The Eastern Red Hills Centre, Skye Central Complex

From Earthwise
Jump to navigation Jump to search
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.

Chapter 7 The Eastern Red Hills Centre[edit]

(A) Introduction[edit]

The Eastern Red Hills Centre (Figure 9) is the last of the four recognised foci of subvolcanic activity on Skye and occupies the ground between Loch Ainort in the NW, and Heast in the SE. This relatively large area contains numerous intrusive and extrusive units, although a significant proportion of the rocks which crop out are pre-Tertiary in age (see Chapter 2), acting simply as hosts to the intrusions.

The presence of a distinct intrusive centre in the area around Beinn na Caillich (now defined here as the Eastern Red Hills Centre) was considered likely by Harker (1904), although he did not define in detail the various components. Further thoughts on the validity of an Eastern Red Hills Centre were presented by Richey (1932), who defined an 'outer ring granophyre', composed of the granites exposed on Beinn na Cro and Beinn an Dubhaich, which surround the boss-shaped 'central mass' of Beinn na Caillich, also composed of granite. Richey also drew attention to a large suite of composite sills which form an incomplete arcuate belt, co-focal to the granite of Beinn na Caillich.

Subsequently, Stewart (1965) presented details of the time-relationships of the various units which he considers to be part of the centre. Similar to the Western Red Hills Centre (see Chapter 6), unlayered gabbros pre-date the granites, although in this case an additional early intrusion is the Kilchrist Ring-dyke, which is a hybrid (mixed-magma) rock. At a later stage the three large subvolcanic granites of the district were emplaced, followed by the suite of composite sills and dykes.

More recently, J.D. Bell (1976) has outlined the time-relationships of the rock-units of the centre and the following table is modified from that work:


INTRUSIVE EVENTS PRINCIPAL ROCK-TYPE
(YOUNGEST)
(dykes pitchstone, felsite, dolerite, peridotite)
Composite sills and dykes granophyre, felsite, basalt
Inner Granite (Beinn na Caillich) amphibole, biotite granite/granophyre, felsite
Outer Granite amphibole, biotite granite/granophyre, felsite
Glas Bheinn Mhor Granite amphibole, biotite granite/granophyre, felsite
Broadford Gabbro gabbro (altered)
Beinn na Cro Gabbro olivine gabbro/eucrite
Kilchrist Hybrid Ring-dyke acid-basic mixed-magma
(OLDEST)


(B) The Kilchrist Hybrid Ring-dyke[edit]

The Kilchrist Hybrids constitute five discrete outcrops of intrusive material in the area between Loch Cill Chriosd (Kilchrist) and Torrin, in the district of Strath, and were first described by Harker (1896, 1904). Three of these, the Eastern, Western and Southern intrusions, form an incomplete ring-dyke which surrounds various pyroclastic rocks (see Sections (8D) and (8E) of Chapter 8), whilst exterior to them are country-rock Cambro-Ordovician and Jurassic sedimentary rocks. The contacts against the country-rocks dip steeply outwards, whilst the inner margins of these hybrid rocks, against the pyroclastic rocks, are relatively flat-lying and in the form of sheets of more fine-grained material. The other two masses form steep-sided intrusions within the pyroclastic rocks and are referred to as the Cnoc nam Fitheach and Coire Forsaidh intrusions. Only the Eastern, Cnoc nam Fitheach and Coire Forsaidh masses form prominent topographic features; the first of which crops out on Creagan Fitheach, whilst the latter two are exposed in the areas defined by their names (Figure 9). The relationships of the Eastern, Western and Southern intrusions, which are possibly connected at depth, to those of Cnoc nam Fitheach and Coire Forsaidh, are obscure. The latter two masses do not appear to be related to a ring-dyke structure, and may simply be discrete masses of the same material intruded into the central block of pyroclastic rocks.

