Ardnamurchan Central Complex, 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. |
Introduction
The Ardnamurchan Central Complex is situated on the mainland north of Mull. Three distinct centres of activity have been recognised and numbered sequentially from oldest to youngest as Centre 1, Centre 2 and Centre 3 (Richey and Thomas, 1930; Gribble et al., 1976). There is no obvious spatial progression as the central complex developed: Centre 2 lies to the west of Centre 1, but Centre 3 lies to the north-east of Centre 2 (P914145a).
The Ardnamurchan Central Complex was emplaced into a metamorphic basement of relatively low-grade Moine psammites and pelites, overlain by Triassic breccias, conglomerates and sandstones, Lower Jurassic sandstones, limestones and mudstones, and Paleocene lavas. The proportion of country rocks to intrusive rocks in the vicinity of Centre 1 is high, whereas centres 2 and 3 are largely devoid of remnants of Pre-Palaeogene country rock between the various major intrusions. The Mesozoic strata and Paleocene lavas have been domed, and dip away from the central complex at about 30°. The updoming occurred at an early stage, since it predates cone-sheet emplacement (see below). No marginal annular folds are preserved.
Centre 1 and the Ben Hiant volcaniclastic rocks
Along the south-east margin of the central complex, on Ben Hiant, a small remnant of the Mull Lava Field is preserved, overlain by about 200 m of volcaniclastic conglomerates and breccias. There are no direct links between these volcaniclastic rocks and the nearby ring-intrusions, which define Centre 1, but they are described here because of their spatial association with Centre 1.
The volcaniclastic rocks are heterogeneous, with matrix-supported clasts derived mainly from the older lava field. Fragments derived from the basement Moine rocks and the Mesozoic cover sequence are rare. One fragment type that cannot be matched with in-situ material is of rhyodacitic ignimbrite with a well-developed eutaxitic fabric. The ignimbrite is clearly the product of explosive activity, although the vent(s) from which the magma was erupted has not been recognised. The clasts within the conglomerates and breccias range in size from several metres, with rare megablocks of several tens of metres, down to sand and silt grade. Bedding is relatively uncommon, but where present, it is typically close to horizontal and tends to be defined by thin layers of tuffaceous siltstone and sandstone. Fragments within these finer grained deposits are predominantly of lava lithologies, especially trachyte, together with Moine psammite and pelite, and Mesozoic sedimentary rocks. The Ben Hiant conglomerates and breccias are interpreted as the products of debris flow and avalanche deposition (Brown, 2003).
Emplaced into the Ben Hiant volcaniclastic rocks are sheets of andesitic pitchstone ('innin-morite'), typically with well-developed columnar jointing. These have previously been interpreted as lavas, although in at least one example on the south-east side of Ben Hiant, the intrusive nature is particularly obvious, with columnar jointing fanning away from the leading edge of the intrusion where it cooled in contact with possibly water-saturated conglomerates and breccias. The volcaniclastic rocks are also invaded by a variety of plagioclase-phyric and non porphyritic dolerite sheets of unclear association. However, the main intrusive sheets, forming the prominent summit of Ben Hiant, are of a distinctive quartz-dolerite and belong to the Centre 1 set of cone-sheets with a point at depth, about 1 km west of Meall nan Con (P914145). The outer members of the set of cone-sheets are inclined inwards at relatively shallow angles (15° to 20°), whereas those towards the interior are inclined at up to 40°. There are also a few multiple and composite (dolerite plus felsite) cone-sheets.
On the north coast of the Ardnamurchan peninsula, around Achateny and Kilmory, are further outcrops of conglomerate and breccia, interpreted as having a similar origin to those at Ben Hiant (Brown, 2003). These rocks are cut by the Centre 1 cone-sheets and were previously referred to as the products of the 'Northern Vents'.
Other intrusions possibly associated with Centre 1 are shown on P914145. All are either sheet- or dyke-like and appear to predate the cone-sheets of Centre 1.
