it has been suggested (Wager and Brown 1968; Wadsworth 1982) that this eucrite body is the ILS equivalent of the Outer Marginal Gabbros and Eucrites, together with the Border Group, which are associated with the OLS, and represents an unlayered marginal facies of a second major phase in the development of the Cuillin Complex.

The dykes, sheets, sills and cone-sheets associated with the Cuillin Complex are described in Chapter 9. The various phases of cone-sheet intrusion have dips similar to those recorded from the layering, and hafre a common focal point below Meall Dearg, at the southern end of Glen Sligachan.

The evidence of 'early granites' is based on the presence of fragments of material within younger vents and is discussed in more detail in Chapter 6. The only granitic body which is considered to be related to the development of the Cuillin Complex is the Coire Uaigneich Granite, which forms a thin, ribbon-like mass around the southern part of the complex (see Section (4L), below).

(B) The Outer Marginal Gabbros and Eucrites[edit]

This group of rocks forms a distinct marginal series in contact with country-rock plateau lavas along the convex portion of the main Cuillin Ridge. Layering is not present, and these rocks are believed to constitute the earliest units of the Cuillin Complex. Weedon (1961), on the basis of detailed field and mineralogical studies, identified four distinct rock-units: (1) The Gars-bheinn Gabbro; (2) The Gabbro with Clouded Feldspars; (3) The Ghrundda Eucrite; and, (4) The Ring Eucrite. Weedon (1961) concludes that the gabbros pre-date the eucrites.

Bevan and Hutchison (1984) conclude that the Ghrundda Eucrite is, in fact, allivalitic in composition and therefore part of the younger Border Group (see Section (4C), below). Furthermore, the presence of rare xenoliths of peridotite within 'clouded feldspar gabbro' north of the Allt Coir' a' Chruidh suggests to these authors that the Gars-bheinn Gabbro post-dates at least part of the Outer Layered Series (see below). A modified version of their proposed sequence of events is as follows:

(YOUNGEST)

Intrusion of the Gars-bheinn Gabbro

Period of cooling

Formation of the Outer Layered Eucrite Series

Formation/emplacement of the Layered Peridotite Series

(?Intrusion of the Gars-bheinn ultrabasic sill)

Intrusion of the Ring Eucrite

(OLDEST)

The following descriptions are mainly from Weedon (1961).

The Gars-bheinn Gabbro crops out on the summit of that name, as well as on Sgurr a' Choire Bhig, where it may be identified as a slightly-altered, fine-grained, smooth-weathering gabbro. It has a green coloration, particularly in the vicinity of darker, fine-grained, basic sheets which are present within the intrusion. Petrographically, the unaltered gabbro is composed of plagioclase (An65–68, but with cores as calcic as An74), and clinopyroxene, in an ophitic textural arrangement. Interstitial patches are present: either glass, or sodic plagioclase, or alkali feldspar. The glassy material may possibly represent an extreme fractionate, with a composition close to that of granite. Northwards, the Gars-bheinn Gabbro grades gradually into the Gabbro with Clouded Feldspars (see below). The upper contact, to the south, with the Ring Eucrite, however, is sharp, being marked by a distinct topographic depression.

The Gabbro with Clouded Feldspars is very similar to the Gars-bheinn Gabbro, but has been distinguished from it on the basis of the alteration present within the plagioclase component, and the schillerization of pyroxenes. It is exposed in the area NE of Sgurr nan Eag and Sgurr a' Choire Bhig. Weedon (1961) suggests that these differences may be due solely to the ingress of fluids at elevated temperatures, which were especially enriched in iron. The emplacement of the Layered Peridotite Series (see above) may be important in this respect, being a potential source of heat to drive convective hydrothermal systems. The boundary between the Gabbro with Clouded Feldspars and the Gars-bheinn Gabbro is not sharp.