The hybrid (mixed-magma) nature of the Kilchrist Ring-dyke is readily noted in the field. Particularly obvious in this leucocratic to mesocratic, medium grain-sized rock are irregular crystals of quartz (2–4mm) fringed by clusters of amphibole and/or pyroxene, together with slightly larger (3–5mm), rounded crystals of white (altered) alkali feldspar. Thin-section studies confirm the field observations and suggest that disequilibrium conditions were responsible for their development. Also present are larger (up to 3cm) xenoliths of fine-grained basic material with irregular and often diffuse margins (similar in many respects to the inclusions present within the Glamaig Granite, see Section (6D) of Chapter 6). From a detailed study of these basic inclusions B.R. Bell (1982) concluded that their presence within the intrusion is a consequence of the mixing of a basic magma with an acid magma.

These hybrid rocks are similar to the mixed rocks of the Marscoite Suite of the Western Red Hills Centre (see Section (H) of Chapter 6) and are probably the result of phenocryst phases within porphyritic acid and basic magmas reacting and dissolving in a 'new' magma derived by the mixing of the two contrasting parent magmas. However, unlike the mixed rocks of the Marscoite Suite, no acid and basic rocks occur in obvious close spatial or temporal association with the Kilchrist Hybrids. Nevertheless, in a more general study of acid-basic mixed-magma intrusions, B.R. Bell (1982) was able to show that by combining specific amounts of the basic and acid components of a typical composite sill of the district (see Section (7H), below), it is possible to derive whole-rock compositions similar to a typical Kilchrist Hybrid.

One interesting feature of the hybrids is the development of narrow, flow-banded, marginal facies, some of which have compositions more acidic than the typical hybrid. These rocks may be examined, for example, in the Allt Coire Forsaidh, where they occur along the inner margin of the Eastern intrusion. This material may have been intruded prior to the main mixed-magma ring-dyke and may represent some of the acid parent involved in the mixing process. Also present within the fine-grained facies are fragments of pyroclastic material, most likely entrained from the Kilchrist deposits.

On the basis of general field relationships, although no definite contacts are visible, the Kilchrist Hybrids probably pre-date the crosscutting Inner Granite (see Section (7G), below) and constitute a small, discrete, subvolcanic centre which entirely pre-dates the granite-dominated Eastern Red Hills Centre (s.s.).

(C) The Beinn na Cro Gabbro[edit]

The Beinn na Cro Gabbro is a coarse-grained, unlayered, basic intrusion with tholeiitic affinities, varying in composition between an olivine-poor gabbro and an olivine-rich eucrite. It consists of several thick sheets of relatively flat-lying material intruded into basic lavas along the N-S -trending ridge of Beinn na Cro (see Section (3D) of Chapter 3). The Glas Bheinn Mhor Granite (see Section (7E), below) has subsequently intruded and altered both the gabbro and the lavas and it is not, therefore, possible to determine the original extent of either rock-type.

The gabbro is most easily examined in the deep gullies NW of the summit of Beinn na Cro, where the complex inter-relationships between the gabbro, lavas and granite are readily noted. Also present are numerous basic dykes which cut both the lavas and the gabbro, but predate the Glas Bheinn Mhor Granite.

There is very little published data on the Beinn na Cro Gabbro, although Harker (1904) does discuss its form briefly. In terms of its petrography and whole-rock composition, it is similar to the Broadford Gabbro (see Section (7D), below).

(D) The Broadford Gabbro[edit]

The Broadford Gabbro crops out over an area of approximately 3km2, NW of Broadford. Along its eastern margin it is bound by Cambro-Ordovician and Jurassic strata, the latter in a faulted relationship, whilst further west the gabbro is cut by the later Outer Granite. In the area around Creag Strollamus the field relationships are complicated by the presence of the Kishorn Thrust Plane (see Section (10B) of Chapter 10) and in consequence there is some doubt about the shape of the intrusion (see below).

Harker (1904) mapped the area in detail and showed that the gabbro is intimately associated with the Cambro-Ordovician carbonates (Ben Suardal Dolostones, see Section (2D) of Chapter 2) which crop out east of the Kishorn Thrust Plane. Throughout its exposure, the gabbro contains numerous small (typically less than 50m across) enclosures of the carbonates, which form small topographic depressions within the harder igneous rock. Harker concluded that the intrusion is boss-shaped, with steeply inclined margins. In contrast, King (1953a) and Bailey (1954a) suggest that the gabbro is probably sheet-like.

