The Western Red Hills Centre, Skye Central Complex

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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 6 Geological sketch-map of the Western Red Hills Centre
Figure 7 Schematic diagram of Marsco showing the distribution of rock-types
Figure 8 Schematic section across Harker's Gully on Marsco (modified from Wager et al. 1965)

Chapter 6 The Western Red Hills Centre[edit]

(A) Introduction[edit]

The Western Red Hills Centre (Figure 6) was first defined as a distinct igneous centre by Richey (1932), when he distinguished the granites of the Glamaig-Marsco-Loch Ainort area from the younger Eastern Red Hills Centre, west of Broadford. The centre covers an area of approximately 35km2 and, at the present level of erosion, is dominated by annular acid intrusions of granite, granophyre and felsite. Significantly, one of the earliest members of the centre is the Marsco Summit Gabbro, constituting the only true basic intrusion. One other important feature of the centre is the presence of a group of rocks referred to as the Marscoite Suite, consisting of narrow ring-dykes of ferrodiorite (an Fe-enriched basic fractionate), a felsite (the Southern Porphyritic Felsite) and various hybrid rocks, which were formed by magma-mixing processes.

A detailed study of the rocks of the Western Red Hills Centre was undertaken by Wager and his co-workers, commencing with a preliminary report by Richey et al. (1946) which discussed age relationships between the granites. The results of this investigation are presented in Wager and Vincent (1962), Wager et al. (1965), J.D. Bell (1959, 1966) and Thompson (1965, 1969). More recently, Thompson (1980) has suggested that aspects of the petrology of the granites, as well as the Marscoite Suite, may be explained in terms of magma-mixing processes during the development of the centre.

J.D. Bell (1976) provides the following useful summary of events:

INTRUSIVE EVENTS PRINCIPAL ROCK-TYPE
(YOUNGEST)
dykes dolerite, pitchstone, aplite
Marsco Granite plagiophyric Fa. Fhed. granite/granophyre
Meall Buidhe Granite plagiophyric Fhed. Amph. microgranite/granophyre
Northern Porphyritic Felsite porphyritic felsite
Marscoite Suite marscoite, ferrodiorite, glamaigite, felsite, andesinite
Southern Porphyritic Granite porphyritic felsite/ microgranite/granophyre
Glen Sligachan Granite alkali feldsparphyric fayalite microgranite
Loch Ainort Granite alkali feldsparphyric Fa. Fhed. granophyre
Beinn Dearg Mhor Granite alkali feldsparphyric Fa. Fhed. granophyre
Eas Mor Granite alkali feldsparphyric amphibole granophyre
Maol na Gainmhich Granite arfvedsonite granite
Glamaig Granite plagiophyric hastingsite biotite granite/granophyre
Marsco Summit Gabbro (olivine) gabbro (altered)
Belig and Meall a' Mhaoil Vents blocks of basalt, gabbro, peridotite, trachyte, sedimentary rocks, granite, dolerite, gneiss
(OLDEST)
(Note: Fa = fayalite; Fhed = ferrohedenbergite; Amph = amphibole)

(B) The Belig and Meall a' Mhaoil vent agglomerates and breccias[edit]

According to the studies of J.D. Bell (1959, 1966), the agglomerates present on Belig and Meall a' Mhaoil constitute the earliest subvolcanic activity in the Western Red Hills Centre. Intrusion of the later granites greatly modified their distribution and probably destroyed much of the original material. J.D. Bell concludes that these accumulations formed inside vents or underground.

Around Belig, three main masses of material have been identified: (1) in the upper parts of Coire Choinnich and on the northern slopes of Belig; (2) around the headwaters of the Eas a' Chait; and, (3) in Coire na Seilg. Other much smaller outcrops are present, further west, on Druim Eadar Da Choire.

The material on Meall a' Mhaoil has a very limited distribution, and has been almost completely destroyed by the later Northern Porphyritic Felsite. No published data are available on these agglomerates, although in many respects they are identical to the material exposed around Belig. The descriptions presented below, therefore, will concentrate on the latter.