In contrast, the two gabbros have sharp junctions with the Ghrundda and Ring Eucrites. These two eucrite intrusions are medium- to coarse-grained, and both contain plagioclases which are more calcic than those recorded from the gabbros. Typically, the feldspars are zoned, with core compositions of Ann, zoned to rims with the composition An65. Reversed zoning is also present. The feldspars are in an ophitic arrangement with clinopyroxenes, which tend to stand proud on weathered surfaces. Olivine and Fe-Ti oxides are also present, although their distributions are somewhat patchy.

The Ghrundda Eucrite crops out in the upper part of Coir' a' Ghrundda, on Sgurr nan Eag (although note interpretation by Bevan and Hutchison (1984), above), whilst the Ring Eucrite is exposed on the steep slopes south of Gars-bheinn at around 300m O.D., and in the many large corries to the west of the main Cuillin ridge. Both eucrites have steeply-dipping contacts.

Detailed geochemical studies of these eucrites have not, as yet, been undertaken.

(C) The Border Group[edit]

The presence of a Border Group to the Outer Layered Series of the Cuillin Complex was first recognised by Zinovieff (1958), and was studied in detail by Hutchison (1964, 1968). The contact with the Outer Unlayered Gabbros and Eucrites (see Section (4B), above) is exposed in Coire Lagan and on the butresses on the south side of the corrie. The material at the margin of the Border Group is a fine-grained tholeiite containing xenoliths of eucrite. Progressing away from the contact, the rock passes into a zone of 'wispy banding' (Hutchison 1968), which in general persists for approximately 30m, before passing into a coarse-grained allivalite ('White Allivalite') which is unlayered. The layer of wispy banding dips steeply inwards, at an average angle of 80°, and may form a marker horizon as it can be traced for a distance of over 5km along strike, to the north and south (Hutchison 1968).

The tholeiite at the margin contains olivine, two pyroxenes and intensely clouded plagioclases, with compositions around An57. Progressing away from the margin the clouding within the plagioclases disappears and their compositions become more calcic (An85). Textural variations are present within the zone of wispy banding, depending upon the inter-relationships of the pyroxenes and plagioclases. Fluxion structures are commonest near to the outer margin, and die out inwards as the rock passes into coarse-grained, unlayered White Allivalite.

The White Allivalite varies in thickness from between 200m on the west side of Sgurr Dearg, to 600m on the SW side of Sgurr a' Mhadaidh. The incoming of cumulus olivine constitutes a demarcation between the unlayered White Allivalite and the subsequent rock unit, the Outer Layered Allivalite Series, which is part of the Outer Layered Series (see Section (4E), below). The inner contact of the White Allivalite dips inwards at an angle of approximately 60°, but as the outer contact dips inwards at a steeper angle, the Border Group must thin upwards and outwards.

Compositionally, the White Allivalite is a coarse-grained (sometimes pegmatitic) rock rich in plagioclase (cores of An90, zoned to rims of An75) intergrown with clinopyroxene. Olivine is present in amounts similar to that of clinopyroxene (10%), whilst orthopyroxene and Fe-Ti oxides are much less common. Cores of individual plagioclases tend to be euhedral, suggesting that at the time of intrusion the parental magma contained a significant amount of primary plagioclase crystals. Hutchison and Bevan (1977) provide evidence that the magmas involved were picritic, rather than basic as has been suggested by Wager and Brown (1968).

Hutchison (1968) envisages the following steps in the development of the Border Group: (1) Intrusion of tholeiitic magma into intensely fractured Outer Marginal Gabbros and Eucrites, which then cools and crystallises calcic plagioclases in random orientations; (2) Injections of a similar magma, causing uplift of a central block and giving rise to the zone of wispy banding when crystallisation took place during periods of movement at the margins. Static conditions resulted in normal, coarse-grained textures. The presence of water-saturated conditions may have caused the development of randomly distributed patches of pegmatite-textured rock; and, (3) A final large pulse of magma, causing uplift in excess of 2000m, to create a large magma chamber from which the (unlayered) White Allivalite and the Outer Layered Allivalite Series crystallised, the latter being banked up against the inner margin of the former.

