Richey, J.E. and Thomas, H.H. 1930. The geology of Ardnamurchan, north-west Mull and Coll. Memoir for geological sheet 51, part 52 (Scotland). Edinburgh: [HMSO for the Geological Survey] Ardnamurchan Central Complex 1:25,000 geological map. British Geological Survey, 2009.
Chapter 21 Tertiary ring-dykes of Centre 3, Ardnamurchan–(Continued)
The ring-dykes that remain to be described comprise the Interior Complex, which is surrounded by the Great Eucrite, and are as follows (see Index Map, p. 201
G. Biotite-eucrite
H. Inner Eucrite
I. Quartz-dolerite veined by Granophyre
J, (J′), and (J″) Quartz-biotite-gabbros
K. Fluxion Biotite-gabbro of Sìthean Mòr
L. Fluxion Biotite-gabbro of Glendrian
M. Tonalite
N. Quartz-monzonite
The area around Achnaha occupied by the Interior Complex is noteworthy on account of its ring-featuring. Its outer limit is set by the conspicuous circle of crags that mark the inner side of the Great Eucrite (see
Time relations between many of the gabbro masses cannot be determined. Two ring-dykes can, however, be proved to be later than the Great Eucrite, viz. the Quartz-dolerite (I) and the Fluxion Biotite-gabbro of Sìthean Mòr (K). An apophysis from the outer side of (K) manifestly cuts the Great Eucrite, but on its inner side no evidence is forthcoming as to its age relations with the Inner Eucrite (H). The Inner Eucrite is with some hesitation regarded as a distinct intrusion from the Great Eucrite, and not merely as a portion of the latter, separated by younger intrusions. An element of doubt also attaches to the Fluxion Biotite-gabbro of Glendrian (L), as to whether it is a distinct intrusion from the similar rock of Sìthean Mòr; for no age relations have been with certainty established between them and the intervening intrusions (H) and (J). The Glendrian Ring-dyke is, however, in sharp contact with the Quartz-biotite-gabbro (J) outside it, and contains small masses of quartz-biotite-gabbro which may be interpreted as xenoliths derived from its walls. If this view be correct, these Glendrian and Sìthean Mòr masses are distinct intrusions. It would seem likely that the various coarse-textured Quartz-biotite-gabbros (J), (J′), and (J″), are all portions of a single intrusion that was split up by the Glendrian Ring-dyke (L) and the Tonalite (M). The Tonalite passes into a fine-grained marginal facies against the gabbros outside it, into which it is clearly intrusive. Cutting the Tonalite in turn there is the small central mass of Quartz-monzonite which marks the final episode of the Later Ring-dyke Complex.
(G) Biotite-eucrite
Biotite-eucrite forms a broad outcrop around the inner margin of the Great Eucrite (E), except to the north. It is noteworthy that a similar discontinuity to the north is a feature of various other ring-dykes of Centre 3. This in itself suggests that the Biotite-eucrite is a distinct intrusion.
The Biotite-eucrite differs lithologically from the Great Eucrite not only in containing biotite, but also in being for the most part more felspathic. The biotite is somewhat sporadic in its occurrence and is locally absent. There is no suggestion in the field, such as well-developed acid veining, that it has resulted by reaction, in situ, between normal eucrite and an incoming acid magma. Sparsely developed acid veining appears to be a late derivative from the mass itself, and is richer in biotite than the normal rock. An example may be seen in the Allt Màm a'Ghaill (S21478)
Perhaps the best locality for judging the distinctness of the Biotite-eucrite from the two intrusions that chiefly bound it, the Great Eucrite and the Inner Eucrite, is alongside the track that leads from Achnaha to Plocaig. A vertical junction with the Inner Eucrite is exposed on the east side of the track, 530 yds. north-west of Achnaha. The Inner Eucrite is easily distinguished from the Biotite-eucrite, since it is in this part of its course of medium grain, fluxioned, and olivine-rich. At the contact seen, no evidence of relative age could be obtained; for neither rock shows any appreciable marginal change against the other. The Biotite-eucrite is frequently exposed alongside the track from this point on to a ford across an easterly branch of the Allt Sanna. Just north of the ford, biotite-eucrite is abruptly succeeded by normal eucrite which is mapped as part of the Great Eucrite. No actual intrusive contact, however, is discernible, but this is not surprising since the two rocks do not differ in texture. Another locality maybe mentioned, viz. alongside the Kilchoan–Achnaha road where it curves. Around a peaty flat, west of Creag an Airgid. There also, though both types of eucrite are found in close association, no intrusive relationship could be discerned. Along the eastern limb of the Biotite-eucrite, as mapped, its margins against the Eucrite intrusions on either side have been only approximately determined. The Biotiteeucrite forms smooth, poorly exposed ground between ice-smoothed rocky country that marks the outcrops of the Eucrites. J.E.R.
