Caledonian minor intrusions, Younger Caledonian igneous rocks, Northern Highlands of Scotland

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From: Johnstone, G S and Mykura, W. 1989. British regional geology: the Northern Highlands of Scotland (4th edition). (Nottingham: British Geological Survey.)

Caledonian minor intrusions, introduction

Throughout the Northern Highlands there are minor intrusions (dykes, sheets and small bosses). A few were affected by the last Caledonian folding; others were sheared and recrystallised during late-stage minor movements when regional temperatures were still elevated; others still are entirely post-tectonic. All, however, are earlier than the deposition of the Old Red Sandstone cover. They are sparsely distributed in the northern part of the area but are found in large numbers south of the Glen Moriston–Glen Shiel through valley. In early descriptions, such as those given in the one-inch Geological Survey memoirs of the northern part of the district, they were thought to represent members of one connected suite. Post-war mapping in the southern area, where they are abundant, has shown that they belong to two suites, the entirely post- tectonic Minette Suite (including the felsites), and the Microdiorite Suite, which is late- to post-tectonic in age. D. I. Smith (1979) has described their distribution, relationships to tectonism and varying metamorphism (see also Talbot, 1983).

The Microdiorite Suite

Rocks of the Microdiorite Suite show a continuous transition from leucocratic felsic porphyrite though quartz microdiorite to microdiorite, and thence through mafic microdiorite to a variable appinitic group consisting mainly of hornblende diorite. The more felsic varieties of the suite cut the more mafic varieties, and both felsic porphyrites and microdiorites show varying degrees of textural or mineral reconstruction. The appinites do not appear to be affected in this manner.

Microdiorites and felsic porphyrites typically occupy joints dipping SE at 35°, along which some degree of late-stage movement has taken place; D. I. Smith (1979) discusses the mechanics of deformation which renders the rocks schistose. In places the intrusion are numerous enough to impart a bedded appearance to the slopes of a hillside when viewed from a distance.

Appinite

This name is now applied to members of the Microdiorite Suite which are coarse-grained, hornblende-rich (occasionally pyroxene-bearing) mafic diorites. They usually occur as thick sheets or small bosses, and the varieties found could all reasonably be derived from the differentiation of a potassium-rich basaltic magma. They commonly form masses of fairly homogeneous rock but, in Sutherland especially, they show gradation into patchy more acid and more basic material, and even into rock in which the basic fraction appears as xenoliths within the more acid fraction. These characteristics led Read and others (1925) and Read (1931) to regard the Sutherland rocks as the results of hybridisation of ultrabasic magma by granitic magma (the Ach’Uaine Hybrids). The appinites are absent from the central part of the Northern Highlands but show a notable concentration in the vicinity of the Rogart Complex (Ach’Uaine types) and the Strontian Granite, where they are cut by veins of the granite and form large inclusions in the main igneous body. These concentrations suggest a genetic relationship with the granite intrusions. In Glen Garry the appinites are cut by veins of the Glen Garry Vein Complex (p.112). A notable concentration of pyroxene- bearing appinite has been found in the area west of Invergarry.

Microdiorite

These rocks are the most numerous representatives of the Microdiorite Suite. They typically form thin sheets (average thickness about 1 m) cutting the foliation of the Moine country rock. They are abundant in the area south-west of Glen Moriston, and D. I. Smith (1979) has delimited the areas in which the intrusions are particularly concentrated. He defines a line north and west of which they are rare (P915484).

Unmodified microdiorites contain zoned andesine and hornblende (both of which form phenocrysts) as well as biotite and subordinate interstitial quartz and potash feldspar. Many of the sheets, however, are schistose along their margin or throughout, but do not show cataclasis. The usual occurrence of the intrusions as swarms of parallel sheets along moderately inclined joints suggests that the schistosity is a feature of ductile deformation developed late in the tectonic history of the area. They appear to have been recrystallised during the process of deformation, and Johnson and Dalziel (1966), Dearnley (1967) and D. I. Smith (1979) note that this metamorphism took place under amphibolite- or greenschist-facies conditions in a manner which shows regular variation across the area (P915484).