The contacts of the agglomerates with the surrounding rocks are not easily examined, either because they are drift-covered, or because both rocks are intensely crushed. Where discernible, contacts are typically steeply dipping or vertical. No evidence of bedding has been recorded.

The pyioclastic material is composed of rounded to sub-angular blocks set in a fine-grained matrix. The size and proportion of blocks to matrix shows a very large range and the type of blocks varies markedly over the outcrop area.

Typically, fragments of sedimentary material are less than 1m across, and fragment sizes decrease westwards.

J.D. Bell (1966) suggests that the blocks fall into one of two categories:(a) material thought to be in situ, or at least close to the original vent (b) acid intrusive material which may have been transported to its present position. Blocks in category (a) include material from the Outer Layered Eucrite Series (exposed further south on Garbh-bheinn, see Chapter 4), Torridonian and Mesozoic strata, basalt, and fragments of cone-sheets, whilst in category (b) are granite, felsite, and quartz porphyry

In addition, specific block-types are more common in certain parts of the outcrop area. For example, Mesozoic strata are lacking in the western portion of the agglomerate, between Coire Choinnich and the northern slopes of Belig, whilst Torridonian strata are well represented. Such a distribution pattern suggests that these vent agglomerates sit astride the Camasunary Fault, which brings Mesozoic and Torridonian strata into juxtaposition (see Section (10D) of Chapter 10).

The matrix of these deposits is composed of comminuted fragments of block materials. Similar to the blocks, the matrix material has been subjected to hydrothermal alteration. For example, olivine is generally serpentinised, whilst pyroxene is chloritised and alkali feldspars sericitised. By comparison, plagioclase is relatively fresh.

Another significant observation is that large (up to 50m across) slabs of material from the Outer Layered Eucrite Series (see Section (4F) of Chapter 4), cut by cone-sheets (see Section (9D) of Chapter 9), are also present within the agglomerates, indicating that the formation of these pyroclastic rocks post-dated the intrusion of some, if not all, of the cone-sheets associated with the Cuillin Complex.

The relative age of the agglomerates is also deduced from the observation that both granites and rocks of the Marscoite Suite (see Section (6H), below) intrude, and frequently chill against, the agglomerates. Furthermore, none of the granite blocks within the agglomerates resemble the surrounding granite intrusions and, therefore, were probably derived from older granites, not now exposed.

J.D. Bell (1966) concludes that these various pyroclastic accumulations formed in response to the introduction of a vesiculating acid magma high into the Earth's crust. Volatiles streaming off the magma brecciated and fluidised the surrounding country-rocks, including any marginal chill facies already crystallised. Continued gas flow caused rounding of blocks and produced the fine-grained matrix material. Upward movement of this particle-gas suspension may have reached the surface, although there is no direct evidence for this preserved within the Western Red Hills district. It is possible that the acid magma which gave rise to these processes is represented by either the Loch Ainort or Marsco Granites, as both of these intrude the agglomerates.

(C) The Marsco Summit Gabbro[edit]

This gabbro forms a distinct cap to the SE part the summit of Marsco (Thompson 1969). It crops out as a relatively flat-lying sheet and is underlain by the Glamaig Granite, the first of the Western Red Hills acid intrusions (see Section (D), below). Numerous veins of fine-grained acid material from the Glamaig Granite intrude and fragment the gabbro, giving rise to a zone of stoping over distances of several metres. In this zone, the gabbro is typically crushed and metamorphosed, although in places a sharp boundary, which is occasionally crenulate, may be observed.

The gabbro has a distinct brown coloration and, unlike the gabbros of the Cuillin Complex, is not intruded by cone-sheets (see Chapters 4 and 9). It is composed of clinopyroxene and plagioclase (with cores of An67–74, zoned to rims of An50, Thompson 1965), in an ophitic arrangement. Olivine is not common and is typically altered. Thompson also reports the presence of orthopyroxene, Fe-Ti oxides, apatite and zircon, together with interstitial hornblende and biotite.