Petrology
(Anal. I,
Microscopically the structure and the mineral-assemblage are those of the eucrites. Olivine occurs in a practically undecomposed condition. It is not a particularly abundant mineral, but when present builds rounded crystals several millimetres in diameter, which are usually of earlier consolidation than the felspar, although they may occasionally enclose small crystals of bytownite of earlier generation. The bulk of the felspar is, however, a much twinned but unzoned basic labradorite. It usually shows some slight mottling with a variable extinction, and is also strung to a varying degree with albite. The analysed specimen (S21250)
Augite of a somewhat dirty brown colour forms large ophitic patches and is moulded upon both olivine and felspar. In addition to the monoclinic pyroxene, hypersthene is a moderately abundant constituent. It occurs in association with olivine, and as fairly large irregular ophitic crystals moulded upon felspar. Iron-ore is not very abundant but is usually present as large ill-formed patches, which, in the analysed rock, are fringed with biotite that, with a- colourless amphibole, forms a narrow reaction zone between the iron-ore and contiguous labradorite.
There is little doubt that the rock, as represented by the analysed specimen, has suffered some slight change from its original condition.
This is suggested by the mottled appearance and local reduction in basicity of the felspars, by the interaction of iron-ore and feldspar with the production of biotite and hornblende, by the relative abundance of rhombic pyroxene, and by the fact that apatite, which is of rare occurrence in the most eucritic types, becomes prevalent in those which carry hypersthene. Some modification of the rock as the result of reheating is also suggested by the occurrence of small granulitic patches within the felspars, which are made up of minute granules of biotite, rhombic pyroxene, hornblende, and magnetite.
Although much of the mass is definitely eucritic and carries olivine, there are portions which are more augitic and are composed essentially of an ophitic diallagic augite and labradorite (S21809)
Two other abnormal types have also been detected. One is a moderately fine-grained rock, occurring as veins (S21478)
Another type, also fine-grained and occurring as narrow strips along the road between Achnaha and Plocaig (S22262)
(H) Inner Eucrite
It cannot be demonstrated whether the Inner Eucrite is a portion of the Great Eucrite or a distinct intrusion. There is, however, a sharp contact with the Biotite-eucrite (G), as described above (p. 322).
In its eastern portion the Inner Eucrite forms a slight curving ridge, and further evidence of its arcuate shape is there provided by the changing directions of longitudinal pegmatite veins that traverse it. The veins are found as far north as the point where the Inner Eucrite and Great Eucrite come together. Exposure here becomes poor, but at one place, west of a patch of peat and 500 yds. south-east of the summit of Meall Clach an Daraich, outcrops are more continuous. A steep junction was noted there between olivine-rich eucrite of fairly fine grain (perhaps the inner margin of Great Eucrite) and coarse-textured felspathic eucrite with large irregular olivine-aggregates (perhaps the outer margin of Inner Eucrite). The relative ages, however, of the two rocks in contact could not be ascertained. Southwards, east of the Kilchoan–Achnaha road, eucrite with pegmatite veins gives place westwards, along the course of the ring-dyke, to eucrite with fluxioned felspars, a type that is difficult here to separate from the adjoining Fluxion Biotite-gabbro of Sìthean Mòr (K). Farther west, however, along the north side of the curving Sìthean Mòr ridge, these two masses are quite distinct in rock-type. The Inner Eucrite next to the Fluxion Biotite-gabbro is here fairly fine grained, is rich in olivine, and contains pegmatite veins, while a few yards away towards its interior it becomes an extremely coarse-textured rock with large plate-like augites. West of Achnaha, normal eucrite forms a low curving ridge that is bordered externally by fluxion eucrite. North of Achnaha, alongside a track leading to Plocaig, this fluxion rock is especially well exposed and contains abundant olivine in crystals or crystal-aggregates arranged parallel to the direction of flow. It may be remarked that the prevalence of olivine and absence ofbiotite usually serve to distinguish fluxion eucrite from fluxion biotite-gabbro.