Along the northern slopes of Creag Strollamus the gabbro extends towards the NW as a dyke-like mass with a width of approximately 200m. This elongate part of the intrusion tapers NW of Creag Strollamus and does not reach the coast. Towards the margins of the intrusion there are doleritic and basaltic facies. The common presence of 'acidified' and 'granite-veined' facies suggested to King (1953a) that some form of hybridisation process had taken place between the gabbro and the later, closely-associated Outer Granite.

The Broadford Gabbro is an unlayered, medium- to coarse-grained rock composed of clinopyroxene and zoned plagioclase (An62–67, King 1953a) in an ophitic to subophitic arrangement. Olivine has not been reported from this intrusion. Local variations in the modal percentages of these two minerals give rise to pyroxene- and plagioclase-rich facies. In general, the gabbro has been severely hydrothermally-altered by the later granites, with the resultant development of secondary mineral assemblages. Commonly, pyroxene is replaced by aggregates of amphibole + chlorite + epidote. Marginal facies of the intrusion locally show the development of shearing and brecciation, possibly due to volatile release during intrusion.

The mechanism of emplacement of this gabbro is not easily envisaged. King (1953a) suggests that it was formed by the metasomatic replacement of basic lavas. Such a hypothesis appears unlikely for several reasons, especially because of thermal constraints.

Another method, which alleviates the 'space problem' and provides a suitable mechanism for explaining the intimate relationship between the gabbro and the enclosures of carbonate, is to suggest that the gabbroic magma was intruded into limestone caves, which had developed at some previous stage. Significantly, Harker (1904) found no enrichment of CaO in the gabbro, indicating that assimilation of Cambro-Ordovician carbonates during emplacement was, in this instance, highly unlikely.

(E) The Glas Bheinn Mhor Granite[edit]

The Glas Bheinn Mhor Granite is the first of the large subvolcanic acid intrusions of the Eastern Red Hills Centre. It crops out on the hill of that name and clearly cross-cuts members of the older Western Red Hills Centre (for example, the Loch Ainort Granite and the Marscoite Suite, see Chapter 6). The eastward extent of this granite has not been determined in detail, although it certainly crops out on the low ground west of the Beinn na Cro ridge. Its outer (western) margin is steeply inclined, dipping to the west, and can be traced from Sron Ard a' Mhullaich, at the head of Loch Ainort, south to the col between Belig and Glas Bheinn Mhor (J.D. Bell 1966).

In hand-specimen, the Glas Bheinn Mhor Granite weathers dull grey and is distinctly porphyritic, with phenocrysts of normally zoned plagioclase (An20–27, J.D. Bell 1966) set in a fine-grained groundmass which is composed of a granophyric intergrowth of quartz and alkali feldspar. The dominant mafic minerals are amphibole and brown mica, although primary pyroxene, commonly replaced by secondary amphibole, has been found (J.D. Bell 1966). Accessory apatite, zircon, sphene, Fe-Ti oxides and fluorite are also present.

Irregularly dispersed throughout the intrusion are small clots of mafic minerals, dominated by amphibole, brown mica and Fe-Ti oxides. These possibly may represent partially-digested inclusions of mafic material: a feature reported from other granites within the Skye Centre and discussed in detail in Sections (6D) and (6K) of Chapter 6.

(F) The Outer Granite[edit]

The Outer Granite of the Eastern Red Hills Centre is defined here as consisting of the acid intrusions which crop out in the arcuate belt which runs from Beinn na Cro in the west, south to Beinn an Dubhaich, and thence around the eastern side of Beinn na Caillich to the Allt Fearna Creag Strollamus area. These separate granite masses were tentatively grouped together by Richey (1932) and subsequent work by Stewart (1965) and B.R. Bell (1982) agree with this interpretation. The intrusion is slightly porphyritic, with phenocrysts of alkali feldspar and quartz, and contains hornblende and biotite. Depending upon the amounts of these ferromagnesian minerals, both grey and reddish-brown weathered surfaces are found. The country-rocks which host the different parts of this annular intrusion exhibit a wide range in composition and age.

Three main parts of the intrusion may be identified: (i) Beinn na Cro; (ii) Beinn an Dubhaich; and, (iii) Allt Fearna–Creag Strollamus.

