Geology of Mull - an outline
Geology of Mull - Palaeogene volcanic districts of Scotland
Text compiled from: The Palaeogene volcanic districts of Scotland British regional geology Fourth Edition by C H Emeleus B R Bell Contributor: D Stephenson
Moine Supergroup
Rocks of the Glenfinnan Group are found at many localities around the margins of the Mull Central Complex, for example in the core of the Craignure Anticline and beneath the Mesozoic rocks on the coast south of Gribun. Screens of Moine rocks occur between inclined sheets and other minor intrusions in eastern Mull. Outside of the Mull Central Complex, fragments of Moine lithologies, including megablocks up to 100 m across, are common in vent infills and in other volcaniclastic rocks (Bailey et al., 1924, fig. 29). Numerous xenoliths of severely altered Moine rocks are present in the Loch Scridain Sill-complex. On the Ross of Mull, Moine rocks, possibly of both the Glenfinnan and Morar groups, crop out south of the Loch Assapol Fault where they are intruded by the late-Caledonian, Ross of Mull Pluton. From the distribution of rocks of the Moine Supergroup on Mull, it is evident that the central complex is underlain by, and intruded into, these rocks.
Dalradian Supergroup
Grey phyllitic to slaty semipelite and black metalimestone belonging to the Appin Group (possibly the Blair Atholl Subgroup) of the Dalradian form the core of the Loch Don Anticline in eastern Mull. They are separated from Moine rocks on Mull by a continuation of the Great Glen Fault.
Caledonian igneous rocks
The Ross of Mull Pluton comprises a number of granitic bodies with a Rb-Sr age of about 414 Ma (Halliday et al., 1979); it intrudes and thermally metamorphoses the Moine metasedimentary rocks of south-west Mull. Cordierite-sillimanite hornfelses have developed from kyanite-bearing pelitic rocks adjacent to the granite and occur as xenoliths. Within the intrusion, the intricate relationships found between quartz-diorite, granite and basic enclaves suggest the co-existence and interaction of basic and silicic magmas.
Minor intrusions of Caledonian age include various felsic types and lamprophyres. The felsic dykes are most common in the vicinity of the Ross of Mull Pluton, and near the Strontian Pluton east of the Morvern lava pile (Johnstone and Mykura, 1989). The Ross of Mull granites have been extensively quarried (p. 173).
Old Red Sandstone
Basaltic and andesitic lavas belonging to the Lower Old Red Sandstone Lorn Plateau Volcanic Formation crop out in the core of the Loch Don Anticline in south-east Mull. Conglomerates and marly sandstones, possibly of similar age, occur on Frank Lockwood’s Island, south-east of Loch Buie in southern Mull.
Intrusions of Carboniferous and Permian age
Minor intrusions of Carboniferous and Permian age are widespread through the southern Highlands and extend into the western Highlands and Islands. There are two main suites: Stephanian tholeiitic rocks and Visean to Early Permian alkali basalts and lamprophyres.
At Gribun in western Mull, Moine rocks are intruded by a dyke of olivine nephelinite exposed at low water which contains a suite of xenoliths and megacrysts of lower crustal and possibly upper mantle origin (Upton et al., 1998). This dyke is similar to numerous other xenolith-bearing intrusions of Carboniferous and Permian age in the western Highlands and Islands and the Midland Valley. The xenoliths and xenocrysts found in these intrusions have furnished valuable information about the nature of the Lower Crust and Upper Mantle at the start of the Mesozoic (e. g. Upton et al., 1998).
Triassic
Many of the New Red Sandstone occurrences mentioned below are assigned to the Triassic on grounds of lithology, unconformable relationships with older rocks, or a conformable one with overlying, fossiliferous Jurassic strata. Fossils are rare in all but the highest of these beds. In the Hebridean area, the New Red Sandstone sequences are thought to belong to the Upper Triassic and accumulated during the earliest stages of basin formation (Steel 1974a, b; Steel et al., 1975).
Up to about 60 m of conglomerate, sandstone and cornstone are found at Gribun and on Inch Kenneth in western Mull, where the striking unconformity with the underlying Moine rocks is exposed on the foreshore and in cliff sections (Plate 4). Clasts of Moine rocks, cherty fossiliferous limestone (Cambro-Ordovician Durness Group), red feldspathic sandstone (Torridonian), vein-quartz, quartzite, granite and rare, red andesitic porphyry (?Siluro-Devonian) indicate both local and quite distant provenance for these beds. Upper Triassic miospores have been recovered from beds near the top of the Gribun succession that also contain indistinct bivalve remains. The uppermost beds are sandy limestones of Rhaetian age, which contain fish scales and bivalves (e. g. Cardinia sp., Chlamys valoniensis, Protocardia rhaetica, Rhaetavicula contorta). In eastern Mull, Triassic conglomerates and sandstones are exposed in the cores of the anticlines that surround the central complex, from Craignure to Loch Don and Loch Spelve. In addition, there are extensive sandstone outcrops on the west shore of Loch Spelve, which extend up Glen Lussa, where the Triassic rocks occur in screens between inclined basic sheets. Clasts in the conglomerates are mainly quartzite and vein-quartz, but pebbles of Moine rocks are abundant locally where these lithologies are in situ nearby. Small outcrops of Triassic strata are also found within the central complex, where their steep dips indicate considerable disturbance by the Paleocene intrusions.
