Bailey, et al. 1924. Tertiary and Post-Tertiary geology of Mull, Loch Aline, and Oban (a description of parts of sheets 43, 44, 51, and 52 of the geological map). HMSO [for Geological Survey]
Chapter 22 Ben Buie, Corra-Bheinn, and Beinn Bheag gabbros
Introduction
The three olivine-gabbros dealt with in the present chapter are lettered E on the one-inch Map, Sheet 44. The Ben Buie and Corra-bheinn Gabbros lie outside the earlier of the two central calderas of
The Ben Buie Gabbro is responsible for the finest rock-scenery of the island, with characteristic dark ice-moulded surfaces of particularly massive appearance. The Corra-bheinn Gabbro behaves similarly, though largely masked by later intrusions. The Beinn Bheag Gabbro gives rise to lines of crag in Beinn Bheag itself, where it is greatly cut up by the Late Basic Cone-Sheets of Chapter 28; but in Glen Forsa it is scenically quite inconspicuous.
A discussion of the many points of interest presented by these three gabbros is grouped below under the following headings: Intrusion-Form, Internal Relations, Relations with Cone-Sheets, Relations with Vent-Agglomerates, and Petrology.
E.B.B., G.V.W.
Intrusion-form
The well-marked agreement of the inner boundary of the Ben Buie and Corra-bheinn Gabbros with the general arcuate distribution of Mull geology must not be lost sight of. In
1. Inside the gabbro, lavas with pillow-structure are easily found, but none has been met with outside (Fig 18, p. 133).
2. Outside the gabbro lies the Derrynaculen Granophyre, which seems to correspond naturally with the Glas Bheinn Granophyre, and this latter skirts the outside of the caldera (
3. Outside the gabbro, the vent-agglomerates of the southern slopes of Ben Buie, with their granophyre- and gneiss-fragments, agree in type with the vent-agglomerate found farther east outside the caldera (
Another type of evidence suggests that the Ben Buie and Corra-bheinn Gabbros (but not that of Beinn Bheag) may have a closer analogy with cone-sheets than ring-dykes. Both masses occasionally show flow-banding, marked by a parallel arrangement of somewhat wavy layers of darker and lighter minerals, and this flow-banding is always inclined at high or moderate angles in rough agreement with the cone-sheets of the neighbourhood. Examples from the Ben Buie Gabbro may be cited as follows:
1. The banding of the complex margin, to be described presently, of Creag na Comhla near Loch Fuaran, is inclined north-east. G.V.W.
2. Good banding is exposed south of Uillt Gharbha between ¾ and 1 mile west-north-west of Ben Buie summit. The inclination is north-east at 30° or 40°.
3. Steep banding striking north-west, or west, is shown by exposures within the angle of the path south-west from Craig Cottage across the river-junction.
4. Banding inclined east, or north-east, sometimes at about 70°, is seen 300 yds. north of the Glen More Road, 150 yds. east of the gabbro-margin.
Banding in the Corra-bheinn Gabbro has been noted somewhat often in the more westerly stream below the Coir' an t-Sailein Quartz-Gabbro, and also on the hill-slopes farther east. The dip of the banding is persistently north-east, and the inclination varies from 25°–80°. (C.T.C)
The Beinn Bheag Gabbro is locally well-banded, where it overlooks Coire Gaibhre from the north, but in this case the inclination of the banding contrasts with that of adjacent cone-sheets. In fact, the banding inclines north-east and the cone-sheets south-west. E.E.B.
Unfortunately, the available mountains are not sufficiently large to reveal whether any of the three gabbros have an inclination in bulk corresponding with the suggestions afforded by the local flow-banding. What they do show is that these great masses are of irregular outline and steeply bounded, and that they undoubtedly continue in depth for some considerable distance without serious diminution. (C.T.C), E.B.E., G.V.W.
The difficulties with which the geologist is faced in accounting for the disappearance of pre-existent country-rock in the case of these great gabbro masses are epitomized between Creach Beinn Bheag and the south-east slopes of Meall nan Capull, where a series of miniature outcrops of fine-grained gabbro appear on the one-inch Map as outposts beyond the main Ben Buie front. Their individual form contasts markedly with that of surrounding cone-sheets through which they abruptly rise. One feels that there must have been a considerable displacement of country-rock, either bodily or piecemeal, either up or down, to clear pipe-like cavities for rising gabbro-magma. Possibly explosion to some extent prepared the way.
Internal relations
Beinn Bheag Gabbro
E.B.B.
