The Srath Na Creitheach 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 5 The Srath Na Creitheach Centre[edit]

(A) Introduction[edit]

Figure 5 Geological sketch-map of the Srath na Creitheach Centre (modified from Jassim 1970)

The Srath na Creitheach Centre (Figure 5) is defined here as a group of three subvolcanic intrusions, the Meall Dearg, Ruadh Stac and Blaven Granites, which post-date the Cuillin Complex and pre-date the Western Red Hills Centre. These intrusions, covering an area of approximately 7km2, crop out in Srath na Creitheach, on the hills of Meall Dearg and Ruadh Stac, and on part of the west side of Blaven. The older, spatially-associated, pyroclastic rocks which crop out to the south and west of the granites are discussed in Sections (8B) and (8C) of Chapter 8.

Very little published data are available on these granite intrusions. Geikie (1894), in a discussion of the time-relationships between the Cuillin gabbros and the granites of the Red Hills, provides evidence that the former pre-date the latter. He also notes that riebeckite is present within one of the granites exposed on Meall Dearg. The conclusions of Geikie are amplified by Harker (1904), who also delineates the outcrop of the 'riebeckite-bearing granophyre' on his unpublished 6 inch to 1 mile map. Anwar (1950, 1955) presents an analysis of a clinopyroxene (ferrohedenbergite) from the other granite exposed on Meall Dearg (see below). Thompson (1965, 1969) briefly examined the granites of the Srath na Creitheach area and concluded that they define an older subvolcanic complex which is distinct from the Western Red Hills Centre further north. He also concludes that two intrusions may be defined, referring to them as the Meall Dearg and Ruadh Stac Granites. According to Thompson (1969), the dome-shaped Ruadh Stac Granite lies below the Meall Dearg Granite and pre-dates the latter. However, Jassim (1970) concluded that the reverse time-relationships are present, with the Ruadh Stac Granite chilling against, and hence post-dating, the Meall Dearg Granite.

The descriptions presented below draw mainly upon the studies of Thompson (1965, 1969) and Jassim (1970).

(B) General field relationships[edit]

It is clear from field observations that the three granites of the Srath na Creitheach Centre post-date the basic and ultrabasic rocks of the Cuillin Complex. Typically, contacts are steep, or vertical, with the development of marginal chill facies, locally containing spherulitic aggregates involving quartz and alkali feldspar. Also, offshoots of the granite intrusions invade the surrounding gabbros. These two features are particularly well-developed at the margin of the Ruadh Stac Granite, south of the summit of Meall Dearg, at Druim Hain.

In sharp contrast, the older gabbros and eucrites are intensely altered over distances of 10–40m from the granite contacts. The plagioclases are opaque and distinctly white, whilst the pyroxenes take on a greenish tinge, with the development of secondary chlorite and epidote.

Xenoliths of altered gabbro and eucrite are common within the marginal facies of the Blaven Granite, are less common in the Ruadh Stac Granite, and have not been reported from the Meall Dearg Granite.

The boundary between the Srath na Creitheach and Western Red Hills Centres runs along the southern face of Marsco at approximately 300m O.D. and is concave to the north. Along part of the contact a screen of crushed gabbro, approximately 400m long, separates the granites of the two centres. It is probable that this material was originally part of the Inner Layered Gabbro Series, and was subsequently caught up in the later granites.

The older pyroclastic rocks, in the immediate vicinity of the granites, have been subjected to intense brecciation and alteration. The various components of the pyroclastic sequence (tuffs, agglomerates, gabbro slabs, see Chapter 8) within this crush zone have reacted differently, with the fine-grained, fragmental material showing the greatest degree of alteration. Marginal felsite veins are common, cutting the pyroclastic rocks and giving rise to areas of 'hybridisation'. This type of alteration is particularly well developed SW of Loch an Athain, where a 50m-wide zone consisting of fine-grained fragments of basic and ultrabasic material surrounded by large patches of quartz and alkali feldspar in a granophyric intergrowth, together with abundant plagioclase and hornblende, can be observed.

(C) The Meall Dearg Granite[edit]

This granite crops out on the upper slopes of Meall Dearg and Ruadh Stac. It is most easily examined on Ruadh Stac, where it clearly overlies the Ruadh Stac Granite. The top of the intrusion has been removed by erosion. In general, the sheet-like Meall Dearg Granite dips to the east at an angle of approximately 30°. It may be identified in the field by its whitish-brown colour on weathered surfaces, whilst when fresh it has a slightly greenish tinge. Two facies of the granite have been recognised, occurring as interleaved sheets. These are: a fine-grained, hornblende-bearing granite; and, a slightly younger, coarser-grained, pyroxene-bearing granophyre. The contact relationships between these closely associated, flat-lying sheets are most readily observed on the northern slopes of Ruadh Stac, when viewed from Marsco.

In detail, the hornblende-bearing variety, exposed half way up the northern face of Ruadh Stac, consists of three distinct sheets. These sheets dip at a very shallow angle to the NE and are between 5 and 30m thick. The rock is aplitic, containing large needles of hornblende. Quartz dominates the rock, constituting up to 40% of the mode, together with approximately 10% of plagioclase-cored alkali feldspar phenocrysts, giving the rock a porphyritic appearance. Jassim (1970) suggests that some of the plagioclases have been derived from an older intrusion and are xenocrysts. In addition, xenoliths of gabbro and granite are present in various stages of digestion.