Jurassic
Lower Jurassic
With the exception of small outcrops of limestone, calcareous mudstone and sandstone on the west coast at Aird na h-Iolaire, and beds of the Blue Lias and Pabay Shale formations at and near Tobermory, Jurassic rocks are restricted to the east and south of the island. They crop out in the cores of anticlines marginal to the Mull Central Complex, from Scallastle Bay almost to Loch Spelve, in many places on the east coast from Loch Don to Loch Buie, and also at Carsaig Bay. The beds range in age from Hettangian through to Bajocian (Mid Jurassic; garantiana Zone) and contain a moderately abundant marine fauna. Near Port nam Marbh, there is a fairly continuous succession from the uppermost 30 m of the Pabay Shale Formation, represented by slightly calcareous siltstones with abundant Gryphaea cymbium and Pecten aequivalvis, to the Bajocian. At Torosay, the Pabay Shale Formation contains the Torosay Sandstone Member, which is over 20 m thick and is cross-bedded in places (Hesselbo et al., 1998).
Middle Jurassic
About 30 m of sandy limestone and calcareous sandstone of the Bearreraig Sandstone Formation are capped by 1 to 2 m of siltstone of the Great Estuarine Group on the east coast at Port na Marbh, south-east Mull. Similar beds crop out fairly continuously beneath the lavas on the west limb of the Loch Don Anticline, and discontinuously on the east limb as far as Duart Bay. On the Ardnamurchan peninsula, Middle Jurassic rocks are limited to massive sandstones and subordinate limestones of the Bearreraig Sandstone Formation exposed in screens amongst the basalt sheets on Maol Buidhe and at Sròn Beag. Despite the effect of thermal metamorphism, typical Aalenian fossils have been obtained from these rocks (Richey and Thomas, 1930).
Upper Jurassic (and upper part of the Callovian)
A small outcrop of baked blue shale just west of Duart Bay has yielded ammonites and other fossils of Kimmeridgian age (mutabilis Zone).
Cretaceous
At Gribun, Cenomanian glauconitic sandstone (6 m thick) is overlain by 3 m of white sandstone and 3 m of silicified chalk with flints (possiby Santonian). The chalk is overlain by breccia containing angular pebbles and cobbles of silicified chalk, sandstone with chert nodules, and 2 m of weakly bedded, red mudstone. The beds above the chalk have been formally designated the Gribun Conglomerate Formation by Hancock (2000), but have been equated with the Clach Alasdair Conglomerate Member and Beinn Iadain Mudstone Formation by Mortimore et al. (2001). The succession is overlain by Paleocene lavas (Plate 4). A rich bivalve fauna is known from the glauconitic sandstone, including Rhynchonella, Exogyra conica, and Pecten asper. Ostrea and Pecten have been found in the chalk. Glauconitic sandstone occurs at Auchnacraig, south of Loch Don, and also at Carsaig where it reaches about 13 m, its greatest thickness in the district.
Palaeogene lava fields and associated sedimentary rocks
Lava sequences
Mull Lava Group
The Mull Lava Group crops out throughout north and west Mull, together with the coastal parts of south Mull (around the margin of the younger Mull Central Complex) and the various offshore islands to the west, including Ulva, Gometra, the Treshnish Islands and Staffa. The successions of flat-lying lavas form substantial mountains, such as Ben More, and commonly develop strong trap featuring (Plate 15). The lavas on the Morvern peninsula, east of Mull, are considered to belong to the Mull Lava Group, as do the lavas east of Ben Hiant, Ardnamurchan, and probably also the isolated outcrops on the south coast of the Ardnamurchan peninsula. Offshore lavas, which extend beneath the Sea of the Hebrides to the Skerryvore Fault, are most likely part of the Mull Lava Group. These flows are probably contemporaneous with flows of the Eigg Lava Formation, which they join in a northern, submarine extension (Fyfe et al., 1993; (Figure 9); Chapter 5).
There has been no detailed regional mapping of the Mull Lava Field since the time of the original survey (Bailey et al., 1924). This account therefore relies to some extent on the synthesis of Richey (1961), together with observations made during the last thirty-five years (e. g. Kerr, 1995b). The lava stratigraphy has been rationalised here, in keeping with that used for the other lava sequences (Table 15).
The remains of the Mull Lava Field as presently exposed, cover an area of about 840 km2 on Mull and Morvern (Emeleus, 1991). The total preserved thickness of lavas on Mull is estimated to be about 1800 m (including an estimated 900 m of olivine-poor tholeiitic basalt lavas now largely within the Mull Central Complex). On the Morvern peninsula, the sequence is about 460 m thick but the lowest lavas thin towards the north (Bailey et al., 1924) and are overstepped by subsequent flows. A thin, laterally continuous sequence of Upper Cretaceous rocks underlies the lavas of Morvern except in the extreme east and north-east of the lava field, where the flows rest directly on pre-Cretaceous rocks (Triassic sandstones and gneisses of the Moine Supergroup). The marginal portion of the lava field was therefore erupted onto, and possibly terminated against, a land surface with a significant topographical relief. The north-west-trending Assapol Fault, in south-west Mull, defines the present-day south-west margin of the lava field.
The volcanic succession on Mull (Table 15) is divided into the Staffa Lava Formation at the base, overlain by the Mull Plateau Lava Formation that consists of the Ben More Main Member and the overlying Ben More Pale Member; these were formerly named, respectively as the Staffa, Main and Pale ‘suites’, of the Plateau Group (e. g. Bailey et al., 1924; Richey, 1961). The Mull Central Lava Formation crops out principally within the Mull Central Complex, and consists of olivine-poor tholeiitic basalt lavas.