Ben Buie Gabbro
Three features of special interest may be noted in connexion with the Ben Buie Gabbro
1. On the south of Loch Fuaran, and particularly along the ridge of rock which runs north-west from Creag na Còmhla, the margin of the gabbro shows a well-marked banding. Some of the bands are rich in magnetite, which is magnetized, and occurs in strings up to 4 inches wide, but of no great lateral extent. Other bands exhibit a pock-marked appearance due to the presence of olivine-rich portions. No single origin can be ascribed to the bands; in some instances they are, no doubt, included, and highly altered portions of cone-sheets; in others, appearances indicate that they are magmatic segregations or the gabbro-complex. G.V.W.
2. Several small allivalite masses (U one-inch Map) occur enveloped in the Ben Buie Gabbro of the Glen More district. The largest and most conveniently placed outcrop is exposed on either side of the road, about a third of a mile west of Craig Cottage. It measures 700 by 200 yds. Wherever seen, the gabbro-allivalite junctions are not quite sharp. In one occurence on Sròn Dubh, too small to show on the one-inch Map, gabbro clearly veins allivalite (S17868)
3. Another type of intrusion, associated with the Ben Buie Gabbro is the Craig Porphyrite (P on one-inch Map), named after the road-side cottage already mentioned. The porphyrite occurs in several detached outcrops shown on
Relations with cone-sheets
As is clear on the one-inch Map, all three gabbros have an outer part comparatively free from intrusions, and an inner part cut to pieces by the Late Basic Cone-Sheets of Chapter 28 (the words outer and inner are used with reference to the centre about which the three gabbros are grouped). In the Ben Buie Gabbro, the relatively simple outer part is far more extensive than the inner more complicated part; whereas, in the other two gabbros, the division is fairly equal
It will be understood from the above that a fairly accurate age-comparison is possible between the Ben Buie and Corra-bheinn Gabbros. Detailed evidence will now be given to illustrate certain features of the time-scale already enunciated in Chapter 21, namely: (1) Main Early Basic Cone-Sheet s—non-porphyritic; (2) Ben Buie Gabbro; (3) Early Basic Cone-Sheets, with small felspar-phenocrysts; (4) Corra-bheinn Gabbro. It will be convenient to consider this time-scale backwards rather than forwards, and to deal first with the marginal relations of the Corra-bheinn Gabbro:
Corra-bheinn Gabbro
Most of the hill-side south-west of the gabbro-margin consists either of basalt-lavas or of Derrynaculen Granophyre, in both cases cut by numerous olivine-dolerite cone-sheets dipping steeply north-east; non-porphyritic and porphyritic dolerites occur, the latter distinguished by crowded small felspar-phenocrysts. Quite commonly, these dolerite-sheets (Early Basic Cone-Sheets of Chapter 21), make up about half the rock just outside the gabbro. Individual sheets may be 20 or 30 ft. thick, and they strike at a considerabl angle against the gabbro-margin, there abruptly to terminate. The relationship is very obvious, since the gabbro, for 200 or 300 yds. in from the margin, is almost entirely free from intrusion by cone-sheets of any kind whatever. Moreover, examination of the marginal zone shows that about a mile south-west by south of Cruachan Dearg, and in various other localities, the cone-sheets just outside the gabbro are veined by numberless irregular granophyre-strings, giving rise to an injection-brece,ia; and these strings are also cut off by the gabbro. In harmony with this, the gabbro, for 20 yds. or so in from its edge, encloses and veins numerous shapeless masses of earlier basic igneous rock. Contact-alteration has masked the original character of these xenoliths; but it is natural to conclude that many of them have been derived from neighbouring cone-sheets.
There is little or no evidence of a recrudescence of Early Basic Cone-Sheets after the Corra bheinn Gabbro. All, or almost all, the numberless basic cone-sheets, shown on the one-inch Map as a complex forming the summit region of Corra-bheinn and Cruachan Dearg, are of types referable to the Late Basic Cone-Sheets of Chapter 28.
Ben Buie Gabbro
Anyone wishing to satisfy himself that the Ben Buie Gabbro is cut by dolerite-sheets with small felspar-phenocrysts, and that these sheets form part of the suite cut off by the Corra-bheinn Gabbro, should commence by ascending Uisgeacha Geala from the bill-track, one-third of a mile north-west of Craig Cottage. Several typical exposures of porphyritic dolerite sheets piercing gabbro are seen, both in the stream, and in the neighbouring slopes on the east. The same relation is again exposed near the gabbro-margin above and below the path about half a mile farther north-west. It is impossible to follow individual sheets with certainty from exposure to exposure, but the sheets, just mentioned, are identical in type with some of the suite, which, another half-mile farther on along the same line of strike, is cut across in toto by the Corra-bheinn Gabbro. This particular locality has been chosen merely because of its situation relative to the Corra-bheinn Gabbro. The mere fact that a certain number of olivine-dolerite sheets carrying small felspar-phenocrysts cut the Ben Buie Gabbro can be illustrated almost anywhere in the outcrop of the latter.
