Mull Central Complex, Hebridean Igneous Province

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Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Mull Central Complex[edit]

Mull Central Complex. P914146
Mull Central Complex: Centre 1, Glen More Centre and the Early Caldera. P914147
Mull Central Complex: Centre 2, Beinn Chaisgidle Centre. P914148
Mull Central Complex: Centre 3, Loch Bà Centre and the Late caldera. P914149

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 (P914146). 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[edit]

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 (P914147). 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[edit]

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[edit]

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[edit]

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 (P914147). The felsites are cut by younger basic intrusions (mainly cone-sheets) which obscure the original geometry.

Early cone-sheets[edit]

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 (P914147).

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[edit]

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[edit]

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[edit]

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 (P914148). 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[edit]

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[edit]

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[edit]

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[edit]

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. Conseqently, their emplacement also constitutes the earliest phase of intrusive activity associated with the youngest centre.


Centre 3, the Loch Bà Centre[edit]

Centre 3, also known as the Loch Bà Centre, was associated with the development of the Late Caldera (P914149).

Late Basic Cone-sheets[edit]

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 (P914149). 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[edit]

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 (P914149), 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[edit]

The Beinn a'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[edit]

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.


References[edit]

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