The presence of a fine-grained, marginal facies to this sheet, with subvariolitic textures involving skeletal plagioclase phenocrysts, suggests that intense chilling of the basic magma occurred locally. This chilling, together with patches of coarse-grained gabbro which are fragmented by the Glamaig Granite, lead Thompson (1969) to conclude that prior to, and during, the intrusion of the granite the Marsco Summit Gabbro was partly crystalline, but with pockets of liquid still present.

Geochemical studies by Moorbath and Thompson (1980) and Thompson (1982) suggest that the gabbro is the product of mixing 80% of a primitive tholeiitic basalt magma (of the Preshal Mhor type, see Section (3D) of Chapter 3) with 10% each of amphibolite-facies and granulite-facies Lewisian Gneiss (see Section (2A) of Chapter 2 and Section (12B) of Chapter 12).

(D) The Glamaig Granite[edit]

The medium-grained, amphibole- and biotite-bearing granite which crops out over a broad tract of ground (approximately 9km2), running from Glamaig in the north, to Marsco in the south, was originally referred to as G1 by Richey et al. (1946) and Wager et al. (1948), but has subsequently been renamed the Glamaig Granite by Wager et al. (1965). The outer margin shows the development of extensive crush planes within a porphyritic felsite chill facies and is thought to be vertical, as is the inner contact against the later Beinn Dearg Mhor Granite. The normal, coarse-grained granite weathers to a dull grey, frequently contains miarolitic cavities, and is characterised in the field by the presence of small, rounded to sub-angular, mafic inclusions (5–50mm). These inclusions constitute up to 5% of the rock and are relatively evenly distributed throughout its mass. Typically, the larger inclusions have distinct outlines, whilst the smaller ones have less clearly defined margins and appear simply as clots of mafic minerals. Also present are leucocratic inclusions of fine-grained material, up to 4m across, typically found within the central part of the intrusion. The Allt na Measarroch dissects the Glamaig Granite from close to its margin, through to its centre, and allows the examination of fresh material of the different types described above.

In the Marsco area, the Glamaig Granite is overlain by the later Southern Porphyritic Granite (see Section (6G), below) on the north side of the hill. On the south side of the hill this relationship is reversed and the Glamaig Granite is overlain by the Marsco Summit Gabbro (see Section (6C), above). These relationships are illustrated in (Figure 7).

In detail, the Glamaig Granite is dominated by equigranular quartz, alkali feldspar and plagioclase (2–8mm), together with small quantities of calciferous amphibole (hastingsite), clinopyroxene, biotite, Fe-Ti oxides and various other accessory minerals (Thompson 1969). Detailed studies indicate that, on Marsco, the granite becomes more mafic in composition near to its contact with the Marsco Summit Gabbro, suggesting that contamination of the acid magma has occurred (see Section (6C), above).

Thompson (1980) suggests that the mafic inclusions dispersed throughout the volume of this granite were introduced in the form of basic magma, which was injected into the base of a magma chamber dominated by acid magma. A consequence of this mixing would be the net transfer of heat from the hotter basic magma to the cooler acid magma, causing the latter to convect rapidly and further entrain the former. This process is discussed in detail in Section (6K), below.

(E) The early northern granites of Maol na Gainmhich and Eas Mor[edit]

These two intrusions were identified by Wager et al. (1965) as having formed early in the evolution of the Western Red Hills Centre. However, no direct evidence of their age relative to the Glamaig Granite is available, although it is clear that both granites were intruded before the Marscoite Suite (see Section (6H), below). Very little published data are available on these intrusions.

The Maol na Gainmhich Granite is a coarse-grained, amphibole-bearing rock which contains only one feldspar, a potassium-rich variety. The amphibole is rich in alkalis and has been identified as arfvedsonite. This intrusion crops out north of Loch Ainort, in the ground south of Maol na Gainmhich, between Ceann a' Chreagain and the coast. In addition, the granite which crops out on the SW coast of the neighbouring island of Scalpay may be from the same intrusion (Wager et al. 1965).

The Eas Mor Granite crops out on the NW slopes of Glamaig, being particularly accessible in the stream bed of Eas Mor, itself. It is ' characterised by phenocrysts of microperthitic feldspar, together with relatively abundant green hornblende, set in a fine-grained, quartz and alkali feldspar dominated, groundmass.