To realize the relationship that, in bulk, the Early Basic Cone-Sheets are cut off by the Ben Buie Gabbro, one has to leave the north side of Glen More and go, for instance, to the gabbro-edge on Tòrr a' Ghoai, about a mile south-east of Derrynaculen Cottage. A great mass of doleritic sheets occurs here just outside the gabbro; as a rule, sheet cuts sheet, but occasionally a lenticle of a pre-sheet granophyre is preserved in an interval. Against these lenticles, and against one another, the sheets, at a hundred yards and more from the gabbro, are for ever showing chilled margins. Sixty or seventy yards from the gabbro, such chilling begins to be lost sight of, and, within fifty yards from the same, it cannot be recognized. The last twenty yards consists of rather fine-grained basic igneous rock—no doubt contact-altered cone-sheets with blurred individuality—intricately veined and mixed with gabbro, and, to a less extent, with granophyre. A visible field-indication of contact-alteration in these dolerite cone sheets is a development of reticulate cracks. The porphyritic type of dolerite-sheet is also represented by a few examples, which retain their chilled edges in proximity to the gabbro, and continuing across its margin actually chill against it. These manifestly later dolerites cut the granophyre-veins and do not show the characteristic crack-system just alluded to. (C.T.C)
The same general phenomena are met with, wherever, between Tòrr a' Ghoai and Creach Beinn, a contact of Ben Buie Gabbro with Early Basic Cone-Sheets is exposed. The evidence afforded by the south face of Ben Buie and Beinn Bheag is summarized in
The minor isolated outcrops of fine-grained gabbro, which are shown on the one-inch Map (three on the south face of Meall nan Capull, two on the south face of Creach Beinn, and one on Creach Bheinn Bheag) furnish additional illustrations; and these all the more striking because in them one can easily see the Early Basic Cone-Sheet Complex, interrupted on the one side of each little gabbro mass, and reappearing on the other. The largest of the Meall nan Capull gabbro outcrops is readily found at the head of Coire na Febla. It is cut by about ten cone-sheets (one of them acid), but these ten are as nothing to the basic sheets that are obviously intersected.
Beinn Bheag Gabbro
The geographical position of this mass precludes a direct age-comparison between it and any appreciable proportion of the Early Basic Cone-Sheets. It is, however, clearly of earlier date than all the very numerous Late Basic Cone-Sheets of its locality.
Relations with vent-agglomerates
The age-relationships of the Ben Buie, Corra-bheinn, and Bheinn Bheag Gabbros to the vent-agglomerates of their neighbourhood may be summarized thus: (1) main agglomerates; (2) gabbros; (3) subordinate agglomerates. Direct field-evidence will be presented to prove that some agglomerates are older and others younger than the gabbros. That the former are the more important follows at once when it is remembered that most of the agglomerates in the region with which we are at present concerned are clearly earlier than the Early Basic Cone-Sheets of Chapter 21, and, therefore, a fortiori, earlier than the Ben Buie and Corra-bheinn Gabbros.
Ben Buie Gabbro
A comparison of the north and south slopes of Ben Buie brings to light a most interesting dual relationship between agglomerate and gabbro
Corra-bheinn Gabbro
The Corra-bheinn Gabbro is manifestly later than the vent-agglomerate that is shown on the one-inch Map traversed by Early Basic Cone-Sheets in Sleibhte-coire (pp. 187, 207). On the other hand, it is earlier than a narrow strip of agglomerate (too small to show) about a third of a mile west-south-west of Cruachan Dearg. This little agglomerate-patch is in contact with the Corra-bheinn Gabbro, from which it has derived most of its material. In addition to pieces of gabbro, it carries a small proportion of some non-local acid igneous rock. (C.T.C.)
Beinn Bheag Gabbro
Petrology
The petrological treatment of the subject will be divided under three main headings: Ben Buie Complex, Corra-bheinn Gabbro, and Beinn Bheag Gabbro.