It is not certain whether this fluxion eucrite forms an integral part of the Inner Eucrite. Gradation from the normal type into the fluxion eucrite seems, however, to take place.
Petrology
This ring-dyke is composed of an olivine-rich rock most nearly comparable with the basic portions of the Great Eucrite (E). It has often approximately the same texture as the Great Eucrite, and appears for the greater part to be unmodified. It consists of fresh olivine, diallagic augite, and felspar, ranging in composition from labradorite to bytownite. The olivine is the usual magnesian-iron variety, which is of early consolidation but, like the olivine of the Great Eucrite, frequently encloses small rounded crystals of early felspar (S21481)
(I) Quartz-dolerite veined by granophyre
As already mentioned (p. 321), this narrow, curving outcrop of quartz-dolerite furnishes the clearest example of a ring-dyke within the Interior Complex. Contacts with adjoining rocks provide evidence of its distinctness as an intrusion and of its steep dyke-like form. Occurring as it does, exactly at the foot of the Great Eucrite scarp, it was there identified at an early stage in the mapping of the Interior Complex, and was the first good evidence to be obtained in Ardnamurchan of the ring-form of the major intrusions.
The characteristics of this composite intrusion are similar to those already described in the case of the ring-dyke of Sgùrr nam Meann (e) (p. 256). The main differences are that biotite is of general occurrence in the doleritic portion of (I), and that the granophyre occurs in thicker net-veins. The restriction of the acid net-veining to the narrow outcrop of the ring-dyke is a most -remarkable feature. The walls of the intrusion are nowhere veined, though the granophyre was intruded subsequently to the consolidation of the quartz-dolerite. Sometimes the granophyre forms distinct masses between the doleritic portion and the wall, or may even occur alone, as at the north-west extremity. The two stages of magma-injection were therefore restricted to the same ring-fracture. The quartz-dolerite was intruded, became consolidated, and was then brecciated before the granophyre magma entered its cracks and fissures. The quartz-doierite grades rapidly in places from the normal basic rock to a more acid type, but is everywhere in sharp or hybridized contact with the veins of granophyre. The granophyre cannot be a late segregation product of the crystallization of the quartz-dolerite, at any rate at its present exposed level.
The replacement of the normal net-veined quartz-dolerite by a breccia composed of the rocks forming its walls has been already briefly described (pp. 212–3). This occurs at the north-eastern termination of the Ring-dyke, and furnishes an indication of its mode of intrusion.
The most extensively exposed contact between the intrusion and its wall is on its eastern side, just north of the outwardly projecting arm of the intrusion shown on the Memoir-map. The outer margin of the Quartz-dolerite, varying greatly in composition, can be traced to a chilled edge against the Great Eucrite. The plane of junction is well exposed on a steep face of eucrite, and though somewhat irregular is, taken as a whole, inclined eastward at about 70 degrees, outwards from the Achnaha Centre (3). Other chilled junctions may be seen farther south and west, for example in the tributary stream south-west of the Allt Màm a'Ghaill, on the outer margin of the Ring-dyke. Along its inner margin in this stream, granophyre forms a mass distinct from the quartz-dolerite portion, and presents a vertical unchilled contact against the Biotite-eucrite within it.