(F) The Beinn Dearg Mhor and Loch Ainort granites[edit]

The Beinn Dearg Mhor Granite crops out on the Red Hill of that name, as well as on Ciche na Beinne Deirge and Beinn Dearg Mheadhonach, and was originally called the G2 granite by Richey et al. (1946). Its inner and outer margins are close to vertical and locally show signs of crushing. Along its boundary with the Glamaig Granite, for example at Am Fuarchoire. it shows the development of a chill facies (porphyritic felsite), which clearly indicates the time-relationships between the two intrusions. At Mam a' Phobuill, the Beinn Dearg Mhor Granite is crushed and altered by the later Southern Porphyritic Granite (see Section (6G), below). Along its inner margin, for example in the Allt Mhic Mhoirein, the Beinn Dearg Mhor Granite is crushed by the later Loch Ainort Granite (see below).

The Beinn Dearg Mhor Granite weathers deeply, to a distinct rusty red, in sharp contrast to the grey coloration of the neighbouring, older Glamaig Granite (see Section (6D), above). Fresh material is most readily examined in road-cuts at the southern end of Gleann Torra-mhichaig, where the rock has a greenish-blue colour. It contains numerous phenocrysts of fresh, glassy anorthoclase, often in a glomeroporphyritic arrangement. The groundmass is composed of granophyric intergrowths of quartz and alkali feldspar. The most common ferromagnesian mineral is the Fe-rich clinopyroxene, ferrohedenbergite, together with some hornblende, Fe-Ti oxides, apatite and biotite. Olivine is not common, but if present, is typically serpentinised.

Around the northern margin of the Beinn Dearg Mhor Granite, south of the Allt Mor Doire Mhic-uin, is a crush zone, with large slabs of crushed, altered gabbro forming part of a screen between it and the Glamaig Granite. These slabs were most likely dragged up along the contact during the emplacement of the later Beinn Dearg Mhor Granite.

The Loch Ainort Granite crops out on the low ground around the head of Loch Ainort, between Maol Ban in the north, and Coire Choinnich in the south, and is roughly L-shaped, with an area of approximately 7km2. Crushing complicates its contacts with the Beinn Dearg Mhor Granite to the west, the Northern Porphyritic Felsite to the north, and the Southern Porphyritic Granite to the south. It is truncated to the east by the first of the Eastern Red Hills acid intrusions, the Glas Bheinn Mhor Granite (see Section (7E) of Chapter 7). In terms of its petrology, the Loch Ainort Granite is almost identical to the Beinn Dearg Mhor Granite (see above). The zone of crushing in the northern part of the granite, between Druim nan Cleochd and Leathad Chrithinn, lead J.D. Bell (1959, 1966) to conclude that the two intrusions are part of the same mass, with upward movement of the inner Loch Ainort Granite, relative to the outer Beinn Dearg Mhor Granite, bringing about the observed field relationships.

In the vicinity of the Allt Coire na Ciche, J.D. Bell (1966) identified a fine-grained, leucocratic rock containing biotite and amphibole. It is light grey and may either constitute a separate sheet of material intruded between the Beinn Dearg Mhor and Loch Ainort Granites, or possibly be a marginal facies of one of them. A small intrusion of similar material is exposed SW of Sron Ard a' Mhullaich, at the head of Loch Ainort. Essentially, these are porphyritic felsites, containing feldspar phenocrysts of the same type present in the granites. In places, the felsites show gradational contacts towards the granites.

(G) The early southern granite of Glen Sligachan and the Southern Porphyritic Granite[edit]

The Glen Sligachan Granite (Figure 7) has a very limited outcrop (750m x 100m), occurring along the northern side of the Marscoite Suite, at the base of Harker's Gully (see Section (6H), below). It weathers to a rusty brown, although when fresh is bluish-grey, with a distinctly porphyritic appearance. The phenocrysts of feldspar consist of plagioclase cores, rimmed with alkali feldspar, together with crystals which are purely alkali feldspar (microperthite). Both ferrohedenbergite and olivine (generally altered) are present in small quantities. This granite is most easily examined in the drift-covered, hummocky ground east of the path, at the foot of Harker's Gully. According to Thompson (1969), the Glen Sligachan Granite was roughly contemporaneous with the Southern Porphyritic Granite.