The sequence built up by the effusion of lavas, predominantly from north-west-trending fissures now represented by the Mull Dyke Swarm. Pauses in the volcanic activity were more common during eruption of the lower part of the succession. They are marked by the development of laterally discontinuous sequences of conglomerate, sandstone, siltstone and coal. However, unlike the lava piles elsewhere in the Hebridean Igneous Province, the current subdivisions of the Mull lavas are based on lithological changes in the lavas and do not always depend on the presence of interflow sedimentary sequences.
Zeolite minerals are common throughout the lavas. These have been shown to have a depth-related distribution (Walker 1971; (Figure 12), similar to the zonal distribution found in Icelandic lavas. The highest zone is characterised by laumontite and overlies a mesolite zone. Based on comparisons with Icelandic zones, it is estimated that the lavas in the vicinity of Ben More were originally over 2200m in thickness, of which about 1000m is preserved. A distinct zone characterised by carbonate minerals occurs in the lavas near Tobermory and in north-west Morvern, becoming wider northwards towards Ben Hiant on Ardnamurchan. It has been tentatively suggested that the carbonate may have been deposited by circulating heated waters from the Ardnamurchan Central Complex (Walker, 1971). The Mull Central Complex is surrounded by a zone of pneumatolysis, which is superimposed on the depth-related zeolite zones (Figure 13).
Staffa Lava Formation
The basal part of the volcanic sequence on Mull and locally on Ardnamurchan is marked by the development of a laterally persistent mudstone, the Gribun Mudstone Member, commonly less than 1 m thick, but thicker at several localities. In Morvern it is known locally as the Beinn Iadain Mudstone Formation (Figure 7). Near Feorlin Cottage at Carsaig, Mull, the mudstone occurs within cavities that developed on the surface of chalk (of possible Turonian age). On the Croggan peninsula south of Loch Buie, at An Garradh, the member consists of about 6 m of buff-coloured calcareous mudstone (marl) and is thought to represent a more aluminous equivalent. These features, together with certain petrographical characteristics (for example the presence of quartz grains), suggest that the mudstone is the product of extreme lateritic weathering of a basaltic ash, which was deposited prior to the effusion of the overlying lavas. The quartz grains were possibly derived from Cenomanian sandstones and introduced either by alluvial or aeolian processes.
At Malcolm’s Point, on the south coast of the Ross of Mull, sedimentary rocks may represent a useful lithostratigraphical marker at the base of the Staffa Lava Formation and above an older, distinctive group of non-columnar flows. The sedimentary sequence comprises a thin carbonaceous mudstone overlain by an upward-fining fluviatile conglomerate–sandstone sequence containing rounded flints (derived from the Turonian Chalk deposits) and more angular fragments of basalt. The sequence is capped by a thin fissile mudstone. The main part of the Staffa Lava Formation comprises a distinctive sequence of tholeiitic basalt flows (defining the Staffa Magma-type or sub-type of Thompson et al. 1986; see Chapter 10). The flows crop out around the sea cliffs of south-west Mull, for example on the Ross of Mull at Malcolm’s Point and Ardtun, at Burg and The Wilderness on the Ardmeanach peninsula, and on Staffa where they overlie hyaloclastite deposits (Plate 13). Flows of the Staffa Lava Formation also crop out at Bloody Bay, north of Tobermory. Significantly, the formation does not occur everywhere at the base of the lava pile; for example it is absent at Beinn na h-Iolaire in west Mull and throughout the Morvern peninsula (Kerr, 1995b).
Fluvial systems that developed between the eruptions deposited largely clastic sedimentary sequences that are collectively termed the Ardtun Conglomerate Member. These include laterally discontinuous conglomerate–sandstone sequences such as those at Ardtun on the north coast of the Ross of Mull, and laterally equivalent, overbank or lacustrine facies that give rise to siltstone–mudstone–coal–limestone sequences, well preserved on the south coast of the Ross of Mull (Boulter and Kvacek, 1989). The most distinctive field characteristic of flows of the Staffa Lava Formation is the development of typically near-vertical columnar jointing, as exemplified by the Fingal’s Cave Flow on Staffa (Plate 13). The locally restricted nature of most flows, with columnar joints that in some instances are near-horizontal, is suggestive of cooling against steep-sided walls of palaeovalleys that cut through a dissected plateau. On the Ardmeanach peninsula, a pillowed facies indicates that the MacCulloch’s Tree flow erupted into shallow water. The flow is underlain by interbedded hyaloclastite breccias, together with dark mudstones and siltstones that contain fragments of carbonaceous material (wood), in a fine-grained matrix. The preservation of a cast of an upright tree, ‘MacCulloch’s Tree’ (Plate18) is the most remarkable feature of the flow, with columnar jointing becoming severely contorted as it approaches the vertical surface of the trunk.
Mull Plateau Lava Formation
The most complete and relatively simplest succession occurs at Ben More, where both the Ben More Main Member and the Ben More Pale Member are present (Table 15). This succession has been the subject of detailed geochemical examination (Kerr, 1995a; Chambers and Fitton, 2000; Chapter 10).
The Ben More Main Member occurs throughout west Mull, as well as on the Morvern peninsula. Reddened, weathered tops to flows, attributable to subaerial weathering, are relatively common, although interflow clastic sedimentary sequences have not been noted to any significant extent. Certain of the reddened deposits on top of flows have been identified as ashy deposits, probably reworked by stream action (Emeleus et al., 1996a). The member is composed of randomly interleaved flows of olivine basalt and hawaiite, together with rarer picrobasalts, mugearites, benmoreites and trachytes. Certain flows, especially the hawaiites and mugearites, are plagioclase macroporphyritic (the Big Feldspar Basalts of Bailey et al., 1924).