In the Allt Màm a'Ghaill, the quartz-dolerite portion is separated from both walls by granophyre, by which the dolerite is abundantly net-veined. In contrast, the granophyre sends broad tongues into the walls of Biotite-eucrite on either side, but does not net-vein them. J.E.R.
Petrology
A considerable portion of the mass has the appearance of a moderately fine-grained quartz-dolerite (S21495)
The acid veins that string through the more basic portions of the intrusion show evidence of basification. They are granophyres (S21536)
(J), (J′), And (J″) Quartz-biotite-gabbros
Quartz-biotite-gabbro occurs in three distinct masses separated by two intrusions both probably of later date (p. 322). The rock is very variable in grain, but for the most part extremely coarse-textured and comparable to pegmatite. In containing iron-ore, in large crystals, and biotite, it differs from eucrite; while quartz can usually be seen in the more acid, felspathic, portions or in drusy cavities. Olivine is frequently present, and in some places the rock approaches eucrite in composition. Epidote is often developed, usually in the more acid quartz-bearing portions.
As already stated, it is considered probable that the three masses here described together are parts of one intrusion. In the case of the two inner masses (J′) and (J″), this is practically certain, for they are separated only by the Tonalite, which is demonstrably of later date. Their former connexion with the outermost and largest mass (J) is not improbable. This mass is separated from (J′) by the Fluxion Biotite-gabbro (L) which appears, by such evidence as is available (p. 334–5), to be later than (J).
While the distinctness of the two larger masses, (J) and (J′), from adjoining intrusions is evident at many places, it should be stated that, in the eastern parts of both, great difficulty was experienced in the field in separating them from adjoining gabbros. The difficulty is, partly at least, a matter of sparseness of good exposure. Possibly, however, had more time been available, the mapping in this part of the area, around Glendrian farm and farther east, might have been improved.
The screen-like mass (J″) is mapped as a unit but, besides the normal coarse-textured gabbro, includes a finer-grained type that may belong to another intrusion (see p. 337). J.E.R.
Petrology
(Anal. VI,
The mass (J′) is a moderately coarse augitic gabbro that has suffered considerable modification. The changes that the gabbro has experienced are the intense albitization of the basic plagioclase (S21538)
(K) Fluxion biotite-gabbro of Sìthean Mòr
This arcuate mass forms the curving steep-sided ridge of Sìthean Mòr, and also extends eastwards across the Kilchoan–Achnaha road. It is separated from the Glendrian Ring-dyke (L), which is similarly composed of fluxion biotite-gabbro, by the Inner Eucrite (H) and the Quartz-biotite-gabbro (J), as already mentioned (p. 321).
The rock is typically medium-grained and dark in colour, and its felspars are markedly fluxioned. Biotite is plentiful, and also iron-ore in small crystals. In the middle part of its course, west of the Kilchoan–Achnaha road, the intrusion, as mapped, consists of three portions, the normal fluxion gabbro on the north, coarse-textured quartz-biotite-gabbro on the south, and an intervening portion composed of the two bordering gabbro-types in alternating bands
In the banded portion, individual bands are usually inclined inwards towards the Achnaha Centre (3), but are sometimes vertical or inclined outwards. Where inclined, the angle is always steep.
The banding phenomenon is quite different from that found in the Hypersthene-gabbro (a) for example (p. 221), which is due to flow-movement during the crystallization of a homogeneous magma. The bands are too wide, and besides they are formed of two distinct rock-types. In this banded zone a third rock-variety occurs which bears a resemblance to the other two, and seems to have resulted from their intermixture; for the banding becomes indistinct in this apparently mixed rock. The banding is best seen at a point 200 yds. west of where the intrusion is crossed by the Kilchoan–Achnaha road. There, on the inner or northern side of the intrusion, the normal type, a medium-grained dark-coloured fluxion biotite-gabbro, occurs alone. On the outer or southern side, coarse-textured unfluxioned quartz-biotite-gabbro is similarly developed as a rock-mass by itself. In the middle, banded zone, the conspicuous white bands of the coarse gabbro vary from an inch or so in width on the north side to, a yard or more on the south side. These broad bands are sharply defined, and are seen to extend for many yards without change of direction or width. A similar variation in the width of the intervening bands of the darker gabbro also occurs, except that in this case the increase in width takes place towards the north. The widest bands are, in fact, found next to the mass, to which they correspond in composition.
