Arran central complexes, Hebridean Igneous Province
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
There are two major centres of igneous activity of Paleocene age on Arran (P914150). These are the North Arran Granite Pluton, whose mountains dominate the north of Arran, and the Central Arran Ring-complex, which forms part of the high ground between Brodick and the west coast. Additionally, outcrops of microgranite and augite diorite at Tighvein, about 5 km south-west of Lamlash, were once regarded as a possible third centre (Tyrrell 1928) but have subsequently been interpreted as mainly sills (Herriot, 1975) (Chapter 8). The igneous geology was fully described by Tyrrell (1928), with further detail on specific sites by Macgregor (1983), McKerrow and Atkins (1985) and Emeleus and Gyopari (1992).
North Arran Granite Pluton
The North Arran Granite Pluton has a near circular outcrop of 10 to 12 km diameter and consists of a coarse-grained Outer Granite and a finer grained Inner Granite (P914150; P914151). Both granites are biotite bearing and are generally non-porphyritic. They contain drusy cavities lined with quartz (smoky or 'Cairngorm', purplish amethyst, yellowish 'Scotch Topaz'), alkali feldspar and mica, together with rare topaz, blue beryl, allanite and zircon. The rare-earth-bearing minerals fergusonite and gadolinite have also been identified (Hyslop et al., 1999). Fluorite, recovered from stream sediments, probably also originated in drusy cavities. For most of its circumference, the Outer Granite is in sharply defined, intrusive contact with rocks of the Dalradian Supergroup, which are thermally metamorphosed and indurated for a distance of several hundred metres from the granite. Small xenoliths of hornfelsed sedimentary rocks are present close to many of the excellently exposed contacts, as in Allt a' Chapuill where the granite is finer grained and texturally variable and veins the country rocks. In general, however, the granites are remarkably free of inclusions. To the west of Corrie, the granite is in faulted contact with Devonian strata. Emplacement of the granite caused major disturbance of the country rocks (p. 151). On the basis of the large-scale deformation, and other evidence, the pluton has long been regarded as an example of diapiric emplacement of granite, an interpretation that has more recently been substantiated and developed (England 1992b; Goulty et al., 2001). However, despite having been forcibly emplaced, there is a complete lack of foliation or other deformation structures in the granite. Within the pluton, the grain-size and textural contrasts between the Inner and Outer intrusions allow the contact to be traced in some detail; well-exposed, sharp intrusive contacts crop out, for example south of Meall nan Damph and towards the northern end of the A' Chir ridge. The younger, Inner Granite is characteristically drusy and finer grained close to the contact and, in stream sections on the west side of Glen Catacol, there is local evidence of multiple, sheeted intrusion in the granite. A well-developed system of joints, more or less parallel to the land-surface, cuts the Outer Granite, giving rise to castellated outcrops as, for example on Goatfell (P580487). The joints are considered to have developed as the granite was unloaded during erosion. Unusually amongst the Paleocene rocks of the Hebridean Igneous Province, the intrusions of the North Arran Granite Pluton are normally magnetised, a feature which they share with quartz-porphyry intrusions in the south of Arran and on Bute (P914126). This has prompted the suggestion that the granites and quartz-porphyry sills came from a common source (e.g. Mussett et al., 1989).
Central Arran Ring-complex
The rocks of this variably exposed ring-complex (P914150) include lavas and minor intrusions of basic, intermediate and silicic composition, tuff, and intrusive masses of granite, gabbro, and dioritic rocks of hybrid aspect. The ring-complex was emplaced into rocks of Devonian to Permian age and, in the south and east, the sedimentary rocks dip steeply away from the complex. Faulted contacts are inferred elsewhere. At its north-east extremity, in and near Glen Ormidale, the ring-complex appears to cut sharply across Devonian, Carboniferous and Permian rocks, which are domed about the North Arran Granite Pluton suggesting that the ring-complex is the younger.
Andesitic and basaltic lavas and volcaniclastic rocks crop out in the Ard Bheinn area (P914150; P914152) where several centres of eruption have been recognised (King, 1955). Tuffs and volcaniclastic breccias contain fragments of igneous rocks (basalt, andesite, trachyte, felsite) and material of sedimentary origin. The latter includes clasts derived from the Devonian rocks but the most spectacular inclusions are several megablocks hundreds of metres across that contain discrete sequences of fossiliferous Mesozoic strata. They include Cretaceous chalk and sandstone, Lower Jurassic mudstone, and Rhaetian mudstone overlying Triassic sandstone. These megablocks, and others made up of of altered olivine basalt lavas, were probably derived from an original country-rock cover, which was shattered during volcanic explosions and caldera formation. The occurrences of fossiliferous Jurassic and Cretaceous rocks are of particular note since they establish the post-Mesozoic age of this igneous activity, as well as proving the former presence over Arran of a cover of Mesozoic rocks younger than the Triassic.
The intrusions of gabbro, granite and granophyric granite form arcuate masses that are interpreted as partial ring-intrusions. Smaller areas of hybrid dioritic rocks are well exposed in the burn at Glenloig Bridge and in the east of the centre on Tir Dubh (the so-called Hybrid Hill), where complex relationships between silicic and basic rocks suggest the co-existence, and mingling, of basic and silicic melts.