Little detailed information is available about the structure and stratigraphy of the Ben More Main Member. Below, two localities are briefly described, in order to indicate some of the complexities and subtleties that are recognised.
The sequence of flows that forms the coastal cliffs at Laggan Bay, near Ulva Ferry, banked up against, and eventually overstepped, a thick heterogeneous accumulation of basaltic ashes, volcaniclastic breccias and debris flow deposits. These most likely mark the site of a vent that penetrated the lava pile relatively early in the development of the Ben More Main Member. The majority of the clasts within the breccia are of basalt and hawaiite; however, rare but conspicuous angular fragments of Moine psammite and Turonian flint can be identified, indicating that material from the subjacent crust has been transported upwards by the magmas or eroded from surface outcrops.
On the west side of the Quinish peninsula, in north Mull, a remarkably well-preserved lava flow crops out for a distance of at least 800 m along the coast between the high and low water lines. The flow preserves both casts (in basalt) and moulds of tree trunks and possibly branches, most of which are flat-lying, and the majority of which trend north-east–south-west. The upper portion of the underlying lava is severely lateritised, representing the soil in which the trees grew. The flow is remarkably well preserved, with surface features and internal structures more typically seen in active volcanic areas, for example, ropy pahoehoe structures, shell-like pahoehoe crusts, and basal or marginal breccias. The remarkable state of preservation of the flow and the presence of fossil trees are taken as clear indication that the Quinish lava was erupted after a significant hiatus in the volcanic activity, and was itself rapidly buried by the succeeding flow.
The Ben More Pale Member crops out around the summit of Ben More, close to the western margin of the Mull Central Complex and, consequently, has been subjected to intense hydrothermal alteration and metasomatism. It overlies the Ben More Main Member, although no sharp boundary has been identified; rather, there is an interdigitation between the two members, over an interval of a few flows, with the increasing preponderance of pale-weathering flows, up sequence. The pale weathering flows are generally of relatively evolved composition, typified by benmoreites (type locality, first identified by Tilley and Muir, 1964). The lower part of the Ben More Pale Member consists predominantly of mugearite, benmoreite and trachyte. The lavas showing the most evolved compositions are typified in the field by a platy jointing and general fissility, and occur in the middle part of the member. The uppermost part of the member marks a return to significantly less-evolved olivine basalt flows (Kerr, 1995a). Intercalated with the lavas are thin sedimentary deposits; on the north side of Ben More, a benmoreite flow is underlain by 30 to 60 cm of fissile black mudstone containing abundant fragments of benmoreite; a similar deposit at the same stratigraphical level occurs on the east side of A’ Chioch. South of the summit of Ben More, Bailey et al. (1924) recorded about 3 m of brecciated black mudstone with plant remains overlying a mugearite flow.
Mull Central Lava Formation
The Mull Central Lava Formation consists of tholeiitic basalt flows that crop out in small areas within the Mull Central Complex and possibly around its margins, where they overlie the Mull Plateau Lava Formation. The formation is about 900 m thick, and many of the lavas may have been erupted within a water-filled caldera, forming a thick succession of pillowed flows. This is the South-east Caldera or Early Caldera of Bailey et al. (1924); the structure is about 10 km in diameter, and is thought to have developed as a consequence of the summit collapse of a central vent volcano on a site now occupied by the Mull Central Complex. Subsequent intrusive activity within the central complex has dismembered the intrusions. Another caldera was also recognised, the so-called North-west Caldera (Bailey et al., 1924, plate III), but flows of the Mull Central Lava Formation within this structure are not pillowed. Throughout the formation, intense hydrothermal alteration has led to severe changes to the primary mineralogy of many of the lavas, with the development of abundant secondary epidote and prehnite, commonly replacing primary minerals, but also in fracture-filling vein systems.
Within the South-east Caldera, Bailey et al. (1924) recognised three zones within the formation (Table 15), but complications due to subsequent structural and intrusive events prevent the thicknesses of these zones being defined. Pillowed flows are restricted to the Outer Zone and Middle Zone. On the basis of the significant thicknesses of pillow lavas still preserved, Bailey et al. (1924) argued that there must have been successive subsidence of the caldera floor.
It is important to note that the lavas of distinctive tholeiitic basalt composition were not always contained by the caldera wall. It is envisaged that some were able to flow outwards, presumably down the flanks of the volcanic superstructure, and possibly to interdigitate with flows of the Mull Plateau Lava Formation that had been erupted from fissure systems.
Dykes, dyke swarms and volcanic plugs
Numerous dykes of Palaeogene age are present throughout western Scotland and the Hebrides. The dykes commonly occur in parallel, north-west- to north-north-west-trending regional swarms, becoming more numerous and varied in direction near to and within the central complexes (Speight et al., 1982)
The dykes in and near the central complexes are generally less than 2 m in thickness. However, injection of dykes side by side into the same fissure can result in thick multiple intrusions, which are most common close to the central complexes. Large numbers of multiple dykes also occur close to the Mull Central Complex. The distribution of the dyke swarms was controlled by a regional north-east–south-west extensional stress field, and the over-riding control on the orientation of the swarms is considered to have been the orientation of lower crustal intrusions that fed the dykes. Locally, subsidiary swarms are orientated approximately north–south, for example in the Outer Hebrides, and between south Skye and Morvern where the subsidiary swarm forms an en échelon link between the Skye and Mull swarms. Echelons of the Cleveland Dyke are up to 25m thick in County Durham and north Yorkshire, some 350 to 400 km from Mull.