The Southern Porphyritic Granite (Figure 7) is exposed in a broad tract of ground north of the Marscoite Suite, between Mam a' Phobuill in the west and the Allt a' Mheadhoin in the east, and also to the south of the Marscoite Suite, at Fiaclan Dearg, on Marsco. Along the northern margin of the main outcrop (Figure 6), dipping at 60° to the south, the Southern Porphyritic Granite cuts the Glamaig, Beinn Dearg Mhor and Loch Ainort Granites, although well-defined contacts are not open to examination. The southern margin of this outcrop of the granite is almost vertical and there is an extensive development of a marginal chill facies of felsite (see below, and Section (6H), below). The outcrop of the Southern Porphyritic Granite south of the Marscoite Suite has the form of a ring-dyke roof fragment, lying below the older Glamaig Granite, and above the younger Marsco Granite (see Section (6J), below).

This granite is light-coloured and contains phenocrysts of quartz and clouded alkali feldspar, set in a granophyric groundmass. Thompson (1969) reports the presence of small amounts of ferrohedenbergite, olivine, amphibole and biotite. Crushing is a common feature within the Southern Porphyritic Granite, and this led Wager et al. (1965) to suggest that high water pressures developed during its crystallisation.

The marginal felsite facies, along the contact with the Marscoite Suite, is referred to as the Southern Porphyritic Felsite and contains the same phenocryst assemblage as the closely related granite, but is set in a felsitic groundmass (Figure 8). Wager et al. (1965) conclude that the felsite constitutes a thin, separate sheet of material injected into the granite after it had been crushed, but while it was still hot. The felsite was most likely derived from the same magma source as the Southern Porphyritic Granite, but cooled much faster and was probably depleted in volatiles. The significance of the Southern Porphyritic Felsite, in relation to magma-mixing processes in the Western Red Hills Centre, is discussed in Sections (6H) and (6K), below.

(H) The Marscoite Suite[edit]

Rocks belonging to the Marscoite Suite were first identified as a distinct group by Wager et al. (1965), and include: porphyritic felsite, ferrodiorite, and the hybrid rocks marscoite, glamaigite and dioritic glamaigite. In Harker's Gully, on the NW side of Marsco, Wager et al. (1965) were able to deduce the field relationships of the first two of these rocks (Figure 7) and (Figure 8) and suggested that marscoite was formed by their mechanical mixing. On the basis of simple calculations, they concluded that the mixing of 65% of ferrodiorite (an Fe-enriched basic fractionate) and 35% of porphyritic felsite (the Southern Porphyritic Felsite, see Section (6G), above) would result in a magma with the composition of marscoite. Mineralogical evidence for the hybrid nature of marscoite is readily obtained from thin-section studies. For example, this fine-grained, grey rock contains xenocrysts of andesine (with rounded edges), orthoclase (with embayed margins, the so-called fingerprint texture) and quartz (fringed with either pyroxene or amphibole)—all of which can be identified as phenocrysts in the postulated basic and acid parents (the andesine in the ferrodiorite and the orthoclase and quartz in the porphyritic felsite).

The ferrodiorite occurs in two varieties: with or without distinct phenocrysts of andesine (zoned from An50 to An30). The groundmass is composed of turbid crystals of alkali feldspar, interstitial quartz, inverted pigeonite, augite and olivine (Fo22, typically altered to serpentine), together with accessory apatite, zircon, Fe-Ti oxides and pyrrhotite. Present within the ferrodiorite intrusion are large (10–30cm) inclusions of andesinite. These inclusions are also found within the marscoite, but are much less common. Lewisian Gneiss xenoliths are also reported from the ferrodiorite intrusion in Harker's Gully and are described in Section (2A) of Chapter 2. The mineralogy of the Southern Porphyritic Felsite is discussed in Section (6G), above.