Centre 2
Centre 2 was formed by four recognisable phases of intrusive activity, all with a focal point at depth, below Achosnich (P914145). The first phase involved the emplacement of an outer (older) set of basic cone-sheets, emplaced into the Mesozoic cover sequence and the underlying Moine psammites and pelites. These cone-sheets are of similar tholeiitic composition to the Centre 1 set, are typically 1 to 5 m thick, and are inclined at 35° to 45° towards the focal point. In the vicinity of Kilchoan, the distribution density of the cone-sheets is very high (P580485). Cone-sheets were emplaced into cone-sheets, and in the relatively uncommon instances where country rock is preserved as screens, it has undergone extreme thermal metamorphism. Multiple and composite members of the set occur, but are not particularly common. Emplacement of these outer cone-sheets produced a relative central uplift of about 1300 m (LeBas, 1971), although a degree of central subsidence may also have been involved. In the critical area, near Camphouse, where the outer (older) set of Centre 2 cone-sheets and the Centre 1 cone-sheets occur together, exposure is poor, and it is not possible to establish the order of intrusion. The two sets of cone-sheets are geochemically indistinguishable (Holland and Brown, 1972; Geldmacher et al., 1998).
The first major ring-intrusion of Centre 2 is a hypersthene-gabbro (Wells, 1954) (P914145b, 2a). It is clearly younger than the outer (older) set of cone-sheets, which it cuts at several places along its southern margin, particularly on the south side of Beinn nan Codhan. The hypersthene-gabbro shows excellent, if uncommon, mineral layering. Layers rich in pyroxene, plagioclase and iron-titanium oxides dip inwards towards the focal point but cannot be traced far. Augen structures and upward-penetrating finger structures occur in the section north of Sanna Bay and suggest that significant post-cumulus modifications have taken place. The thermal effects of the hypersthene-gabbro are substantial, with the formation of various rocks of rheomorphic origin from a variety of basement and Mesozoic lithologies. At Glebe Hill, strongly hornfelsed rocks contain a high-grade thermal metamorphic mineral assemblage, with sapphire, spinel and plagioclase, produced from a highly aluminous parent, possibly a Paleocene claystone, a Jurassic mudstone, or pelitic material from the Moine basement.
The gabbro that crops out at Lochan an Aodainn close to Sonachan is a thoroughly recrystallised olivine-gabbro, severely shattered throughout its mass and with plagioclase crystals that are clouded due to the presence of abundant inclusions of opaque minerals; the clouding has been attributed to thermal alteration. The outcrop pattern is largely controlled by the surrounding younger intrusions and, consequently, its ring shape did not result from its mode of emplacement (P914145b, 2b). The degree of fracturing and alteration present clearly indicate that it was emplaced early in the development of Centre 2.
The quartz-dolerite of the Sgurr nam Meann Ring-dyke is a hybrid intrusion, consisting of dolerite veined by microgranite and felsite. It crops out in an arc over a distance of about 6 km south and south-east of Sanna Bay (P914145b, 2c). This intrusion, most likely a steep-sided, outward-dipping ring-dyke, was formed by the turbulent mixing of basic and silicic magmas (Blake et al., 1965; Skelhorn and Elwell, 1966). Typically, the silicic material either forms veins (millimetre to centimetre wide) within large masses of dolerite, resulting in relatively angular outlines to the dolerite 'clasts', or acts as 'matrix' to more rounded masses of dolerite (P580486). In the latter situation, thin, irregular (crenellated) chilled margins to the dolerite indicate that the basic material was liquid when it came into contact with the silicic magma. Thus, on mixing, the hot basic magma underwent rapid cooling and solidification against the cooler but still liquid silicic magma, forming 'pillows' of dolerite with chilled, tachylitic margins. Contraction cracks formed on the edges of the 'pillows', and silicic magma was then injected into these and beyond, resulting in veining and fragmentation of the dolerite.
Within Centre 2 there are several other less complete ring-intrusions that predate the inner basic cone-sheets (see below). They comprise quartz-dolerites and gabbros, a granite and various felsites.
The third phase of intrusive activity associated with Centre 2 was the emplacement of the inner (younger) basic cone-sheets. These intrusions are steeply inclined inwards, at up to 70°, towards the focal point below Achosnich. They are typically plagioclase-phyric dolerites.