The majority of the dykes in the linear regional swarms are of basaltic or slightly more evolved composition. In and around the central complexes there are, additionally, dykes of silicic, intermediate and, less commonly, ultrabasic composition (Gibb, 1968, 1969). The basaltic dykes of the regional swarms are predominantly of tholeiitic basalt or tholeiitic olivine basalt, or of mildly alkaline olivine basalt. Dyke compositions may remain fairly uniform over considerable distances, as has been well demonstrated in regional dykes that extend across the Southern Uplands and into north Yorkshire (Macdonald et al., 1988).
Tholeiitic basalts and related, more evolved, lithologies predominate in the Mull Dyke Swarm. However, alkali olivine-dolerites, locally termed ‘crinanites’, are common as far southeast as Loch Fyne, and the swarm contains silicic and intermediate dykes in the vicinity of Oban and on Mull.
The majority of the plugs are basaltic in composition, but a few of trachyte composition have been recorded. On Mull, a large trachyte plug intrudes lavas and is, itself, intruded by north-west-trending dykes at Druim Buidhe, south-east of Tobermory.
Sills and sill-complexes
Loch Scridan Sill-complex
The Loch Scridain Sill-complex intrudes the Moine basement, the Mesozoic sedimentary sequence and the overlying Paleocene lavas in south-west Mull, particularly on the Ross of Mull and on the north side of Loch Scridain. The sills are typically betwen 0. 5 and 6 m in thickness but exceptionally exceed 10 m, and commonly have well developed chilled margins. They are of tholeiitic affinity, ranging in composition from tholeiitic basalt, through andesite and dacite, to rhyolite (including glassy variants). Basaltic compositions are the most common, with about 20 per cent of the intrusions being of intermediate composition, and a relatively small number of rhyolitic composition. Many of the sills are xenolithic, with cognate and accidental (upper crustal) material represented; two of the best examples are to be found at Killunaig and Kilfinichen Bay, on the south and north sides of Loch Scridain, respectively (Plate 22). Composite sills occur, commonly involving crystalline and glassy rock types of significantly different compositions as in, for example, the classic xenolithic sill at Rudh’ a’ Chromain on the south side of the Ross of Mull. The sills may be related to the initial stage of the development of the Mull Central Complex (Centre 1 or Glen More Centre, see p. 126; Dagley et al., 1987).
The sills were described in detail by Bailey et al. (1924). These early studies were concerned mainly with the complex high-temperature mineral assemblages that developed within crustal xenoliths prior to and/or during sill emplacement. More recently, Brearley (1986) described the melting reactions of the Moine basement materials, and Kille et al. (1986) inferred that the sill magmas were actively convecting during emplacement, maintaining high temperatures at the contacts, which resulted in localised thermal erosion of the pelitic Moine wall-rocks.
The Loch Scridain sills may be divided into three distinct geochemical groups (Preston et al., 1998a; Chapter 10). The basic sills (Group 1) are markedly xenolithic, containing both cognate and crustal types (Plate 22). The cognate xenoliths are of ultrabasic and basic composition, and are most likely of cumulate origin (Preston and Bell, 1997). There are two broad groupings of the crustal xenoliths (Preston et al., 1999):
siliceous xenoliths derived from psammites of the Moine Supergroup or, less commonly, as in the Rudh’ a’ Chromain Sill, sandstone and conglomerate xenoliths from the local Mesozoic country rocks* aluminous xenoliths, of which there are three types, each dominated by glass produced by melting of the xenolith, hence the term buchite
The aluminous xenoliths include:* mullite buchites — a mass of mullite needles (3Al2O3.2SiO2) set in clear glass and pale lilac in hand-specimen
- cordierite buchites — small crystals of cordierite and mullite needles set in clear glass and virtually black in hand specimen
- plagioclase-rimmed mullite buchites — a core of mullite-rich glass surrounded by a thick rim of white-weathering, coarse-grained plagioclase. (An87–60)
Clear blue corundum (sapphire) also occurs. Pockets of quenched, isotopically contaminated basic glass with skeletal plagioclase and clinopyroxene occur trapped between the plagioclase crystals. The highly aluminous composition of the buchite glasses is consistent with their derivation from a clay-rich sediment or its metamorphic equivalent, most likely the pelites of the Moine Supergroup (Dempster et al., 1999).
Central complexes
Our understanding of the order of intrusive events within the Mull Central Complex is still largely due to Bailey et al. (1924). Subsequent work has been restricted to a small number of studies of some of the main intrusions and a brief summary and field guide by Skelhorn and Longland (1969). Three centres are recognised and numbered sequentially 1, 2 and 3; the first and last are considered to have been related to the development of calderas: Centre 1 to the Early Caldera or Glen More Centre and Centre 3 to the Late Caldera or Loch Bà Centre. Centre 2, the Beinn Chaisgidle Centre, is composed of various cone-sheet and ring-dyke intrusions.
During formation of the Mull Central Complex, there was a gradual shift of activity from Centre 1 through to Centre 3 (Figure 27). Movement was in a south-east to north-west direction, by a few kilometres, parallel to the trend of the regional dyke swarm. Large annular folds surround the central complex (see p. 150).
Centre 1, the Glen More Centre
Within the Mull Central Complex and partly acting as country rock to the intrusions, are remnants of pillowed basaltic lavas. The relationship between these lavas and the main lava field on Mull (p. 75;(Table 15)) is unclear, although Bailey et al. (1924) concluded that the pillowed material constitutes the stratigraphically youngest part of the lava field and formed within a caldera, hence the Glen More Centre is also referred to as the Early Caldera (Figure 28). The pillowed flows are referred to as being of the ‘Non-Porphyritic Central Magma Type’ or the ‘Central Mull Tholeiites’ (Chapters 6 and 10).