(i) Beinn na Cro—has a steep outer contact against the older Glas Bheinn Mhor Granite (see Section (7E), above), and an almost vertical contact against a screen of pyroclastic rocks and rhyolitic lavas (see Sections (8D) and (8E) of Chapter 8 and Section (3E) of Chapter 3, respectively), which separates it from the younger Inner Granite (see Section (7G), below).

(ii) Beinn an Dubhaich—a much studied granite which is wholly intruded into a broad anticline of Cambro-Ordovician carbonates, and which contains numerous enclosures of these country-rocks. There is little evidence of stoping or tectonic disturbance associated with the intrusion of this granite. Dykes which cut the large enclosures of country-rock show little sign of deviation from the regional trend (see Section (9B) of Chapter 9). Local variations in the attitude of the granite country-rock boundary have led to different interpretations being placed on the general shape of the Beinn an Dubhaich intrusion. Harker (1904) suggested that it was a steep-sided mass, and similar conclusions have been reached by Stewart (1965), Raybould (1973) and B.R. Bell (1982). In contrast. King (1960) and Whitten (1961b) favour a sheet-like structure, intruded along the Kishorn Thrust Plane, dipping at a shallow angle to the SE (see Section (2D) of Chapter 2 and Section (10B) of Chapter 10). With the first interpretation the enclosures of Cambro-Ordovician country-rocks represent roof-pendants, whilst those who favour the sheet-like model claim they represent irregularities in the Cambro-Ordovician floor, which lies below the base of the granite sheet. Drilling through one of these enclosures of country-rock (reported in Raybould 1973) encountered granite below, which adds considerable support to the 'steep-sided contact' model. In addition, Hoersch (1979), on the basis of a ground magnetometer study, concluded that the intrusion is a steep-sided stock and extends to depth. She also suggests that lobes of the granite penetrate the area to the north.

Tilley (1949) reported the presence, locally, of alkali pyroxene -bearing marginal facies of the Beinn an Dubhaich mass, associated with the development of skarns in the carbonate country-rocks (see Section (7I), below) and Raybould (1973) found four distinct varieties of granite within the intrusion, namely: porphyritic microgranite, granophyre, pale green granite, and hornblende granite. Tuttle and Keith (1954) and Tuttle and Bowen (1958) examined the inversion temperatures of quartz and feldspar crystals from the intrusion and concluded that during crystallisation high-temperature features had been 'quenched-in', such as would be expected if the granite had crystallised from a rhyolitic magma, and had not been formed by lower-temperature metasomatic processes. The thermal and metasomatic effects of this granite on the surrounding country-rocks are also discussed in Section (7I), below.

(iii) Allt Fearna–Creag Strollamus—consisting of a number of small granite, micro-granite and porphyritic felsite intrusions which cut various pre-Tertiary country-rocks, as well as the older Broadford Gabbro. The boundaries of these masses are complicated by the presence of the Kishorn Thrust Plane, which dips at a shallow angle to the east. Harker (1904) and King (1953a) note that in places granite has been intruded along the thrust plane. King (1953a) further suggested that much of the granite formed by in situ metasomatic replacement of original Torridonian strata, but this hypothesis was rejected by Stewart (1965) on several grounds, but principally because of vertical contacts between the granite, the Broadford Gabbro and Cambro-Ordovician carbonates, as seen in tributaries of the Allt Fearna. Where granite and gabbro are in contact, zones of hybrid material have developed (see Section (7D), above). The contact between the main intrusion of this granite (in the Allt Fearna–Creag Strollamus area) and the Inner Granite (see Section (7G), below) may be traced from Buaile nan Aodan (SE of Beinn na Caillich), towards the plateau area around Lochain Beinn na Caillich, where it follows the sharp break in slope at around 250m O.D., west to Creagan Dubh. This change in slope provides the most useful field evidence as to the position of the boundary between these two granites. More detailed studies indicate that the Inner Granite is frequently chilled against the older Outer Granite, thereby allowing the contact to be readily distinguished (see Section (7G), below).