Within Harker's Gully, on the north side (Figure 8), the Southern Porphyritic Felsite, marscoite and ferrodiorite occur as relatively thin, steeply-inclined sheets, which constitute part of a large, outwardly-dipping, composite ring-dyke which can be traced, albeit discontinuously, from Marsco in the west, to Coire Choinnich in the east. On Marsco, at the level of the so-called 'Shelter Stone' (Wager et al. 1965), the following field relationships can be deduced. First, marscoite chills against, and rarely veins, the Southern Porphyritic Felsite, although their bulbous mutual boundary suggests that the latter had not totally crystallised. Also, the marscoite grades over approximately 10m into porphyritic ferrodiorite, suggesting that the two intrusions were emplaced almost synchronously, and that in situ mixing (both mechanical and diffusional) has destroyed any sharp boundary between the two.

On the south side of the gully the ferrodiorite is coarser-grained and non-porphyritic. Typical ferrodiorite may be collected from the overhanging Shelter Stone. Also on this side of the gully, the ferrodiorite is in contact with the Marsco Granite, the last of the Western Red Hills major intrusions (see Section (6J), below).

Further studies by Thompson (1969) identified quartz xenocrysts within the ferrodiorite, suggesting that it too is a hybrid rock. Also, at the top of Harker's Gully, above the roof of the Marsco Granite, Thompson found a complete section through this composite ring-dyke. The section consists of: felsite-marscoite-ferrodiorite-marscoite-felsite, and confirms the previously deduced sequence of events as: felsite, followed by marscoite, followed by ferrodiorite.

Members of the Marscoite Suite in the northern part of the Western Red Hills Centre crop out as three separate intrusions, referred to by Wager et al. (1965) as the Glamaig, Meall Buidhe and Moll Shore intrusions. In addition to marscoite, two other hybrid rocks are identified within these composite intrusions: glamaigite and dioritic glamaigite.

The Glamaig intrusion consists of an inclined sheet cutting the granites and lavas SW of the summit of Glamaig and on Sron a' Bhealain. The attitude of this sheet is most readily determined in the Allt Daraich, where it dips at 45° to the NW. On Sron a' Bhealain, the dip is much less (7°), but is in the same general direction. The Meall Buidhe intrusion is exposed on the ground between the hill of that name and the Abhuinn Torra-mhichaig, further west, and forms an almost-vertical sheet intruded into older rocks. The Moll Shore intrusion is in many ways similar, and is readily examined along the road-cuts south of Maol na Gainmhich. The disposition of these intrusions suggests that they have the same steep-sided, incomplete ring-dyke form as the Marscoite Suite exposed in the southern portion of the centre (see above, and Section (6K), below).

The margins of all three of the northern intrusions consist of chilled marscoite, which grades over 1m into 'normal' marscoite. After a further 10–20m the rock takes on a heterogeneous, streaky (or net-veined) appearance, passing into, over the next 10m, a hybrid rock referred to by Wager et al. (1965) as glamaigite. This is the rock which Harker (1904) had earlier described as a xenolithic granophyre, believing it to have formed by the mixing of marscoite and a granitic magma (see below). Towards the centres of the three intrusions more homogeneous facies of this mixed rock are preserved, and Wager et al. (1965) referred to these as dioritic glamaigite.

Intimately associated with the centre of the Meall Buidhe glamaigite is an acid intrusion known as the Meall Buidhe Granite. The petrology of this somewhat variable granite is described in Section (6I), below.

Sections through the three intrusions are of the symmetrical form: marscoite-glamaigite-dioritic glamaigite-glamaigite-marscoite.

Xenoliths and xenocrysts are common within the northern marscoiteglamaigite intrusions. Andesinite is the commonest xenolith type, together with glomeroporphyritic patches of xenocrystic plagioclase. Also present are smaller, rounded masses of fine-grained basic material containing plagioclase phenocrysts with compositions of An50 and whole-rock compositions approximating to that of hawaiite.