(D) The Layered Peridotite Series (Sgurr Dubh Peridotites)[edit]

This group of rocks represents the first cumulates associated with the Cuillin Complex. The Layered Peridotite Series is at least 500m thick, although the base of the sequence is not exposed. They crop out in an arcuate band running from near Loch Scavaig, in the south, to Coireachan Ruadha, in the north. South of An Garbh-choire the peridotites are in direct, almost vertical, contact with Unlayered Gabbros and Eucrites (see Section (4B), above); the Border Group (see Section (4C), above) being absent. These relationships may be explained by a downward movement of a central portion of layered rocks (including the peridotites), bringing them into contact with non-layered rocks (the Outer Marginal Gabbros and Eucrites). The thermal status of the peridotites at the time of their emplacement (movement) has been discussed by Wager and Brown (1968), who suggest that the alteration present in the Gabbro with Clouded Feldspars developed in response to the emplacement of this hot peridotite mass, although Hutchison (1966b) concludes that the clouding is a much later event.

Details of the field relationships and petrology of the peridotites are given by Weedon (1956, 1965). These olivine-rich rocks are easily distinguished from the eucrites and gabbros by their reddish-orange oxidation surfaces. Furthermore, the peridotites are relatively soft and easily eroded. These rocks are readily examined at An Garbh-choire, and in the area extending to the SE. To the north, the ground rises up towards Meall na Cuilce, where rocks of the Outer Layered Eucrite Series crop out. The nature of the contact between the peridotites and the eucrites is discussed in Section (4F), below.

Weedon (1965) identifies three 'zones', each named after the dominant rock-types within it. They are: Zone I, dunites and peridotites; Zone II, allivalites and feldspathic peridotites (named depending upon the relative amounts of plagioclase present); and, Zone III, brecciation and segregation veins within Zones I & II.

Rocks of Zone I form the lowest part of the sequence. They are at least 200m thick, and are exposed in the Allt Beag, near Loch Scavaig. Layering is not particularly well-developed due to the presence of equigranular olivine throughout. In the area nearest Loch Scavaig, the dominant rock-type is dunite, whilst to the west, further up the sequence, peridotites occur with the incoming of intercumulus plagioclase. The olivines have compositions typically around Fo83, whilst the interstitial plagioclase is recorded as An" (Hutchison and Bevan 1977; Bevan 1982). Apart from olivine and plagioclase, small quantities of chrome spinel and pyroxene are also found, occasionally forming distinct (1–2cm-thick) bands.

Zone II rocks are dominated by feldspathic peridotites and allivalites. The presence of significant amounts of plagioclase allows the development of excellent layering. These features are best observed between An Garbh-choire and Caisteal a' Garbh-choire. Features typically associated with cumulates are readily identified: phase layering, graded 'bedding', truncation structures, slumps, and current 'bedding'. Elongate, dendritic olivines are also found, forming a delicate interlocking structure most clearly seen on weathered surfaces. Layering becomes less obvious further up the Zone II sequence, whilst small, rounded xenoliths, probably cognate to the peridotites, become a common feature. Compositions of the olivines and plagioclases within the Zone II rocks have not been studied in detail, although optical studies by Weedon (1965) and electron probe micro-analyses by Hutchison and Bevan (1977) suggest that a slight amount of normal cryptic variation is present within the sequence.

The brecciation and segregation veins of feldspathic peridotite which consititute Zone III form an important component of the Zone I rocks around the headwaters of the Allt a' Chaoich. These veins are typically 2–5cm wide and form an anastomosing series of stringers throughout the host rocks. Their presence suggests that a significant amount of post-depositional movement of inter-cumulus liquid took place through the crystal (cumulate) pile. Further evidence of disturbance of the peridotites is seen in the form of slumps and folds within the succession, and in places layering dips atypically at angles in excess of 70°, towards the centre of the complex.