The irregularly-shaped acid intrusion which crops out at An Sithean, SSW of Broadford, is considered to be part of the Outer Granite and is, in places, extremely fine-grained (essentially a porphyritic felsite).

(G) The Inner Granite[edit]

The Inner Granite is the last of the large subvolcanic intrusions of the Eastern Red Hills Centre and crops out on the three prominent summits WSW of Broadford, namely, Beinn na Caillich, Beinn Dearg Mhor and Beinn Dearg Bheag. This intrusion has a near-perfect circular outline, but is 'truncated' by a NE-SW -trending fault-line at Creagan Dubh (north of Beinn Dearg Mhor). The contact of the Inner Granite with the surrounding country-rocks is always steep and there is commonly the development of a fine-grained chill facies (see below). Along its western, southern and eastern margins the granite intrudes various pyroclastic rocks (see Sections (8D) and (8E) of Chapter 8), whilst to the north it is in contact with the Outer Granite (see Section (7F), above).

Miarolitic cavities are a common feature within this intrusion. They are particularly well-developed in the summit areas of the three hills noted above, presumably because they are close to the roof zone. Present within these cavities are euhedral crystals of quartz, alkali feldspar and, occasionally, fluorite. Two mineralogical facies of the granite are readily identified in the field. First, a normal, coarse-grained facies, consisting of phenocrysts (3–5mm) of quartz and alkali feldspar, set in a groundmass which is dominated by the development of a granophyric intergrowth involving the same two minerals. The alkali feldspar is generally clouded. Also present are laths of oligoclase (An20–25). The main ferromagnesian minerals are hornblende and biotite, often occurring as small clusters associated with Fe-Ti oxides. Zircon, sphene, and apatite are present as accessory minerals. The fine-grained chill facies of this intrusion has only developed locally, and typically does not exceed 5m in thickness. It is most readily examined in the gorge at the head of the Allt Slapin, north of Torrin, where the burn cuts through the contact between the granite and various brecciated acid intrusions (see Section (9O) of Chapter 9). This marginal fades is a felsite which, locally, veins the country-rocks. Occasionally, extreme chilling against the country-rocks has led to the development of spherulitic masses (1–2mm) composed of quartz and alkali feldspar set in a granular, felsitic groundmass. The same phenocrysts as in the normal granite are present, although they are typically smaller (1–2mm). The main ferromagnesian phases are microphenocrysts (0.5–1mm) of the Fe-rich clinopyroxene, ferrohedenbergite, and an almost pure Fe-olivine, fayalite. Alteration of the ferrohedenbergite to aggregates of chlorite, epidote and actinolite, and of the fayalite to serpentine and/or chlorite, is common.

The differences in the mafic mineral assemblages present within the two facies of the granite may be explained in terms of the bulk composition of the magma involved. Essentially, the acid magma when intruded was close to its liquidus temperature, containing relatively few phenocrysts. Experimental studies (for example, Bowen and Schairer 1935 and Morse 1980) suggest that under such conditions acid magmas with low CaO contents and high Fe/Mg ratios will crystallise olivine and pyroxene in preference to hornblende and biotite. In addition, the relative amount of volatiles will be low during the early stages of crystallisation and this will further restrict or prevent the formation of hydroxyl-bearing minerals. Subsequent crystallisation would decrease the Fe/Mg ratio of the magma and increase its relative CaO and volatile contents. Eventually, conditions would prevail that allowed hornblende and biotite to become the stable ferromagnesian minerals. Compositional aspects of the granites are discussed further in Section (12D) of Chapter 12.

(H) The composite sills and dykes[edit]

Composite sills and dykes constitute an important component of the Eastern Red Hills Centre, cropping out over an arc at least 15km long, from Rubha Suisnish in the south, to Rubha na Sgianadin in the north, and possibly continuing through Scalpay to Loch Sligachan. The radius of the circle, of which this arc forms almost a half, is approximately 5km and is co-focal with the centre of the Inner Granite, suggesting a close spatial (and possibly genetic) relationship between the composite intrusions and the granites. These intrusions were first described in detail by Harker (1904), who notes over twenty discrete exposures, all of which occur within strata of Liassic age (see Section (2F) of Chapter 2).