From simple calculations, Wager et al. (1965) suggest that the northern marscoites of the Glamaig, Meall Buidhe and Moll Shore intrusions were generated by the mixing of 40% porphyritic felsite magma (Southern Porphyritic Felsite) with 60% basic (hawaiitic) magma.

The inhomogeneous nature of the glamaigites suggests that they too may have been formed by various mixing processes. For example, detailed microscopic studies show that the dark patches characteristic of glamaigite, most readily identified on weathered surfaces, and the lighter-coloured 'matrix' material have mineralogies similar to that of typical marscoite. Thus, glamaigite has at least two distinct parts, both essentially marscoites, and both containing xenocrysts of andesine, quartz and alkali feldspar. It is clear, therefore, that the inhomogeneous and homogeneous forms of glamaigite have had long, complex histories which have not, as yet, been studied in detail.

(I) The Northern Porphyritic Felsite and the Late Northern Granite of Meall Buidhe[edit]

The Northern Porphyritic Felsite crops out over an area of approximately 2km2, south of Meall Buidhe, and has many features in common with the Southern Porphyritic Felsite (see Sections (6G) and (6H), above). However, no equivalent granite can been correlated, at the present level of erosion, with this felsite. Similar to the Southern Porphyritic Felsite, the Northern Porphyritic Felsite contains phenocrysts of quartz and alkali feldspar (and in the same relative proportions), but differs in terms of its greater absolute amount of these phenocrysts. A further distinctive feature is the presence of small (2–10cm), often angular, fragments of basic material dispersed throughout this intrusion. This material is similar, but not identical, to the inclusions found within the Glamaig Granite (see Section (6D), above). The felsite intrudes the Meall a' Mhaoil agglomerates (see Section (6B), above) and has a thin crush zone where it is in contact with both older country-rock lavas and the Glamaig Granite.

The Meall Buidhe Granite occurs within the northern marscoiteglamaigite intrusion of Meall Buidhe, which crops out in the area between Meall Buidhe and the Abhuinn Torra-mhichaig (see Section (6H), above). The best exposures are in the Allt a' Bhealaich Bhric, where its close relationship with these hybrid rocks is readily noted. It is a microgranite, containing phenocrysts of oligoclase rimmed with altered alkali feldspar, set in a groundmass dominated by quartz and alkali feldspar. Also present are small quantities of pyroxene, a green amphibole, a green biotite, and a small amount of serpentinised olivine. These mineralogical features led Wager et al. (1965) to conclude that this intrusion is similar to the Marsco Granite, the youngest of the granites of the Western Red Hills Centre (see Section (6I), below).

(J) The late southern granite of Marsco[edit]

This intrusion crops out on the southern and western sides of Marsco (Wager et al. 1965; Thompson 1969). It is readily identified by its nonporphyritic character, is pale blue when fresh (pale brown when weathered), and may be examined on the southern slopes of Marsco, at the level of the Shelter Stone (Figure 8). The northern margin of this granite, against the slightly older ferrodiorite intrusion of the Marscoite Suite (see Section (6H), above), dips steeply to the south. These time-relationships are suggested by rare veins of Marsco Granite which penetrate the ferrodiorite, and by the disruption of members of the Marscoite Suite by the Marsco Granite in Coire nan Laogh. However, Thompson (1969) notes that where Marsco Granite is in contact with ferrodiorite there is evidence that the latter had not completely crystallised. Along such contacts are hybrid rocks, suggesting that some form of mixing process has taken place, albeit locally. The flat-lying roof of the Marsco Granite crops out at approximately 500m O.D., although in places apophyses of the intrusion vein and stope the older, overlying Southern Porphyritic Granite at higher levels (Figure 7).

A significant feature of the Marsco Granite is the presence of miarolitic cavities, especially within the roof zone of the intrusion. They are lined with quartz, alkali feldspar, amphibole and Fe-rich mica (Thompson 1969). The primary mineralogy of the intrusion consists of quartz, alkali feldspar and plagioclase (An24), together with clinopyroxene (ferrohedenbergite) and olivine in the lower parts of the intrusion, and amphibole near to the roof zone. Accessory apatite, zircon, allanite and Fe-Ti oxides are also present.