Ben Buie Complex
(Anal.. I.;
The title Ben Buie Complex is used to cover the Ben Buie Gabbro (Eucrite) of the earlier part of this chapter, and, along with this great mass, the attendant small intrusions of allivalite and porphyrite, and also the banded granulitic marginal assemblage developed near Loch Fuaran. The word gabbro as applied to the bulk of the Ben Buie Complex is generally replaced in the sequel by the narrower term eucrite, since the Ben Buie Gabbro, unlike the Corra-bheinn and Beinn Bheag Gabbros, is predominantly of eucritic character. The allivalite-masses, it will be remembered, are supposed to be of older date than the eucrite that envelops them; in this case, the same general relations hold at Ben Buie as Dr. Harker found in Skye and Rum, where ultrabasic plutonic rocks have been succeeded by others of less basic, but allied character. The Craig Porphyrite is clearly later than the eucrite; but, as pointed out already, it should be regarded, according to its field-relations, as part of the same complex.
The chemistry of the allivalite-eucrite assemblage has already been discussed (p. 21) in relation to analyses from Ben Buie and Rum.
The Ben Buie Complex has escaped pneumatolysis to such an extent that it retains a large proportion of its olivine in a fresh state. This is in keeping with the observations that the two most massive Early Basic Cone-Sheets also contain fresh olivine (p. 239).
Allivalite
Several small masses (p. 245), by reason of the dominating presence of olivine and anorthite
Olivine in these rocks is usually in an unaltered condition, and occurs chiefly as rounded to sub-idiomorphic crystals that often attain large dimensions. The crystals are characteristically colourless in thin section, but are locally rendered almost opaque by inclusion of dust-like particles and dendritic growths of magnetite. In its structural relations, the olivine commonly occurs in poecilitic fashion imbedded in large plate-like masses of felspar. As was pointed out by Dr. Harker, in those members of the allivalite group, which are richest in olivine, this mineral is the first to separate from the magma, but as the proportion of olivine decreases we find it moulded upon, and often enclosing, an earlier-formed felspar (S2645)
The felspar of these ultra-basic rocks is optically negative and has extinction angles that prove it to be near anorthite in composition. It is always twinned according to both carlsbad and albite laws, but pericline lamellation is not such a common feature as in the more normal gabbroic rocks subsequently described. Most frequently, it is unzoned, but occasionally, more especially when augite is present in appreciable quantity, it is zoned with an optically positive felspar of basic labradorite composition.
The constant presence of a chromiferous spinellid as an accessory is an essential character of this group of rocks. Generally, it builds well-shaped crystals of fair size which appear coffee-brown by transmitted light, and are referable to picotite (S15631)
This spinellid is an early product of consolidation, but, seemingly, did not always precede the separation of the other constituents. It may be observed enclosed in olivine in very limited quantity, but shows a distinct tendency to segregate in the more felspathic portions of the rock; and veins of anorthite and picotite
The rocks as a whole are remarkable for their freshness, and seldom show any alteration other than a partial serpentinization of their olivine along cracks; some slight rise in temperature, probably on the intrusion of the subsequent eucrite, may account for the occasional development of fibrous and almost colourless hornblende at the margin of the olivine-crystals (S15624)
As has been stated above, the genetic connexion of these rocks with the eucrites is established by the occasional presence of augite and of a plagioclase of less basic composition. Augite is never abundant in the rocks of undoubted ultrabasic character, but occurs in small quantity in association with olivine (S15629)
Eucrite
(Anal. I.;
Passing now to the more prevalent, and presumably later, type of intrusion, we find that, although it presents much variety in texture, and in the relative abundance of constituents, there is a fairly constant mineral-assemblage. It is characterized, usually, by an abundance of olivine, and by the presence of an ophitic diallagic or normal augite, and of a felspar near bytownite in composition
The olivine presents the same general characters as in the peri-dotitic rocks described above. It is usually an abundant constituent, and many of the eucritic rocks approximate to the allivalites by reason of the prevalence of this mineral and the basicity of their felspar (S18451)
A rhombic pyroxene occurs but rarely; in some examples, it appears to be a colourless enstatite, but in others it has the pleochroism of hypersthene (S16713)
The felspar is always more basic than normal labradorite, and most frequently ranges in composition from basic labradorite to near anorthite. It is generally of later consolidation than olivine, which it poecilitically encloses, but occasionally part is of an early generation and may occur within the olivine or with olivine moulded upon it (S17386)
Accessory minerals are comparatively rare in these rocks, apart from scattered patches and crystals of magnetite, and, in the more basic rocks, a chromiferous spinellid. Apatite is of infrequent occurrence, but when present builds moderately large crystals (S15623)
Before leaving the subject of the normal Ben Buie Gabbro, a word may be said concerning the banding which locally is a feature of the mass. This banding is due in part to textural, and in part to compositional, variation (S15626)
Banded granulites, Loch Fuaran
Sharply separated petrographically from all other components of the Ben Buie complex, is a group of granulitic and banded rocks which occurs to the west of Ben Buie, in the neighbourhood of Loch Fuaran (p. 245). These rocks are shown on the one-inch Map as part of the Ben Buie Gabbro (Eucrite), but their appearance suggests recrystallization; and, although in some cases they reproduce the characters of that type of micro-gabbro known as beerbachite, they are most probably metamorphosed masses of an earlier gabbro, or remnants of other early basic intrusions. Similar occurrences were encountered by Dr. Harker<ref>A. Harker, op. cit., pp. 115, 116.</ref> in Skye.