Early granites
The oldest of the main intrusions of the Glen More Centre are the steep-sided granites of Glas Bheinn and Derrynaculean, which possibly form parts of ring-dykes or steep-sided stocks. Emplacement of these granites was, in part, controlled by ring-faults, with central collapse. Brecciation, due either to gas escape or ring-faulting, is common throughout the granites, especially in the Derrynaculean mass. In addition, the Glas Bheinn intrusion was emplaced into the core of the somewhat imperfectly developed marginal Loch Spelve Anticline. Both granites show significant hydrothermal alteration, with primary pyroxene being chloritised or uralitised. Marginal facies of the Glas Bheinn Granite contain partially assimilated siliceous material, most likely derived from country-rock Triassic sandstones.
Explosion breccias
Several masses of explosion breccia occur along the trace of the ring-fault which is used to define the extent of the Early Caldera. The best examples occur within the south-east sector of the bounding fault, on the eastern side of Sgurr Dearg. The breccias contain subangular to rounded fragments of Paleocene lavas, Mesozoic sedimentary rocks, Moine gneisses and a wide variety of coarse-grained igneous rocks (gabbro, granite, etc.). Moine gneisses are generally absent from the breccias inside the main caldera-bounding fault, indicating that the basement lies at a deeper structural level beneath the caldera and that the explosive brecciation occurred at a fairly shallow level in the crust. Fragmented rhyolitic rocks with flow-banded and perlitic textures also occur in the breccias, which were most likely formed by gas streaming from silicic magmas. Surface volcanic deposits related to this explosive activity are not recognised, due to the level of erosion.
In Coire Mór, on the east side of the central complex, is an outcrop of generally unstratified volcaniclastic breccia containing subangular to rounded blocks of various Paleocene igneous rocks and Pre-Paleocene sedimentary rocks. Also present are large masses of flow-banded rhyolite. Similar material occurs at Barachandroman at the south side of Loch Spelve. The Coire Mór rocks were interpreted by Bailey et al. (1924) as surface accumulations and contemporaneous rhyolite lava flows, but Richey (1961) preferred a model of subsurface gas brecciation, akin to the explosion breccias of Sgurr Dearg.
Early felsites
The flow-banded Beinn Mheadhon, Torness and Creag na h-Iolaire felsites are approximately contemporaneous, and predate the emplacement of the explosion breccias. The Beinn Mheadhon Felsite is located outside the caldera-bounding fault, but the other two are inside (Figure28). The felsites are cut by younger basic intrusions (mainly cone-sheets) which obscure the original geometry.
Early cone-sheets
A set of early cone-sheets was emplaced into the Glas Bheinn and Derrynaculean granites, the explosion breccias and the early felsites. These cone-sheets are predominantly basic, although a small proportion of intermediate and silicic intrusions is also recognised. They dip inwards at approximately 45° towards a focal point below Beinn Chaisgidle and approach an aggregate thickness of 1000 m, with individual sheets up to 10 m thick. Consequently, significant central uplift will have occurred as a result of their emplacement. The main outcrop can be traced in an arcuate belt that runs from Glen Forsa in the north, close to Loch Spelve, and thence across Glen More to Derrynaculean (Figure 28).
The precise timing of the emplacement of the intermediate and silicic intrusions relative to the dominant basic intrusions is unclear, although it is evident that they did overlap. This is confirmed by the presence of a number of composite (basic–silicic) cone-sheets. Movement of the fault defining the Early Caldera had ceased by the time the early cone-sheets were emplaced.
Gabbros
Gabbros Subsequent to cone-sheet emplacement, two large gabbroic bodies were intruded into the central complex: the Ben Buie Gabbro in the south-west, outside the main ring-fault, and the Beinn Bheag Gabbro inside the fault in the north-east quadrant of the centre. Emplacement of the magmas involved in the formation of the Bein Buie intrusion may have utilised the main ring-fault. However, Skelhorn and Longland (1969) suggested that the Ben Buie mass and possibly the Corra-bheinn Gabbro of Centre 2 were originally circular in plan, and that central subsidence has removed much of the intrusion(s) to a deeper structural level. Furthermore, the inward dips of the mineral layering in the Ben Buie Gabbro increase from about 15° to 20° near the intrusive contacts with earlier rocks, to angles in excess of 35° close to the main ring-fault on the east and north-east sides of the intrusion. The increased dips may have resulted from movement on the fault. The outer, south-eastern, margin of the Ben Buie Gabbro dips outwards at a shallow angle; however, the inward dipping nature of the modal layering of the intrusion may indicate that the base might be at no great depth. The chilled margin of the Ben Buie Gabbro is of tholeiitic basalt composition, akin to that of the Mull cone-sheets, further suggesting a genetic link (Skelhorn et al., 1979). The cumulate nature of the gabbros results in layers ranging in composition from olivine-dominated assemblages (peridotite), through typical olivine-gabbro assemblages, to plagioclase-dominated assemblages (troctolite and bytownite troctolite). Layers rich in chrome-spinel are common in the more ultrabasic lithologies (Henderson and Wood, 1981). Bailey et al. (1924) defined various facies based upon grain-size and mineral proportions within the Beinn Bheag Gabbro, together with a marginal facies veined with silicic material and a brecciated facies. Xenoliths are common throughout the intrusions, both cognate (peridotite, gabbro, troctolite, etc, essentially unaltered and not recrystallised) and granular-textured rocks, interpreted to be the products of thorough recrystallisation of earlier formed parts of the intrusion, or of country rock basaltic lavas. The two gabbro intrusions were subsequently invaded by various cone-sheets, basic through to silicic, which belong to Centre 2 (see below).