Subsequent ring-faulting, along the inner margin of the peridotites, possibly formed the magmatic disconformity between these ultrabasic rocks and the overlying Outer Layered Eucrite Series (see Section (4F), below).

(E) The Outer Layered Allivalite Series[edit]

These rocks have an estimated thickness of 1800m and have been subdivided into five units. With the exception of the lowest unit (Unit 1), the basis of the divisions is: plagioclase-pyroxene cumulates passing upwards into plagioclase-olivine cumulates. The latter show good, small-scale layering. Plagioclase dominates throughout this group of rocks and shows a small degree of normal cryptic variation (An82–87), along with olivine (Fo81–84). Zinovieff (1958) lists two main features used to identify and distinguish the five units, namely, the general absence of layering at the base of each unit, and the presence of olivine only in small amounts. Additionally, within the lowest part of the series, olivine constitutes approximately 40% of the rock, whilst at the highest levels it is only half that amount.

Hutchison (1968) and Hutchison and Bevan (1977) have shown that the Outer Layered Allivalite Series is banked up against the unlayered White Allivalite of the Border Group (see Section (4C), above) and constitutes typical cumulates deposited on the margin of the funnel-shaped Cuillin magma chamber. Hutchison and Bevan (1977) suggest that the Unit 1 allivalites, at the lowest exposed part of the sequence, may be equated with the Zone II allivalites of the Layered Peridotite Series (see Section (4D), above); extensive ring-faulting bringing about the present field relationships. They envisage a series of events in which, after the formation of the Border Group rocks, there was a period during which dunite cumulates developed, and these were subsequently overlapped by peridotite cumulates (the Zone I rocks in Section (4D), above). This was followed by the development of allivalites, when plagioclase became a fractionating phase. The absence of pyroxene in the Unit 1 allivalites of the Outer Layered Allivalite Series suggests that these rocks are more likely related to the Layered Peridotite Series (Wager and Brown 1968; Hutchison and Bevan 1977). The boundary between Units 1 and 2 of the Outer Layered Allivalite Series, the latter rich in cumulus pyroxene, may represent the incoming of a new batch of magma into the Cuillin chamber.

After the downward faulting of the inner ultramafic rocks (dunites and peridotites of the Layered Peridotite Series), relative to the Unit 1 rocks of the Outer Layered Allivalite Series and the Border Group rocks, the present distribution of rock-types is readily explained.

(F) The Outer Layered Eucrite Series[edit]

This group of rocks achieves a thickness of 1600m, and is eucritic because of the presence of plagioclase with the composition of sodic bytownite, together with clinopyroxene. Detailed studies of these rocks were undertaken by Carr (1952, 1954), Weedon (1956, 1961), Zinovieff (1958) and Jassim (1970). On the basis of Carr's work, three zones were identified, each with distinct cumulus minerals. These are: Zone I, plagioclase-olivine-clinopyroxene orthocumulates; Zone IL olivine-rich adcumulates; and, Zone III, olivine-magnetite adcumulates.

The Outer Layered Eucrite Series is exposed over a large sector of the complex, starting in the west (SW of Sgurr nan Gillean) and forming a crescent-shaped mass passing through Loch Coruisk, Sgurr na Stri and Blaven, ending in the east at Garbh-bheinn. In the western part, the eucrites are in contact with the Outer Layered Allivalite Series and the Layered Peridotite Series, where a narrow, unlayered, marginal facies has been identified, whilst in the vicinity of Sgurr na Stri this unlayered facies is in contact with country-rock plateau lavas. In the Blaven–Garbh-bheinn sector the layered eucrites are in direct contact with the country-rocks. This outer boundary of the eucrites dips inwards at approximately 70°. Layering within these eucrites always dips towards the centre of the complex, and generally increases from values of approximately 20° in the lowest (outer) parts of the series, to dips as high as 40° towards the top of the series, at the inner contact with the Druim nan Ramh Eucrite (see Section (4H), below). Generally, layering is more prominent towards the top of the series.