Though intrusions with igneous mineral assemblages are found throughout the area, there are some strongly metamorphosed microdiorites south of Glen Garry which seems to have involved in the regional (?D3) folding of the Moine rocks. It thus appears that there may be more than one period of intrusion, and that the latest intrusions escaped the deformation and metamorphism which affected the earlier ones. As yet no clear evidence of sequential intrusion has been found.

Felsic porphyrite

The felsic porphyrites have a much more restricted occurrence than the microdiorites and are apparently confined to the central part of the area south-west of the Glen Moriston–Glen Shiel valley (P915485) although it is possible that sparse representatives lie undetected outside that zone. The felsic porphyrites, where undeformed, are porphyritic microgranites with phenocrysts of oscillatoryzoned albite-oligoclase, biotite and, in places, hornblende, set in a granoblastic fine-grained groundmass of quartz, potash feldspar and acid plagioclase in equal proportions. Like the microdiorites, they are commonly schistose, having been deformed and recrystallised to produce amphiboliteor greenschist-facies mineral assemblages; amphibolite facies is dominant. Garnet- bearing varieties are found between Glenfinnan and the east end of Loch Garry.

Early representatives of the Microdiorite Suite

Rocks very similar to members of the suite, but earlier than the main phase of intrusion, are found in two areas. In a restricted area near the west end of Loch Garry microdiorites were apparently foliated during the last period of regional Caledonian folding. Around the Cluanie Granite a restricted swarm of acid dykes was emplaced earlier than the intrusion of the Cluanie Granite. Both these, and the Cluanie Granite itself, are cut by representatives of the main phase of intrusion of the Microdiorite Suite. They are distinguished from the younger felsic porphyrites by their greater abundance of phenocrysts of acid plagioclase, biotite and hornblende.

The Minette Suite and felsites

In contrast to rocks of the Microdiorite Suite, members of the Minette Suite are widely, if sparsely distributed throughout the Northern Highlands. They extend beyond the Caledonides across the Moine Thrust and cut Torridonian and Lewisian strata of the Foreland. They usually form vertical-sided dykes with sharp, chilled edges and have an average thickness of about 3 m (D. I. Smith, 1979). Many are traceable for several kilometres, commonly in an E–W direction. Most are typically minettes, lamprophyres rich in phenocrysts of biotite ± augite. A small number of hornblende lamprophyres (vogesites) are represented. The only place where these lamprophyres are sufficiently numerous to constitute a swarm is around the Ratagain Complex where they are displaced 6 km in a sinistral sense by late movement on the Strathconon Fault.

Thick dykes of felsite, mainly trending E–W for many kilometres, are also found in the Northern Highlands. Their relationship to the Minette Suite is uncertain, but around the Ratagain Complex smaller dykes of felsite are apparently represented in the same swarm as the Minettes. Minettes cut felsites and vice versa, but it is not certain if these localised dykes belong to the same phase of intrusion as the more extensive and thicker bodies.

Breccia-filled fissures

In the areas where intrusions of the Microdiorite Suite and the vein complexes are abundant there are a number of breccia-filled fissures, as well as some more extensive areas of broken and disoriented blocks. The main constituents of these breccias are blocks of country rock, mainly of local derivation, rotated but jammed together. They are set in a very minor matrix of more or less felsic microdiorite. In places even this minor igneous matrix seems to be lacking. The breccias are similar to explosion breccias described from Kentallen by Bowes and Wright (1967). J. D. Peacock (personal communication) considers that they may have formed by a filter- press mechanism following explosive activity, and were transported in a liquid rather than a gas. In the Cluanie area these breccias are Younger Caledonian igneous rocks 117 veined by later felsic porphyrites and members of the vein complexes to form a second-generation agmatite.

North of Glen Garry, within the area of the Glen Garry Vein Complex, local areas of disoriented blocks of Moine psammite are found; they may measure several tens of metres across, but are not fissure-fillings. They may be similar in origin to the fissure breccias.

Selected bibliography