Harker (1904) identifies various types of minor intrusions which are composed of acid and basic components, in an intimate association. The suite of sills and dykes described here are referred to by Harker (1904) as being of the Cnoc Carnach Type. They are typically composed of three distinct units: upper and lower basic portions which show evidence of chilling against the country-rock strata, and a central acid portion (granophyre, microgranite or felsite) which shows no evidence of chilling against the fine-grained, basic material. All of the sills are either flat-lying or dip at a shallow angle (approximately 15°) towards the Inner Granite. The basic components have thicknesses of 0.5–2.5m, whilst the acid centres have much greater thicknesses, often 50m, or more. In many instances, erosion has stripped off the upper basic unit and part of the acid centre. For example, the sills which crop out on Cam Dearg, Beinn nan Cam and Cnoc Carnoch, itself, are in part eroded down to this level. Numerous faults disrupt and complicate the relatively simple field relationships of these intrusions.

Evidence for the intrusion of the acid magma after the basic magma includes the presence of fragments of the latter within the former, together with veins of felsite and microgranite cutting the basic portions (for example, within the upper basic unit of the Cnoc Carnach intrusion, along its western side).

The composite dykes associated with the sills are presumed to have acted as feeders to the sills. They have similar geometries, with marginal, fine-grained, basic portions flanking a thicker, central, acid unit. Numerous examples of these dykes are preserved, although the intrusion at Loch Fada, north of Cam Dearg, provides the most clear-cut evidence for the close genetic relationship between the dykes and the sills.

Petrological features of these intrusions are most readily understood from a study of the 5m-thick composite sill which crops out at Rubh' an Eireannaich, on the west side of Broadford Bay. The descriptions presented below are based on the studies of Harker (1904), Buist (1959) and B.R. Bell (1982, 1983). The sill has upper and lower basic units (each 1.3–1.4m thick) and a central porphyritic felsite (approximately 2.4m thick). Unlike the majority of the sills, boundaries between the basic and acid parts are completely gradational, typically over a distance of 10- 20cm. The upper and lower contacts, with the Liassic country-rock strata, exhibit strong chilling, producing a glassy (now devitrified) basic rock. This grades over a few millimetres into a fine-grained, dark grey rock which contains phenocrysts of labradorite and xenocrysts of oligoclase (with rounded edges) and alkali feldspar (exhibiting the so-called fingerprint texture, indicating disequilibrium), all set in a groundmass dominated by augite, plagioclase (andesine-labradorite) and Fe-Ti oxides. The central portion of the sill is light grey to greenish-white and may be described as a porphyritic felsite, with phenocrysts of oligoclase and rare alkali feldspar, set in a felsitic groundmass composed of quartz and alkali feldspar. Hydrothermal alteration of the central felsite is, in places, severe, as evidenced by the presence of secondary calcite, chlorite and pyrite. Between the contrasting basic and acid rock-types. 10–20cm-thick hybrid zones exhibit a complete compositional spectrum. Nearest the basic units, xenocrysts of oligoclase and alkali feldspar within the hybrid are set in a groundmass dominated by augite, plagioclase and Fe-Ti oxides. Towards the acid centre of the sill, these minerals grade imperceptibly into phenocrysts (with no evidence of rounding or magmatic corrosion), set in a felsitic groundmass.

These features suggest strongly that two periods of hybridisation of acid and basic magmas have taken place. First, there was the incorporation of an acid magma into a basic magma, within a magma chamber which existed at a depth greater than the present level of erosion, resulting in the resorption of any phenocrysts which were present within the acid magma. The rounded and corroded oligoclases and alkali feldspars within the basic portions of the composite sills represent this process in an arrested state. The absence of quartz xenocrysts within the basic units suggests that either quartz was not present as a phenocryst phase in the acid magma involved, or that it was totally dissolved during the mixing. Second. there was in situ hybridisation, caused by diffusion of cations between the two contrasting magmas after their emplacement, and whilst they were still close to their liquidus temperatures. The extent and nature of this hybridisation process has not, as yet, been fully quantified.