A variation in the character of the white gabbro bands is noteworthy. Where these are wide, the rock is coarsely or indeed very coarsely crystallized, and there is no fluxion arrangement of the minerals. In contrast, the associated bands of the darker gabbro show perfect fluxion arrangement of the felspar crystals, parallel to the edges of the bands. But, where the white gabbro is in thinner bands, their crystallization is finer and the felspars frequently show fluxion parallel to the banding. It is in this part of the banded zone that the banding often becomes indistinct, and the intermediate mixed type of gabbro is developed.
From the above observations the conclusion is drawn that two distinct magmas found their way into the same ring-cavity, and along their mutual junction formed a banded gabbro. The condition of these two magmas was different. The fluxion gabbro was evidently in a partially crystallized state while it flowed in interbanded fashion with the other magma. The narrower bands of the white gabbro, however, must have started to crystallize before flow-movement had ceased, since their felspar-crystals are often fluxioned. As already stated, it is in this part of the banded zone that intermingling of the two magmas appears to have often taken place, producing the intermediate type of gabbro. In the outer, southern part, where the magma that gave rise to the broad bands of white gabbro remained liquid till flow-movements had ceased, the individual bands are in perfectly sharp contact. It seems necessary to assume that the two magmas were here inhibited from mixing with one another. This may perhaps be in some way connected with the fact that the magma which formed the darker gabbro was in a partially crystallized state while the other remained altogether fluid.
Marginal relations
The Sìthean Mòr intrusion is of importance because of its marginal relations to the Great Eucrite (E). Half a mile west of the Kilchoan–Achnaha road an apophysis from it can be traced out into the Eucrite
Further details concerning the apophysis may be added. At a point l00 yds. west of the Kilchoan–Achnaha road it extends out from the Fluxion Biotite-gabbro for a short way into the Eucrite, and then runs westwards for a distance of 250 yds. parallel to the parent mass, which it then rejoins. At this end it is very narrow, being only about 12 ft. wide, and the fine-grained marginal type occurs alone. Exposures are discontinuous for a few yards where the apophysis should connect on to the main mass, but they are so alike in rock-type, fine grained, fluxioned, dark, and with plentiful biotite, that their continuity cannot be doubted.
The small xenoliths of eucrite are found all along the outer marginal portion, which is well exposed. They are usually of normal eucrite, but at about the middle of the apophysis they consist of pegmatitic eucrite with large plate-like augites. At this point similar coarse-grained eucrite bounds the apophysis on its northern side, and it is therefore concluded that the xenoliths are derived from the eucrite forming the walls. A short way west of this the xenoliths are especially well seen, just east of a stone-wall that crosses the apophysis
Though the junctions of the Sìthean Mòr intrusion with both the Biotite-eucrite (G) and Inner Eucrite (H) can be determined to within a few feet, no actual contacts have been found. Consequently the relative ages of these three intrusions are not known. Where the eastern arm of the apophysis branches from the main mass it is, however, in intrusive contact with a narrow outcrop of biotite-bearing eucrite or gabbro that would appear to belong to the Biotite-eucrite. There are therefore some grounds for considering the Biotite-eucrite to be earlier than the Sìthean Mòr intrusion.
Segregation-veins with ill-defined margins were noted in the Fluxion Biotite-gabbro in a small burn just east of Sìthean Mòr, south of where the Kilchoan–Achnaha road crosses a stream–the Allt Uamha na Muice. The rock contains large crystals of biotite and is akin to tonalite (S21540)
Petrology
This mass, from a petrological point of view, has much in common with the Biotite-eucrite (G), which it adjoins. These two intrusions, although demonstrably distinct, have undoubtedly a similar history, both being fundamentally eucritic in nature. The variable character of the Sìthean Mòr mass is presumably due to the hybridizing influence of an acid magma on a partially crystallized basic magma of eucritic composition. It presents characters that were encountered locally in the Biotiteeucrite, and it possibly represents a more advanced stage of hybridization than the adjoining mass exhibits. The mineral assemblages however, are similar in both cases.