Cryptic variation is present within these eucrites, with plagioclase ranging from An75 in the lowest exposed part of Zone I, to An67 at the top of Zone III. Likewise, olivine varies from Fo74 to Fo67. A significant feature of these cumulates, especially in Zones II and III, is the presence of 'calcic-phase phenocrysts' of plagioclase, with a relatively constant composition of An85 (Carr 1952). It is likely that their presence indicates the incoming of a new batch of porphyritic magma at that stage in the development of the magma chamber.

Carr (1952) also discusses the presence of various types of 'blockstuff' (xenoliths) within the Outer Layered Eucrite Series. These are principally blocks of dunite and peridotite, but also include allivalite, eucrite, gabbro, dolerite and basalt, and are found particularly in the Meall na Cuilce area (Weedon 1956, 1961). Their incorporation into the eucrites suggests that erosion of already consolidated material within the magma chamber took place, probably as a result of convective currents. To achieve this type of erosion it is likely that faulting, of a central subsidence or marginal uplift type, occurred within the magma chamber.

The inner margin of the Outer Layered Eucrite Series is marked by the strongly discordant Druim nan Ramh Eucrite which cuts across rocks of all three zones.

(G) The Outer Layered Gabbro Series[edit]

Zinovieff (1958) recognises a group of layered gabbros in Coire Riabhach and part of Coire a' Bhasteir which he assigns to an Outer Layered Gabbro Series. The layering which is present within these rocks distinguishes them from the Outer Unlayered Gabbros and Eucrites to the north and west. Layering is only occasionaly seen, dipping at an angle of approximately 15° towards the centre of the complex. These layered gabbros are cut by the Druim nan Ramh Eucrite to the south.

(H) The Druim nan Ramh Eucrite[edit]

The Druim nan Ramh Eucrite is a coarse-grained, basic rock devoid of layering and has a semicircular outcrop which separates members of the Outer and Inner Layered Series. It was first described by Carr (1952), who referred to it as an Invading eucrite'. Subsequent work by Zinovieff (1958) showed that this unit was a ring-dyke, as the rocks inside and outside the structure could be correlated. On this basis, Wager and Brown (1968) suggested that large displacements of a central block took place prior to, and during, the injection of the eucrite. Such displacements, however, would have to be of the order of 1000m in an upward direction—contrary to the general trend of central subsidence throughout the evolution of the complex.

An alternative hypothesis is that the Druim nan Ramh Eucrite represents a marginal group of rocks, equivalent to the Outer Marginal Gabbros and Eucrites and Border Group of the Outer Layered Series (see Section (4A), above), and marks the onset of a second major phase of activity within the complex related to the development of the Inner Layered Series.

The Druim nan Ramh Eucrite has almost vertical contacts and an average width, at the present level of erosion, of 200m. It is truncated to the south, in Srath na Creitheach, by pyroclastic rocks, and to the north by the Marsco Granite of the Western Red Hills Centre. It is best observed on Druim nan Ramh. As a general rule, the eucrite tends to inweather relative to the cumulates which it cuts. The outer contact is extremely irregular, consisting of veins of material injected into rocks of the Outer Layered Series. Zinovieff (1958) notes that individual veins are often enriched in one mineral, for example, olivine or pyroxene, suggesting that the magma came in as some kind of crystal mush. Progressing inwards, a xenolithic facies is encountered, containing blocks of material from the Outer Layered Series, particularly allivalite and eucrite. The inner contact is much sharper and it is possible to distinguish the eucrite from the Inner Layered Allivalite Series (Zinovieff 1958).