B.R. Bell (1983) has shown, however, that, for all elements determined, complete compositional gradients exist between the basic and acid portions of the Rubh' an Eireannaich composite sill. In addition, rareearth-element patterns are similar for the basic and acid portions, indicating that the two magmas were co-genetic and, therefore, that the hybridisation processes involved were some form of re-mixing event.

In Section (7B). above, it was suggested that compositions very similar to those of the Kilchrist Hybrids could be generated by combining specific proportions of the basic and acid members of the composite sills found in the district. The compositional similarities between these two groups of hybrid (mixed-magma) intrusions, within the same intrusive centre, suggests that they may be genetically linked, although no comparative studies have. as yet, been published.

(I) Thermal and metasomatic effects of the granites[edit]

Metamorphic and metasomatic effects caused by the various intrusions of the Eastern Red Hills Centre are recorded in the Cambro-Ordovician and Jurassic strata of the district (see Sections (2D) and (2F) of Chapter 2). The features within the Cambro-Ordovician rocks are, however, more widespread and will be considered first.

The whole of the Beinn an Dubhaich and parts of the Allt Fearna–Creag Strollamus mass of the Outer Granite (see Section (7F), above) intrude Cambro-Ordovician carbonates. Both thermal metamorphic and metasomatic alteration of the carbonates have resulted and these features are discussed in detail by Tilley (1947, 1948a,b,c, 1949, 1951) and Hoersch (1977, 1979, 1981).

The Cambro-Ordovician strata within the Strath district are described in Section (2D) of Chapter 2. Three units are identified: (1) The Strath Suardal Dolostone (oldest); (2) The Ben Suardal Dolostone; and, (3) The Beinn an Dubhaich Dolostone, which is only exposed in the vicinity of the granite. Apart from simple recrystallisation, within the carbonates are cherty nodules which, to varying extents, have also been involved in metamorphic and metasomatic processes. Hoersch (1981) showed that these nodules (70% quartz, 20% dolomite and 10% calcite) reacted with dolostone (99% dolomite and 1% calcite) to form reaction rims of calcsilicate minerals. In terms of the incoming index minerals, which appear with increasing temperature as the contact with the granite is approached, Hoersch (1981) has identified four distinctive zones: (1) talc-bearing (lowest temperature, 350–425°C); (2) tremolite-bearing (425–440°C); (3) diopside-bearing (440–520°C); and, (4) forsterite-bearing (highest temperature, 520–600°C).

The first signs of metamorphism in the dolostones is the growth of talc and calcite at chert-dolostone (nodule-matrix) boundaries (Zone 1). This metamorphic reaction will continue until one of the reactants (generally the chert nodule) is totally consumed. Within the second zone tremolite has developed, occurring within nodules irrespective of whether silica has been, or has not been, totally used up. Within the third zone, the incoming of diopside is typically at the expense of any talc remaining within the nodule structure, together with some of the tremolite. Finally, nearest the granite, small forsterite grains have developed within diopside crystals (Zone 4). Within 100m of the contact the nodules take the form of alternating layers of forsterite (plus a small amount of calcite) and pure calcite. Tilley (1951) suggests that this may be due to a Leiesgang phenomenon (secondary nested rings or bands caused by rhythmic precipitation within a fluid-saturated rock), although Hoersch (1981) points out that these bands may be the product of original zonation patterns present within the nodules.

Only the inner-most isograd (separating the diopside and forsterite zones) can be traced with confidence in the field, running from the northern end of Loch Cill Chriosd, east to the area south of Ben Suardal, thence SW towards the area west of Loch Lonachan (Hoersch 1981). Because of the discontinuous nature of the exposure the other isograds cannot readily be traced. Furthermore, the sub-surface shape of the granite (see Section (7F), above) may mean that distances from the boundary, at the present level of erosion, do not actually represent true distances from the contact (for example, consider the possibility that the granite occurs at no great depth below Ben Suardal).

During the metamorphism the pressure was approximately 500bars (equal to a depth of 2km). Detailed mineralogical data and thermodynamic calculations indicate that the proportion of CO2 present within the metamorphic fluid greatly influenced the reactions which took place (Hoersch 1981).