A series of specimens collected from the western shores of the loch are of fine-grained compact rocks that consist mainly of granules of strongly pleochroic hypersthene, granulitic augite, and small elongated crystals of labradorite, is a granulitic matrix of clear labradorite and magnetite (S16864)
As in Skye, the true nature of such rocks is a matter of some uncertainty; but, from a variety of characters that they present, it is clear that they have resulted from the recrystallization of earlier basic rocks. A somewhat coarser type of granulitic rock that may be termed a granulitic hypersthene-gabbro (S16722)
Again, an augite-granulite (S16716)
From Garbh Shlios, north of Loch Fuaran, a rather coarser rock has been collected from just outside the margin assigned on the one-inch Map to the Ben Buie Gabbro. The specimen may be styled a granulitic gabbro, and is mainly composed of augite, hypersthene, and recrystallized labradorite (S16521)
Granulite-inclusions
Turning to granulitic rocks that can definitely be proved from their field-relations to be inclusions of earlier rocks, we find that a dolerite-sheet to the west-south-west of Sròn Dubh (S17902)
Segregation-veins
The Ben Buie Gabbro, in common with most gabbros of similar type, is sometimes traversed by narrow anastamosing segregation-veins of lighter colour. South-west of Craig, such veins are well-developed, and are seen to be granophyre, sometimes almost pegmatitic in character (S15628)
Craig Porphyrite
This type of rock, (p. 245), which clearly must be regarded as part of the Ben Buie complex on account of the field-relations, is distinctly abnormal in character, and presents peculiarities that have resulted from the absorption of gabbro-material by an invading magma of less basic composition. Many features of this rock are paralleled in the hybrid zones that result from the interaction of granophyres and gabbros (Chapter 33). For the most part, as developed on Sròn Dubh, it is a fine-grained rock of dark colour, characterized by a micro-porphyritic development of its ferromagnesian and more basic felspathic constituents
The matrix is a turbid mass of felspathic matter into the composition of which alkali-felspar enters to a considerate extent. Generally speaking, the amount of matrix is not large (S16525)
There can be little doubt that we are dealing here with a rock that has formed from a magma charged with more or less completely digested basic material. Most of its constituents, with the exception of certain of its felspars, have separated from the melt, and give the rock a somewhat delusive normal appearance.
Corra-Bheinn Gabbro
The olivine-gabbro of Corra-bheinn, as developed to the west of Cruachan Dearg and in Glen Clachaig, is a moderately coarse ophitic rock, less basic in general than the eucrites of Ben Buie. It is composed of augite, olivine, and labradorite, with subordinate iron-ore, Olivine is usually present, but most often as pseudomorphs of irregular form composed of serpentine, fibrous hornblende, or both (S14307)
Here, again, a more basic magma seems to have preceded the introduction of the main mass, but the phenomenon is on a smaller scale, or at any rate is less evident, than in the case of the Ben Buie Complex. We find in the Corra-bheinn Gabbro small basic patches (S16500)
About a quarter of a mile to the north-east of Corra-bheinn, what appears to be the Corra-bheinn Gabbro, sliced by Cone-Sheets, has the character of a quartz-gabbro (S17961)
Felspathic veins (S16541)
The older dolerites, where in contact with, and invaded by, the gabbro, have been granulitized in a manner similar to those involved in the Ben Buie Gabbro. They have developed a marked granulitic structure with the production of rhombic pyroxene, and frequently contain large areas of recrystallized felspar (S16528)
Beinn Bheag Gabbro
The mass, which occupies the slopes of Beinn Bheag to the east of Beinn Talaidh and stretches down into Glen Forsa (
The typical coarse rock (S18657)
The finer black-and-white gabbro (S18661)
We will now consider the series of more acid rocks that occurs intimately associated with the coarse gabbro on either side of the summit of Beinn Bheag