The pyroxene-bearing component of the Meall Dearg Granite has a much coarser appearance and also constitutes three sheets. The pyroxene is an Fe-rich monoclinic variety, ferrohedenbergite, which occurs in association with the Fe-rich olivine, fayalite (typically altered to chlorite + haematite). The groundmass is dominated by quartz and alkali feldspar in a granophyric intergrowth. These sheets have weathered more deeply than the hornblende-bearing variety and are slightly lighter in colour.

Although texturally distinct, the bulk mineralogies of the two varieties of the Meall Dearg Granite are very similar. The differences in the mafic mineral assemblages within the two facies are most easily attributed to differences in the relative water contents of the magmas during crystallisation. Thompson (1965) reports the presence of fluorite as an abundant accessory mineral in both facies of the Meall Dearg Granite.

(D) The Ruadh Stac Granite[edit]

The Ruadh Stac Granite has commonly been referred to as "The Riebeckite Granophyre" (Geikie 1894; Anwar 1955), although recent studies (Thompson 1965, 1969, 1976) suggest that the compositional range of the amphiboles is broader, from arfvedsonite to ferrorichterite.

This intrusion weathers, often deeply, to a pale brown, and underlies the Meall Dearg Granite, commonly with intervening slabs of gabbro present along the contact (see Section (5E), below). It crops out on both Ruadh Stac and Meall Dearg, up to heights of 300m O.D. Its base is not exposed. Fresh material is most readily examined in the stream beds of the Allt nam Fraoch-choire and the Allt Teanga Bradan, north and south of Ruadh Stac, respectively.

The texture present within this granite is extremely variable, from aplitic, through porphyritic, to granophyric. Sparse phenocrysts of subhedral perthite occur in a groundmass which is either usually granophyric or granitic in texture. Amphibole constitutes approximately 5% of the rock.

(E) The contact between the Meall Dearg and Ruadh Stac granites[edit]

Although some uncertainty surrounds the time-relationships of these two granites, some field-relationships are not in dispute. For instance, the junction is readily identified because of the distinctly different weathering characteristics of the two intrusions; the Meall Dearg Granite has a rugged, irregular appearance, whilst the Ruadh Stac Granite is darker, more fractured, and weathers to much smoother surfaces. Secondly, gabbro slabs are present along the contact between the two intrusions and are similar to those described in Section (5B), above.

A "needle-sharp" contact just above the gabbro slabs on the NW slopes of Ruadh Stac indicated to Jassim (1970) that the Ruadh Stac intrusion chilled against the Meall Dearg intrusion. Thompson (1965, 1969) suggests the reverse. Further studies are needed in order to resolve this anomaly.

(F) The Meall Dearg acid breccias[edit]

On Meall Dearg, acid agglomerates and breccias intrude the Meall Dearg Granite. They are considered by Jassim (1970) to be the result of volatile release closely associated with the incoming of the Ruadh Stac Granite. The main exposures are located approximately 400m NE of the summit, trending NW-SE, with a width of 50–100m and a length of 400m, tapering at both ends into narrow dyke-like masses.

The material ranges from coarse agglomerates with spheroidal weathering characteristics in the lowest exposures, to more fine-grained, angular fragments, best described as a breccia, near the top. The clasts are readily identified as the pyroxene-bearing material of the Meall Dearg Granite (see Section (5C), above).

Related to these rocks are numerous breccia dykes, generally less than 1m wide, which are common within the Meall Dearg Granite, but only rarely seen within the Ruadh Stac Granite. Fragments in these dykes are similar to those in the agglomerates and breccias described above, and are contained within a groundmass of comminuted acid material.

(G) The Blaven Granite[edit]

The Blaven Granite crops out on the lower, western slopes of Blaven, from the northern end of Loch na Creitheach, north to Coire Dubh. On its east side, the granite is in contact with rocks of the Outer Layered Eucrite Series, whilst to the west it cuts pyroclastic rocks. Both contacts are steep, dipping to the NE.

A marginal facies of more felsitic material is commonly present, often up to 10m wide. Along the granite-eucrite contact numerous slabs of eucrite are present within the granite. These slabs are up to 100m in length and show signs of extreme alteration.

The Blaven Granite is generally light-coloured when fresh, of medium grain-size (1–2mm), and contains phenocrysts of alkali feldspar. The dominant mafic mineral is a green hornblende, typically altered to chlorite, which occurs in irregularly-shaped clots.

(H) Shape and mechanism of intrusion of the granites[edit]

From the studies of Thompson (1965) and Jassim (1970), it is clear that the Ruadh Stac Granite is dome-shaped and underlies the sheet-like Meall Dearg Granite. According to Jassim (1970), the Meall Dearg Granite has an 'inverted L' shape, forming an incomplete ring-dyke, with the steep limb located east of the summit of Ruadh Stac (against the Outer Layered Eucrite Series), and the flat-lying roof-rocks exposed on the summits of Ruadh Stac and Meall Dearg. The presence of two varieties of the Meall Dearg Granite (see Section (5C), above) indicates multiple injections of magma.

The almost-vertical, sheet-like shape of the Blaven Granite suggests that the level of erosion is deeper for this intrusion, when compared to the other granites of the centre, with the outcrop pattern being that of the limb of a very incomplete ring-dyke.

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