The dominant mineralogy of the Druim nan Ramh Eucrite is calcic plagioclase (An70–82) intergrown with clinopyroxene. Olivine and Fe-Ti oxides are also present, but are not abundant.

Further work remains to be completed on this intrusion, as its precise field relations and petrology may be critical to the elucidation of the relationship between the Inner and Outer Layered Series. In view of this, in the account of the Inner Layered Series which follows (see Sections (4I), (4J) and (4K), below), no preference is shown to either of the two genetic models discussed above.

(I) The Inner Layered Allivalite Series[edit]

Very little published information is available on the Inner Layered Allivalite Series. According to Zinovieff (1958), three units can be identified, which he suggests are simply a repetition of Units 3, 4 and 5 of the Outer Layered Allivalite Series (see Section (4E), above). The estimated thickness of the whole group is of the order of 500m, and individual units consist of plagioclase-pyroxene cumulates at the base, passing upwards into finely laminated plagioclase-olivine cumulates. This is very similar to the mineralogical characteristics of the Outer Layered Allivalite Series. Where layering is present, it dips towards a focal point similar to that deduced for members of the Outer Layered Series (below Meall Dearg, at the southern end of Glen Sligachan), with values around 25° at the outer margin (with the Druim nan Ramh Eucrite), increasing to approximately 40° at the top of the exposed sequence. These rocks are best examined on the high ground east of Sgurr nan Gillean and in the vicinity of the summit of Sgurr Hain.

(J) The Inner Layered Eucrite Series[edit]

This group of rocks is of the order of 450m thick and is exposed in the Harta Corrie, where the series is in direct contact with the Inner Layered Allivalite Series, which it overlies at a steep angle (Zinovieff 1958). Layering is not well-developed and these rocks are truncated to the east by a large mass of pyroclastic rocks on Meallan Dearg, and to the NE by a faulted contact with the younger Inner Layered Gabbro Series. Poor exposure, plus the effects of later intrusions, largely prevents a detailed investigation of these rocks and there is, at present, no published mineralogical data available.

(K) The Inner Layered Gabbro Series[edit]

The Inner Layered Gabbro Series constitutes the youngest group of cumulates preserved within the Cuillin Complex. They are best exposed on the ridge of Druim Hain, west of Srath na Creitheach, as to the south, in Coire Riabhach, exposure is poor. Within the latter region, however, it is possible to identify a vertical shatter zone, approximately 50m wide, along which downward movement of the layered gabbros has brought them to their present position. Carr (1952) estimates that this displacement is of the order of 300m. Rocks within this zone are intensely crushed, resulting in a tuffaceous product, and contain xenoliths of peridotite. These shattered rocks are also cut by numerous irregular intrusions of dolerite and basalt.

At least 750m of gabbros are preserved on Druim Hain. The arcuate distribution of the layering is readily identified on vertical aerial photographs, with the focal point lying below Meall Dearg to the north. The lowest rocks of the series show the poorest development of layering and constitute at least 200m of the sequence. The incoming of a distinct lamination marks the main portion of the layered succession, at least 300m thick. These rocks exhibit excellent rhythmic layering, occasionally showing grading, with magnetite enrichment at the base of individual units passing upwards into plagioclase-rich tops. The attitude of the layering varies from approximately 30° in the lower parts, up to 70° at the top of the preserved sequence. Normal cryptic variation has been identified within both the plagioclases and the olivines, with ranges of An58–69 and Fo58–66, respectively. Apatite also occurs as a cumulus phase, indicating that the magmas involved reached relatively evolved compositions. The highest exposed rocks (SSE of Meall Dearg) show occasional developments of coarse, pegmatitic facies.

These gabbros contain calcium-rich plagiocases (An85), similar in composition to plagioclases ('calcic-phase phenocrysts') in the Outer Layered Eucrite Series (especially Zones II and III, see Section (4F), above). Their presence suggests that the supply of new batches of porphyritic magma into the chamber may have continued until at least this stage in the evolution of the complex.