Where the mass is obviously eucritic, it is a moderately coarse-grained basic gabbro (S21482)
Veins in the mass may be described as basic granophyre (S21540)
A specimen collected from the white-weathering gabbro of the banded zone (p. 330) is gabbroic in texture (S22628)
The abnormal character of the mass of quartz-biotite-gabbro on the outer side of the banded zone is evident in specimens collected (S21475)
The interesting apophysis of the Fluxion Biotite-gabbro (K), which stretches out into the Great Eucrite and which has been referred to in detail above (p. 331), has a fine-textured sharp margin against the Great Eucrite. The junction of the two rocks can be examined in the area of a thin section. The marginal portion of the Sìthean Mòr mass (S22252)
The Eucrite (E) at the junction shows signs of crushing which are absent from the fine-grained rock in contact with it, but this crushing has been followed by a granulitization of the broken-down material. The basic felspars are intensely albitized both marginally and by veins of soda-felspar that traverse them in all directions. This intense albitization is undoubtedly connected with the later intrusion. Augite has passed over to uralitic hornblende, and there has been a small development of biotite.
At the actual contact, there has been a certain amount of give and take between the two rocks involved. On the Eucrite side there has formed a narrow zone of somewhat coarsely crystalline rhombic pyroxene and biotite with a recrystallized felspar of moderately acid composition. The rhombic pyroxene is pseudomorphed by fibrous hornblende, as also is the olivine occurring in the eucrite. This amphibolitization is presumably part of the process of thermal alteration to which both rocks have been subjected. The interior of this apophysal mass (S22253)
(L) Fluxion biotite-gabbro of Glendrian
The almost complete ring-ridge formed of this intrusion is one of the striking features of the Achnaha district (see
Sharp contacts between the Fluxion Biotite-gabbro and Quartzbiotite-gabbro outside it are exposed, on the south margin, along a scarp overlooking the Kilchoan–Achnaha road (
The distinctness of the Fluxion Biotite-gabbro on its inner side from the coarse Quartz-biotite-gabbro (J′) is most evident where their junction has been mapped across a stream 500 yds. southwest of Glendrian farmhouse. In a steep scar forming the right bank, the Quartz-biotite-gabbro can be followed in exposures to within a few yards of where the Fluxion Biotite-gabbro outcrops. The actual contact of the two rocks is not seen.
Petrology
(Anal. I,
The analysed specimen (S21249)
As mentioned above (p. 334), it is not uncommon to encounter large included masses of normal gabbroid material, which are quite unmodified. These Mr. Richey regards as of xenolithic origin, and microscopic evidence supports his view. One such mass from the eastern portion of the Fluxion Biotite-gabbro's outcrop (S21806)
The Fluxion Biotite-gabbro consists generally of large fluxionally arranged plates of intensely zoned and albitized basic labradorite, much augite frequently fringed with uralitic hornblende, and abundant titano-magnetite that occurs chiefly in association with the augite. Occasionally, the felspars are unzoned and appear to have escaped any serious albitization. In such cases, they have an optically negative sign and are of bytownite composition (S21489)
Apatite has undergone a concentration in the acid portions of the rock, similar to that observed in the tonalitic and quartz-monzonitic masses, and forms crystals nearly 2 mm. in length (S21249)
There is little doubt that this Fluxion Gabbro owes many of its somewhat unusual physical and mineralogical characters to the intervention of an acid magma. It is difficult to say to what extent the rock-mass is to be regarded as of hybrid nature, but that it was in a modified condition before intrusion appears to be more than probable. H.H.T.