(K) Mechanisms of intrusion and volcanic processes[edit]

Much progress towards linking field and petrological aspects of the Western Red Hills intrusions has been made through the detailed studies of Thompson (1969, 1980).

Briefly, he makes the following observations: (1) Two distinct structural groups of granites are present, one elongated N-S (Glamaig, Beinn Dearg Mhor and Loch Ainort Granites), the other and later, oriented E-W. Members of the N-S group were intruded sequentially inwards, whilst the E-W group were emplaced sequentially outwards; (2) All of the intrusions are approximately co-focal about a point east of Loch Ainort; (3) The outcrop patterns of the early granites have the appearance of steep-sided ring-dykes which have been disrupted and partially destroyed by the later granites, whereas the later granites have shapes more akin to short arcs of circles, which taper towards their edges; and, (4) Permissive emplacement of the intrusions is strongly suggested by the general lack of disturbance of the surrounding country-rocks.

Thompson (1969, 1980) suggests the following cauldron subsidence model for the development of the Western Red Hills Centre. After the formation of the vent agglomerates and breccias (see Section (6B), above) and the intrusion of the sheet-like Marsco Summit Gabbro, the first of the granites was emplaced as a steep-sided mass in the Glamaig area. This was followed along its concave side by the Beinn Dearg Mhor and Loch Ainort Granites, which are petrographically and compositionally very similar. At this stage, it is postulated that the subsided, central block was jammed against the country-rocks along part of the ring-fracture zone, possibly in the vicinity of Glen Sligachan. Successive tilting of this block about an E-W oriented axis permitted the intrusion of the various members of the Marscoite Suite and the later granites. With each successive tilt of the block, similar, but not identical, intrusions would be emplaced in the northern and southern sectors of the centre. Thus, the distinctive differences between members of the Marscoite Suite preserved in the two parts of the complex might be explained.

In their study of the Marscoite Suite, Wager et al. (1965) conclude that the mixing events associated with the hybrid rocks marscoite, glamaigite and dioritic glamaigite occurred within a magma chamber at a level deeper than that exposed at present. They suggest that the two contrasting magmas, a lower basic one (either ferrodioritic or hawaiitic) and an upper acid magma (porphyritic felsite), existed as discrete components within a chamber because of their different densities. Convective currents within each of these magmas caused mechanical mixing to take place along their mutual boundary. The subsequent emplacement of batches of either the acid parent, the basic parent, a homogeneous hybrid (marscoite), or an inhomogeneous hybrid (glamaigite), in either the southern or northern sector of the centre, provides a suitable mechanism for explaining the various rock associations described in Section (6H), above.

More recently, Thompson (1980) and B.R. Bell (1983) have suggested that these mixing events may have been triggered-off by the injection of primitive, basic magma into a chamber already containing either acid magma or acid magma ponded above fractionating basic magma. Mixing between these contrasting magmas would produce the hybrids marscoite and glamaigite.

Another feature of this type of mixing process would be the entrainment of blebs of the basic liquid into the acid magma. Thompson (1980) suggests that the basic inclusions which typify the Glamaig Granite (see Section (6D), above) (and possibly the Northern Porphyritic Felsite) originated in this way.

The emplacement of these annular, hybrid intrusions (Glamaig Granite, marscoite and glamaigite) may also indicate a foundering of the central caldera block. Such an event would have allowed additional magma-mixing. Heat transfer from the basic magma would have caused extreme vesiculation of any unmixed acid magma still present within the chamber, resulting in the intrusion of magma along ring-fractures and surface venting. For instance, hybrid (mixed-magma) volcanic products are preserved within the sequence of pyroclastic rocks at Kilchrist, south of Beinn Dearg Bheag, 10km east of the Western Red Hills Centre (see Sections (8D) and (8E) of Chapter 8). It is likely that processes such as those described above were involved in their formation, although it is not possible to relate these deposits directly to the specific events involved in the formation of either the Glamaig Granite, marscoite, or glamaigite.

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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