Xenoliths present within the gabbros include: peridotite, eucrite, and dolerite.

Any rocks which may have formed at a later stage than the preserved portion of the Inner Layered Gabbro Series have subsequently been removed by erosion.

(L) The Coire Uaigneich Granite[edit]

The only rock of truly granitic composition associated with the Cuillin Complex is the Coire Uaigneich Granite. It crops out along the southern margin of the complex, between Sgurr na Stri and Coire Uaigneich, in a mass which is narrow, discontinuous, and ribbon-shaped. It is most easily examined on the beach on the west side of the Abhainn Camas Fhionnairigh, where it forms a wave-cut platform. On these clean surfaces variations between coarse- and fine-grained facies are readily noted and partially-digested xenoliths of sandstone can be seen. In hand-specimen, needles of hypersthene (often replaced by chlorite) are also conspicuous, in a pale granitic groundmass. In the Sgurr na Stri area, the granite is intruded along the contact between Torridonian strata and metamorphosed basic lavas, and dips to the NW at an angle of between 35 and 50°. The only other place where the field relationships of the granite are easily examined is in Coire Uaigneich, itself, where it is intruded into Jurassic sedimentary rocks.

Near to its margins, fine-grained facies of the granite are locally developed, grading rapidly into coarser material towards the centre of the intrusion. Details of the age relationships between the granite and the Cuillin Complex are still in doubt. Carr (1952) concludes that it predates the complex, whilst Almond (1960) suggests that it formed at a later stage.

The presence of sandstone xenoliths within the Coire Uaigneich Granite lead Wager et al. (1953) to study the intrusion in more detail. Within the granite itself, they report quartz paramorphs, after tridymite, and suggest that the whole-rock composition of the intrusion falls within the primary quartz field of the system Qz-Ab-Or, when PTOTAL = PH2O (= lkbar). In consequence, they conclude that this particular granite represents a partial melt of Torridonian sandstone (see Section (4M), below, and Section (2B) of Chapter 2). Experimental studies by Brown (1963) provided further evidence for this model, suggesting that the melting events probably took place at a depth of approximately 1km.

Meighan (1976, 1979) proposed an alternative model, suggesting that the intrusion originated via fractional crystallisation of a basic magma, possibly associated with the Cuillin Complex, and subsequently underwent silicification. Important to his arguments are differences in major- and trace-element chemistry between the granite and typical Torridonian sandstones from the area. Meighan also suggests that the so-called paramorphs of quartz, after tridymite, are, in fact, the hydrothermal replacement of plagioclase by quartz.

More recently, Dickin and Exley (1981) have re-investigated the intrusion in detail, using major-, trace- and isotope-element data. They conclude that the intrusion has not undergone significant changes in bulk composition, and that it formed by the mixing of two distinct magmas: one being a partial melt of Torridonian sedimentary material, the other an acid differentiate of a basic magma (from the Cuillin magma chamber) enriched in incompatible elements such as Zr and Y. The mixing involved two parts of Torridonian melt for every one part of the differentiate.

(M) Thermal effects of the Cuillin Complex[edit]

The introduction of large volumes of basic and ultrabasic magma into the crust brought about several changes in the associated country-rocks. Investigations into the nature of the Torridonian sedimentary rocks in Camasunary Bay by Almond (1960, 1964) and Jassim (1970) provide considerable evidence of the significant amount of heat transferred from the complex into the country-rocks. In the vicinity of the Abhainn nan Leac, on the east side of the bay, 'normal' Torridonian strata are encountered, consisting of sandstones, arkoses and occasional grits (see Section (2B) of Chapter 2). Progressing inwards, towards the margin of the complex, these rocks show evidence of alteration and distortion, most readily observed in the small stream east of An-t Sron (200m from the contact). The rocks take on a slightly bleached appearance and in thin-section there is ample evidence that recrystallisation has taken place. Within 100m of the contact (for example, NE of the small outcrop of Jurassic marble on the west side of the bay), rheomorphism (mobilisation) of leucocratic layers within the Torridonian strata has taken place. This rock-type is in many ways similar to that of the Coire Uaigneich Granite (see Section (4L), above). These leucocratic veins cut the basic minor intrusions present within the country-rock. Furthermore, these minor intrusions show evidence of distortion (sinuous contacts and brecciation) at their margins. Thin-section studies indicate that reactions have taken place between the rheomorphosed Torridonian strata and the minor intrusions, producing hybrid material.