The final phase of intrusive activity was the emplacement of a further set of incomplete ring-intrusions, mainly of quartz-gabbro, some of which have a fabric due to the alignment of plagioclase (the so-called fluxion structure, or lamination). Whether certain of these intrusions are late-stage members of Centre 2 or early members of Centre 3 is not clear.
The formation of the Glas Eilean Vent (P914145a), cropping out on the east side of the narrow promontory and tidal island on the east side of Kilchoan Bay, was later than the intrusion of the outer basic cone-sheets, but contemporaneous with the development of the younger intrusions of Centre 2. The outcrop of the vent material is partly fault-bounded, with Moine basement rocks along the south-east margin and basaltic lavas to the west. The fragmental material within the vent consists of blocks of Moine psammite and pelite, Jurassic sandstone and limestone, and various basic igneous rock-types (porphyritic and non-porphyritic dolerite and basalt, some perhaps derived from the outer cone-sheets). Much of the fragmental material appears to have been brecciated almost in situ, and not subsequently transported any significant distance. The matrix that binds the blocks together forms vein-like features, up to 0.5 m across, with fine-grained margins against the blocks. It is composed of comminuted block material together with pumiceous and shard-like fragments of devitrified (silicic?) glass. The vein-like features form an anastomosing network, and were probably formed when pyroclastic material was emplaced during rapid volatile escape. The devolatilisation process may have been responsible, to a large extent, for the initial and subsequent brecciation of the blocks.
Centre 3
The youngest intrusive centre within the Ardnamurchan Central Complex comprises annular gabbroic intrusions with near-complete ring geometries. The dips of the inter-intrusion contacts are most likely inwardly inclined and give rise to a nested set of funnel-shaped intrusions. Fabrics due to plagioclase alignment ('fluxion structures') help to distinguish between some of the intrusions. Medium-grained intermediate and silicic intrusions associated with the centre are of hybrid origin, containing components derived by fractional crystallisation of basic magmas, together with partial melts of country rock (Gribble et al., 1976). The focal point of Centre 3 is 1 km east-north-east of Achnaha, directly below the composition-ally evolved intrusions (P914145). The centre is slightly ovoid, measuring about 7 km east-north-east and 6 km north-north-west. The margin of the centre is defined by various relatively small gabbros with outcrops that define sectors of ring-intrusions. However, the largest of the Centre 3 intrusions and the one which helps to define its overall geometry and give it topographic expression, is the Great Eucrite (P914145b, 3e), a bytownite olivine-gabbro, which forms an arc of high ground (Plate 37). The outcrop width of the Great Eucrite is about 1 km and the external diameter of the intrusion is between 4.5 and 6 km. Thus, if it were a true ring-dyke, dipping steeply outwards (compare with P914136b), unrealistically large amounts of central subsidence would have been required. More likely, the Great Eucrite, together with other gabbroic intrusions of Centre 3, constitute a composite intrusion of lopolithic or funnel-like shape. One of the most remarkable features of the Great Eucrite is that, given its relatively large volume, it is largely devoid of internal structure; layering, mineral fabric, xenoliths and pegmatites are uncommon, implying that the intrusion had a very simple emplacement history.
Within the interior of Centre 3 is a hybrid ring-dyke that was formed by the partial mixing of basic and silicic magmas (P914145b, 3b). This narrow, steep-sided intrusion crops out over about 270° of arc, suggesting off-centre subsidence of the central block or some form of rotational or 'trap door' subsidence mechanism for its emplacement. Certain of the gabbroic ring-intrusions within the inner part of Centre 3 crop out over the same sector as this ring-dyke.
The innermost part of Centre 3, directly above the focal point, is composed of a small area of amphibole-rich tonalite and a central mass of quartz-monzonite with large biotite crystals. Given their position within Centre 3 and their hybrid mineralogical and compositional characteristics, it is likely that these evolved rocks are the product of reaction between fractionating basic magmas of the centre and partially melted roof rocks. Radiometric age determinations on the tonalite and Great Eucrite give ages of about 59 Ma (P914126).
There are very few minor intrusions associated with Centre 3. They are limited to sparse dolerite cone-sheets cutting the older members of the centre in the south, and a few north-north-west-trending dykes of the regional swarm that cut the younger ring-intrusions.