(M) Tonalite
The Tonalite is in plan an oval-shaped mass, the longer axis of which is directed east-north-east, in conformity with other ring-dykes of Centre 3. Its margin has, however, many minor irregularities, as may be seen where it makes contact with the gabbros outside it, along a scarp that borders the road a quarter of a mile south-east of Achnaha. The striking difference in composition between the Tonalite and the surrounding gabbros enables its margin to be accurately traced.
The rock is everywhere of the same general type, with conspicuous biotite set in a greyish felspathic ground-mass, the dominant felspar being lath-shaped plagioclase. The rock varies in grain from place to place, though medium textures are most usual. Between an elongate screen-like mass of Quartz-biotite-gabbro (J″) and the outer wall, fine-grained types are chiefly met with, while at a point north-west of the western end of this screen the rock is unusually coarse.
Along its outer margin the Tonalite becomes much finer grained and at the same time more acid in composition. A gradual passage outwards from typical tonalite to this more acid margin can be established on the north-west and northern sides of the mass. The fineness of texture is a result of chilling, and not due to the change in composition, for on the north side the acid marginal type becomes finer grained close to the margin than it is farther away. The exposures concerned are along the eastern arm of a stream-loop which the junction crosses. In rock-type this acid margin is quartz-monzonite. Reference may be directed to Chap. 15, p. 209, where the significance of such acid margins is discussed.
The screen-like mass of Quartz-biotite-gabbro (J″), enclosed in the south-eastern part of the Tonalite, is seen at one point in contact with the Tonalite, which is chilled against it. This contact is located on the south side of the gabbro-screen, midway between two north-westerly hollows that cross the west part of the mass, and mark the course of two basic dykes. The western half of the gabbro is composed of a fine-grained type, and the eastern half of extremely coarse pegmatite-like gabbro. No contacts could be found between these two rocks owing to lack of exposure, but they may well be separate intrusions. Both may be classed as quartzbiotite-gabbro, though the more easterly rock is certainly more acid and must approach tonalite in bulk composition. Fluxion biotite-gabbro is a third rock-variety found in the screen, and forms its narrow south-western end.
The quartz-biotite-gabbro and fluxion biotite-gabbro of the screen correspond in position to similar gabbros, (J′) and (L), that form the opposite, outer wall of the Tonalite. They are evidence of the slightly transgressive nature of the Tonalite mass relatively to other ring-dykes, as is further brought out by the mapping of the Glendrian Ring-dyke (L). This intrusion, otherwise a complete ring, is broken through by the Tonalite for a short distance on its south-west side.
Acid veins traverse the gabbro walls of the Tonalite, and are abundant alongside the Achnaha road, a quarter of a mile southeast of Achnaha. They were classed as granophyre in the field. They have not been examined microscopically, but are of interest as being the acid expression of the Tonalite magma.
An example of hybridization in situ is well seen at the southwest termination 'of the gabbro-screen described above. There, xenoliths of the gabbro are contained in the Tonalite, and may be observed in all stages of dissolution. Patches of basified-looking tonalite probably represent completely digested xenolithic material. It may be remarked that no other instance of such hybridization was noted within the Tonalite. J.E.R.
Petrology of the tonalite
(Anal. II,
The Tonalite, with its related central intrusion of quartz-monzonitic type, is of especial interest, not only because it is one of the largest masses of this character met with in the Tertiary Province, but because it exhibits peculiarities of petrogenetic importance which point more or less conclusively to a hybrid origin.
In its most homogeneous and most normal occurrence, such as to the north and north-east of the Quartz-monzonite, it is a moderately fine-grained holocrystalline rock of grey-green colour, in which felspar and elongated crystals of hornblende occur in about equal proportions. Elongated much-twinned plagioclase felspars, up to a quarter of an inch in length, are common, but the bulk of the felspar is of later formation and occurs as a matrix to the other constituents. This later felspar frequently has a decided pink tinge.
Biotite is usually subordinate to hornblende but occurs in small deep-brown flakes.