Lower Jurassic carbonate-bearing sedimentary rocks, exposed between An-t Sron and the valley on the east side of Blaven, have been metamorphosed and metasomatised to give high-grade marbles containing the calc-silicate minerals spurrite and .rankinite (Wyatt 1952). Endogenic effects of this process have lead to the gabbro taking on a whitish appearance.

Within the Lower Tertiary plateau lavas, particularly on the Strathaird Peninsula and in Camasunary Bay, alteration zones in the form of haloes around the Cuillin Complex have been identified. These are described in Section (3D) of Chapter 3.

(N) The volcaniclastic pipes of the Cuillin Complex[edit]

Zinovieff (1958) records over forty pipe-like structures of volcanic breccia and agglomerate throughout the western portion of the Cuillin Complex. Other similar material has been identified in the eastern sector of the complex (Dundas, in a personal communication to J.D. Bell 1976), as well as to the south of the complex, on the north side of the Sound of Soay (Harker 1904).

These masses of pyroclastic material pierce, and hence post-date, the Layered Series (see Section (4A), above). They are readily identified in the field by their distinctly fragmental appearance and consist of blocks of ultrabasic and basic rock-types set in a doleritic or tuffaceous matrix, derived locally from the Layered Series.

Zinovieff (1958) has identified three main groupings of these pipes: (1) The Coire na Creiche Group; (2) The Sgurr nan Gillean Group; (3) The Lota Corrie Group. Other smaller, geographically-localised groups are present throughout the Cuillin Hills.

The largest single outcrop is found on Meallan Dearg; in the lower part of Harta Corrie. Close to the outcrop margins, the main pyroclasts are of tuff, grading rapidly inwards into a coarse agglomerate. Crush zones are common. The dominant block-types are of allivalite, eucrite and gabbro, often angular, set in a predominantly doleritic, but sometimes tuffaceous, matrix.

The size, degree of rounding and sorting of pyroclasts within these pipe-like structures appears to depend upon the structural height at which the material exists. Zinovieff (1958) proposed the following model to explain all of the observed features. First, dyke injection causes brecciation of the country-rock gabbros whilst under compression. Increased vapour pressure due to rapid cooling of this magma causes fluidisation of the blocks which have formed, lifting them upwards in a coherent fashion. During this process, abrasion of blocks, mixing, and further brecciation takes place, resulting in the development of a tuffaceous matrix. Material of this type penetrates upwards to the highest levels (for example, the material exposed on the upper parts of Sgurr nan Gillean). Finally, the fluidised mass collapses, due to the lack of a sufficient pressure gradient to drive the system, and intrusive agglomerates result.

The pipe on the southern slopes of Sgurr na h-Uamha shows many of the features outlined above (Zinovieff 1958). At its base the dominant pyroclastic material is a breccia, with numerous associated basic dykes. The matrix constitutes approximately 10% of the total volume of the pipe. This passes upwards into an agglomerate with rounded clasts, set in a tuffaceous matrix. Material in the uppermost preserved part of the pipe is highly vesiculated, suggesting a final escape of gas from the system.

References[edit]

Appendix 1: Glossary of petrological names and terms[edit]

Appendix 2: Glossary of fossil names[edit]

Appendix 3: Glossary of place names and grid references[edit]

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