Along the granite-carbonate contact skarns developed sporadically, with widths typically less than 3m. They have formed as a result of the movement of various species from the crystallising acid magma and the heated country-rock carbonates, with deposition occurring within the contact zone. At the contact, the granite contains an alkali-rich pyroxene (in contrast to the normal mafic minerals, hornblende and biotite, see Section (7F), above). Tilley (1949) considered the pyroxene to be the product of a net removal of Al from the granitic magma into the carbonate country-rocks. In order to form the various skarn mineral assemblages outlined below, Si, Al, Fe, B and F (within a fluid phase) must have moved out from the granite (Tilley 1951).

In his detailed investigation of these skarns, Tilley (1951) described over ten discrete outcrops from around the Beinn an Dubhaich mass of the Outer Granite (see Section (7F), above). Moving towards the granite, carbonates which have simply been metamorphosed (forsterite-bearing) typically give way to zones rich in forsterite and diopside (1–2cm), and then pass into a hedenbergite-plagioclase-wollastonite rock of metasomatic origin.

Tilley (1951) recognises two main groups of skarns, characterised by their different mineral assemblages: Group (I), at the granite—country-rock contact and not containing any fluorine- or boron-bearing minerals; and, Group (II), on the country-rock side of Group (I) assemblages and containing fluorine- and boron-bearing minerals, together with magnetite and monticellite. Group (I) skarns can be independent of Group (II) skarns, but this is not common. Group (I) assemblages include: grossular garnet + wollastonite and hedenbergite + plagioclase (see above). The commonest Group (II) assemblage consists of magnetite + forsterite + chondrodite + clinohumite + diopside + grossular garnet + clinochlore. The boron- and fluorine-bearing minerals within the Group (II) skarns include: fluoborite, szaibelyite, datolite, ludwigite, and harkerite (boron-bearing); and, chondrodite, clinohumite, cuspidine and fluoborite (fluorine-bearing). The source of the boron and fluorine is presumed to have been the crystallising acid magma.

The commonest (and most obvious) skarns involve magnetite, examples being the deposits south of the old manse at Kilchrist, and at Kilbride, south of Ashbank.

Studies of the granite-carbonate contact by ground magnetometry identified over 25 'anomalies', suggesting that several magnetite-bearing skarns may be present at no great depth below the present level of erosion (Whetton and Myers 1949).

In the Beinn an Dubhaich area dolerite dykes intrude the country-rock carbonates but are cut by the later granite. These dykes have been subjected to Ca-metasomatism during the alteration of the carbonates (see above). This metasomatism is readily noted within the dykes south of the old manse at Kilchrist, where primary pyroxene has been replaced by a secondary brown amphibole.

Mineral assemblages of metasomatic origin are also found within the marginal facies of the granite. In addition to the alkali-rich pyroxenes, as described above, Tilley (1951) records pods and veins of clinopyroxene (diopside to hedenbergite), plagioclase (oligoclase to andesine), fluorite, grossular and andradite garnet, idocrase, epidote and allanite from within the granite south of the old manse at Kilchrist.

The only other metamorphic or metasomatic effects attributable to the granites of the Eastern Red Hills Centre are found within the Jurassic Bearreraig Sandstone Formation (see Section (2F) of Chapter 2) south of Dunan, between Glas Bheinn Bheag and the Allt Strollamus. The boundary between the granite and the country-rock strata was originally described by Day (1931), who concluded that metasomatic processes were involved during the formation of the associated hornfelses, and that the contact is intrusive. Black (1955), however, concluded that it is a fault. Furthermore, Black suggested that the contact-rocks are not true hornfelses which developed in response to high heat flow from the granite, but are cataclastic in origin (essentially a fault breccia). The net additions of Fe, Mg, Ti and H2O which he determined were considered to have been deposited from volatiles streaming along the granite–country-rock boundary.

Smith (1960) re-investigated these contact rocks and defined six zones of increasing metamorphic grade, ranging from strata showing very slight amounts of recrystallisation, through to hornfelses, nearest to the granite, containing feldspar porphyroblasts. To produce such effects Smith (1960) concluded that during the development of these rocks there was a net removal of K, Na, Mg, Fe and H2O from the granite, as it crystallised, into the country-rocks.


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]

At all times follow: The Scottish Access Codeand Code of conduct for geological field work