The structure of the rock is granitic, and the constituents, as proved by microscopic examination, are plagioclase felspar, hornblende, biotite, augite, perthite, and quartz; with some apatite, magnetite, and sphene (S21248)
The plagioclase occurs as much fissured, resorbed, and intensely albitized crystals of labradorite, zoned with oligoclase and fringed with perthitic orthoclase (S21468)
To the south of the monzonitic mass, the tonalite in its central portions is of the same type as that described above, except perhaps that augite is a little more abundant (S21541)
Between the screen of biotite-gabbro and the eastern margin of the mass, the usual type of tonalite is encountered (S21469)
Towards the outer margins the rock becomes finer in grain and more acid in composition. It is noticeable that biotite ultimately becomes the dominant ferromagnesian constituent, and with a certain amount of augite gives to the mass a quartz-monzonitic or mica-felsite character. Towards the western and northern margins the tonalite retains its dominantly hornblendic nature for some distance (S22260)
The xenocrystal nature of the Tonalite, the presence of hornblende and biotite of late generation, and the lateral passage into rocks of quartz-monzonitic type along its peculiar acid margin, are features that all point to this intrusion being a gabbro-granophyre hybrid. It is, however, equally certain that the hybridization was an accomplished fact before the intrusion of the mass into its present position, as in the case of the marscoite masses of Skye; but, instead of the acidification of a basic magma, as in the Skye occurrences, we are presumably dealing here with the basification of an acid magma.
The Quartz-biotite-gabbro Screen (J″)
This screen-like mass within the tonalite is definitely of gabbro-origin. It has, however, suffered greatly at the hands of an invading acid magma, and like other biotite-gabbros shows unmistakably the effects of hybridization. In fact, the changes that have taken place are just such, given full expression, as would lead to the formation of tonalitic and quartz-monzonitic types. For instance, a moderately coarse augitic gabbro (S21543)
(N) Quartz-monzonite
As a rock the Quartz-monzonite may be distinguished from the surrounding Tonalite by the following criteria, Biotite is more plentiful and in larger crystals than in the Tonalite. The felspathic portion of the rock is finer textured, and it is possible to distinguish in the hand-specimen the darker plagioclase from a lighter and compact orthoclase-quartz portion. The latter constitutes about a half to a third of the rock.
The little central mass of Quartz-monzonite is separated as a distinct intrusion froth the Tonalite, on account of an intrusive junction seen on its north-west side. There, a sharp contact between the two rocks exists, and while the Tonalite does not change in grain, the Quartz-monzonite is markedly finer textured for an inch or so away. The Quartz-monzonite is situated at the actual centre of the Interior Complex, and is the latest intrusion of the whole ring-dyke suite.
Petrology
(Anal. III,
Microscopically
The amount of labradorite is small, for it exists only as somewhat infrequent centres to some of the larger plagioclase-crystals that are deeply zoned with less basic varieties, and which usually have oligoclase peripheries.
The biotite is a deep-brown variety. It frequently encloses crystals and patches of oligoclase felspar, but the perthitic alkali-felspar and quartz are moulded upon it. It is more or less devoid of other inclusions, except occasional apatite, and does not exhibit pleochroic halos. When decomposed it passes into chlorite with, sometimes, the separation of granular sphene.
The augite is scattered indiscriminately throughout the rock, generally as small ill-formed crystals but occasionally as prisms and stumpy individuals with fairly good crystal-outlines. It has the wide extinction and moderately large axial angle of a normal aluminous augite, but its colour is similar to that of the recrystallized augite of the modified gabbros that have been frequently referred to in earlier pages.
The perthitic felspar builds the largest individual crystals in the rock, and embraces all the other constituents. It is usually intensely turbid and in it, and in the oligoclase, is concentrated the bulk of the apatite. As in most other Ardnamurchan rocks of this nature, it is clear that the constituents of apatite are concentrated in the alkaline and more siliceous portions of the magma, and the period of crystallization of the mineral is mainly concurrent with that of biotite and acid plagioclase, including the albite-perthite. It is seldom encountered in the quartz except as crystals projecting from neighbouring felspars.
This monzonitic mass is genetically related to the tonalite described above; and chemically, as will be seen from the analyses (