Loch Uisg Granite–Gabbro Intrusion
This intrusion consists of two discrete lithologies. The gabbroic component varies between an olivine-gabbro and an olivine-dolerite, whereas the granite has well-developed granophyric texture and is quite severely hydrothermally altered. The junction between the two comprises a zone of hybrid material formed by magma mixing. This asymmetical, composite intrusion appears to have the geometry of a flat-lying sheet emplaced into lavas, although at the western end of Loch Uisg the upper contact of the granite dips steeply to the north. At the eastern end of the intrusion, part of the roof is formed by volcaniclastic breccias, exposed at Barachandroman (see above), within which the more muddy rocks are thermally altered and thoroughly recrystallised. Emplacement of the Loch Uisg intrusion postdates the formation of the annular folds and the development of the explosion breccias of the Glen More Centre. The relationship with the early basic cone-sheets is less clear since the intrusion is cut by some sheets but in turn cuts others. Overall, the intrusion would seem to be a relatively late component of the Glen More Centre.
Centre 2, the Beinn Chaisgidle Centre
After the development of the Glen More Centre, the focus of igneous activity shifted several kilometres towards the north-west, to the area around Beinn Chaisgidle (Figure 29). Centre 2, also known as the Beinn Chaisgidle Centre, is dominated by thin, steeply inclined, outward-dipping ring-dyke intrusions varying in composition from basalt through to rhyolite, and inwardly inclined basalt and dolerite cone-sheets.
Corra-beinn Gabbro
The Corra-beinn Gabbro is the most westerly of the large gabbro masses in Mull, and is mainly, but not wholly, outside the main bounding fault of the Early Caldera. It contains layered structures that dip to the north-east at 25° to 80°. The gabbro may be a late member of Centre 1; however, since it truncates early basic cone-sheets that in turn intrude the Ben Buie Gabbro it is tentatively assigned to Centre 2.
Ring-dykes
The ring-dykes are typically of silicic composition, with steeply inclined margins. Thicknesses vary between 50 and 500 m. They range from relatively coarse-grained rocks such as granite, through to microgranite and rhyolite. Basic ring-dykes are much less common, and vary from gabbro through to dolerite. Some of the ring-dykes are composite, with a range in composition from silicic to basic, but without obvious internal contacts.
Glen More Ring-dyke
The Glen More Ring-dyke is probably the best known example of a steeply inclined, compositionally variable hybrid intrusion in the Hebridean Igneous Province. The ring-dyke crops out from the river in Glen More northwards to the summit of Cruach Choireadail, over a vertical distance of almost 500 m. It grades upwards in composition from olivine-gabbro through dioritic rocks to a somewhat melanocratic microgranite. The primary mineralogy has largely been replaced by secondary, hydrothermal minerals. It is perhaps the most useful of the Centre 2 intrusions to study in order to observe the processes of differentiation and ring-dyke formation. Bailey et al. (1924) and Koomans and Kuenen (1938) interpreted the vertical variation in composition as the product of in-situ differentiation by liquid–crystal fractionation, whereas others (Holmes, 1936; Fenner, 1937) concluded that the dioritic rocks resulted from the mixing of silicic and basic magmas (see Chapter 10).
Cone-sheets
Most commonly, the basic intrusions are inwardly inclined cone-sheets of basalt and dolerite, in some instances veined by remelted parts of the silicic ring-dykes that they intrude. The cone-sheets are usually less than 10 m thick and dip inwards, generally at 20° to 50°, towards a focal point beneath Beinn Chaisgidle. Thus, complicated relationships between typically silicic ring-dykes and the typically basic cone-sheets are found throughout Centre 2; these relationships are well developed in the Allt Molach stream section in Glen More.
The final intrusive phase unambiguously associated with Centre 2 was the emplacement of the quartz-dolerites that make up the Late Basic Cone-sheets. Emplacement of these cone-sheets continued as the focus of intrusion migrated north-west towards Loch Bà and Centre 3 became established. Consequently, their emplacement also constitutes the earliest phase of intrusive activity associated with the youngest centre.
Centre 3, the Loch Bà Centre
Centre 3, also known as the Loch Bà Centre, was associated with the development of the Late Caldera (Figure 30).
Late Basic Cone-sheets
These cone-sheets were clearly emplaced during the latter stages of the development of Centre 2 and the earlier part of Centre 3, since plutonic intrusions belonging to both the centres truncate, and in turn are intruded by, cone-sheets belonging to this set. Those that are clearly associated with Centre 3 are symmetrically disposed about an axis trending north-west, parallel to the length of Loch Bà (Figure 30). In places, the density of cone-sheet emplacement is very high, with very little country rock preserved. Central uplift must have been significant.
Glen Cannel Granite
This granite was the first major silicic intrusion to be emplaced within Centre 3. The mildly alkaline granite forms an oval, dome-shaped mass with a north-west-trending long axis. The intrusion contains abundant gas cavities (druses) and is preserved predominantly within the subsided block inside the late-stage Loch Bà Ring-dyke. The granite cuts numerous Late Basic Cone-sheets within the central subsided block, but outwith the block, to the south-east, it is cut by similar cone-sheets. Thus, it appears that there was an overlap of the intrusive events, or that different intrusions make up the granite, or that more than one set of cone-sheets exists. The granite is partially roofed by volcaniclastic rocks, masses of quartz-dolerite and intrusive felsites. From the disposition of the felsites along the edges of the granite (Figure 30), they might be regarded as a chilled marginal facies were it not for exposures on the east side of Bìth Bheinn and Creag Dubh, south of Loch Bà, which show that the granite is in sharp intrusive contact with the felsites.
Beinn a’ Ghraig and Knock granites
The Beinn à Ghraig Granite is located outside the Loch Bà Ring-dyke along its north-west margin. It is of similar petrographic type to that of Glen Cannel, but is considered to be younger as it cuts Late Basic Cone-sheets on Beinn a’ Ghraig, but is itself cut by only one or two cone-sheets. The Knock Granite is of similar age, taking the form of a steep-sided, elongate mass, 50 to 300m wide, separated from the north-west margin of the Beinn a’ Ghraig intrusion by a screen of hornfelsed basaltic lavas. Similar, most likely related, granitic, dioritic and hybrid ring-intrusions occur to the north-east of Loch Bà, in the vicinity of Toll Doire, Maol Buidhe and Killbeg. Although the country-rock lavas have been invaded by the Late Basic Cone-sheets, and are hornfelsed, they do not appear to have been significantly folded or faulted. This suggests the relatively passive emplacement of the granitic magmas, probably by a combination of subsidence and stoping.
Loch Bà Ring-dyke
This ring-dyke is the final major silicic intrusion of the Loch Bà Centre. It has an external diameter of about 8 km and a width varying from 400 m down to zero in those areas where the trace of the ring-fault is marked only by brecciation of the country rocks. In general, the ring-dyke walls are close to vertical, although steep outward dips occur along the north-west portion. The Loch Bà Ring-dyke is cut by late members of the north-west-trending regional dyke swarm, but is unique amongst the major intrusions of the Mull Central Complex in being entirely free of cone-sheets.
The intrusion was first described by Bailey et al. (1924) and its petrology and origin have subsequently been investigated by Walker and Skelhorn (1966) and by Sparks (1988). The later studies recognised the hybrid nature of the intrusion, involving dominant silicic rock (rhyolite with sparse phenocrysts of sodic plagioclase, sanidine, hedenbergite, fayalite, magnetite, ilmenite and zircon) containing inclusions (typically less than 10 cm long) of phenocryst-poor basic material ranging in composition from ferrobasalt through to dacite. The inclusions constitute less than 10 per cent of the ring-dyke, are commonly glassy, and range in shape from rounded to lenticular, the latter with distinctive ragged ends. The rhyolite is partially devitrified with an obvious flow banding and the preservation of fiamme (eutaxitic texture) is indicative of a pyroclastic origin. Given the glassy, hybrid nature of the intrusion, even where it is 400 m wide, and the development of textures typical of welded tuff, it is evident that its emplacement involved mixing of magmas during the eruption of pyroclastic material (Chapter 10). Space for this intrusion was most likely created by the combined action of gas brecciation and central subsidence, by a relatively small distance, of the pre-existing block inside the ring-dyke.
Structure of the lava fields
The detailed stratigraphy of the Mull Lava Field has not been determined (Table 10), and therefore only the broad outline of the structure is known. The lavas of north-west Mull, Ulva, the Ardmeanach peninsula and the eastern part of the Ross of Mull, and the thick Ben More succession are generally flat-lying or dip gently, although evolved lavas south-east of Ben More are involved in folding associated with the emplacement of the central complex (see below). Away from the central complex, the base of the lavas crops out in the south at Carsaig Bay, in the west at Gribun and in the north at intervals on the coast between Tobermory and Bloody Bay. This may indicate that the lavas form a broad north-north-west-trending syncline in north-west Mull. Several north-north-west- to north-west-trending faults occur on the island of Ulva, towards the western end of the Ardmeanach peninsula and south of Loch Scridain. On the Ross of Mull, near Ardtun, the Paleocene lava outcrop terminates at the west-north-west-trending Loch Assapol Fault, where they are thrown down against Moine metasedimentary rocks.
Structures associated with the central complexes
One of the most notable structural features of the Hebridean Igneous Province is the set of concentric folds that almost encircle the Glen More and Beinn Chaisgidle centres on Mull (Bailey et al., 1924). The folds are developed in the surrounding older lava sequence and the various subjacent pre-Paleocene rocks. They include the Loch Spelve and Loch Don anticlines and the Duart Bay and Coire Mòr synclines (Figure 27). South of Loch Don, Dalradian metalimestones and phyllites form the core of a north-trending anticline, flanked successively by late-Silurian andesitic lavas, Lower and Middle Jurassic sandstones and mudstones, and Paleocene lavas. These major structures continue to the north and north-west, where Jurassic rocks form the core of the Craignure Anticline as far as Craignure Bay, with Moine rocks exposed in the core between Craignure Bay and Scallastle Bay. Elsewhere, the folding generally involves Paleocene basaltic lavas and, on the eastern flanks of Ben More, mugearites near the top of the lava succession. It is suggested that the folds formed in response to the early intrusive events in the central complex, but their age is not well understood. Furthermore, the area enclosed by the folds is domed and the folds may owe their origin to gravity-driven movement as the dome developed. From evidence in the Loch Don area, it has been suggested that doming and folding were initiated prior to eruption of the Paleocene lavas (Cheeney, 1962;Walker, 1975a), but Bailey (1962) considered the evidence to be inconclusive. Near Sgurr Dearg, the folds are crosscut by, and thus predate, volcaniclastic breccias of the central complex.
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