Scourian-Laxfordian Relationships in the Barra Isles

Scourian-Laxfordian relationships in the Barra Isles

PETER W. FRANCIS

CONTENTS I Outline of geological relationships ..... I62 2 The Eastern Gneisses ...... x63 3 The Oitir Mhor Zone ...... x7I 4 The Western Gneisses ...... I7I 5 Correlations ...... x74 6 Synthesis ...... x74 7 Some aspects of dyke deformation ..... x76 8 Discussion ...... ~78 9 References ...... x8x

SUMMARY The Lewisian rocks of Barra are a Laxfordian occurred in the supra-structure under amphi- supra-structure of amphibolite facies gneisses bolite facies conditions, which continued after and a Scourian infra-structure characterised deformation ceased. In the infra-structure, by orthopyroxene bearing gneisses. Both units evidence for a pre-Scourie dyke pyroxene contain representatives of the Scourie Dyke granulite facies metamorphism is preserved. suite; in the supra-structure they are highly The Scourie dykes of the infra-structure have deformed and folded; in the infra-structure pyroxene granulite facies mineral assemblages they are relatively undeformed, unfolded, and resulting from intrusion into hot or dry country retain original discordant relationships. The rock gneisses. infra-structure also has several suites of in- Probably a very old cover to basement rela- trusive igneous rocks earlier than the Scourie tionship is preserved in the area, which was dyke suite, the most widespread being dykes of first deformed in the Scourian orogeny, and dioritic composition. then subsequent Laxfordian deformation of the The same sequence of Laxfordian deforma- interface between the two major units produced tion may be recognised in both units. Folding overall structure of the area.

WHEN THE TERM 'LEWISIAN' WaS first applied to the Precambrian gneisses of Northwest Scotland, little was known of the geology of the islands in the Outer Hebrides, that are among the few areas of Britain never to have been mapped systematically by the Geological Survey. One of the principal results of recent work by a group from Imperial College, London, has been the recognition of areas of conspicuously low Laxfordian deformation and the interpretation of these areas in terms of the overall structure of the Hebrides. This paper concerns the southernmost part of the chain, centred on the Island of Barra, where a Scourian assemblage which is affected by relatively slight Laxfordian deformation, underlies a series of more highly deformed gneisses, that are regarded as a dominantly Laxfordian assemblage. The contrast and relationship between these two important units is described, and the use of deformed dykes in their interpretation is illustrated.

Jl geol. Soc. Lond. vol. x29, t973, pp. I6X-I89, 6 figs, 4 plates. Printed in Northern Ireland.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I62 P. W. Francis

The first comprehensive account of the Outer Hebrides was by Jehu & Craig (I925). The first correlation came from Kursten (I957) who recognised at three localities two phases of deformation that he compared with the Scourian and Laxfordian episodes of the Scottish mainland. Dearnley (I962) distinguished a central area in which Scourian rocks survived from areas strongly affected by Laxfordian deformation and metamorphism and drew further comparisons with the Scourian and Laxfordian complexes distinguished on the mainland (Sutton & Watson, I95I ). His recognition through most of the Outer Hebrides of basic dykes which he correlated with the Scourie dykes which intrude Scourian rocks and are affected by Laxfordian modifications on the mainland was of particular importance. The possibility that pre-Scourian gneisses existed in the islands was first raised by Dearnley & Dunning (I968) and was explored in more detail by Coward et al. (1969, 197 I). The Barra Islands have been mapped by Hopgood (I964, I965, I97I; see also Bowes & Hopgood I969). Hopgood recognised a number of early intrusive types and the occurrence of pyroxene beating metamorphic assemblages in some eastern localities. Both these observations are confirmed by the present work as is the demonstration that the gneisses have undergone repeated deformation. The sequence of episodes recognised by Hopgood, and the inferences concerning the large-scale structure, differ very greatly from those that are developed below.

I. Outline of geological relationships The Lewisian rocks of the Barra area are considered as three geological units, the Eastern Gneiss, the Western Gneiss and the Oitir Mhor zone. The terms 'Eastern' and 'Western' gneisses have been employed by Jehu and Craig, Dearnley and other authors in a broadly geographical sense to distinguish between the rocks which lie respectively above and below the Outer Hebrides thrust, that major eastward dipping dislocation which traverses the Outer Hebrides. In Barra, the Eastern Gneisses differ in lithology and history from the main part of the Western Gneisses and so constitute a geologically distinct unit. A small part of the area west of the thrust, which has been named the Oitir Mhor zone, is occupied by rocks almost identical with the Eastern Gneisses. These rocks have a natural contact with the main part of the Western Gneisses and it is suggested that their relationships with the Western Gneisses provide a key to the regional structure. The Eastern Gneiss unit of Barra is made up of banded acid gneisses, often carrying orthopyroxene, which are cut by a variety of small igneous bodies. Isotopic dating of samples from early and late sets of intrusions indicate that they were emplaced at about 26oo m.y. (Francis et al. I97I ) and they are therefore considered, with the pre-existing gneisses, to be of Scourian age. A suite of basic dykes with two pyroxene assemblages cutting pegmatites dated at 2462 :k 53 m.y. cut the intrusives mentioned above but are themselves locally metamorphosed, deformed and cut by pegmatites dated at I679 m.y. Both petro- graphically and in their relationships these dykes resemble the basic suite identified throughout the Outer Hebrides by Dearnley (I96~) and correlated by him with the Scourie dykes which pre-date the main Laxfordian events on the mainland.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles t63

The degree of Laxfordian deformation affecting these dykes and other rocks of the Eastern Gneiss unit is moderate or slight except within a narrow zone of amphibolite facies acid gneisses which will be referred to later. In the Oitir Mhor zone, orthopyroxene bearing acid gneisses cut by small intrusions of several kinds, the latest being a suite of pyroxene bearing basic dykes, form an assemblage exactly comparable with that of the Eastern Gneisses. Effects of deformation later than the emplacement of the basic dykes are slight. The rocks of the Oitir Mhor zone pass into the Western Gneiss unit through a very narrow transition zone marked by rapid increases in the deformation of the basic dykes and by the amphibolitization of dykes and gneisses. The widespread Western Gneiss unit is an assemblage of banded and migmatitic biotite or hornblende bearing gneisses in which highly deformed amphibolite sheets represent the basic dykes of the other units. Laxfordian deformation is strong through much of the Western Gneisses. The contrasts between the dominantly pyroxene bearing Eastern Gneisses and Oitir Mhor zone and the dominantly hornblende bearing Western Gneisses are not only lithological but extend also to the style and distribution of successive sets of minor structures. The recognition of such contrasts, has led the author to conclusions radically different from those of Hopgood (I97I), who regarded the structural pattern and history as being approximately uniform throughout the area. 2. The Eastern Gneisses By reason of their relatively slight modifications resulting from Laxfordian deformation and metamorphism, the Eastern Gneisses provide valuable evidence concerning the early history of the assemblage (Table I). Three zones characterised by differing intensities of Laxfordian (post Scourie dyke) modification are distinguished (Fig. I). In the area of least deformation (zone I), Scourie dykes are consistently discordant, are not folded or boudinaged and usually contain mineral assemblages of granulite facies. In the zone of moderate deformation (zone 3), discordant relationships are common but the dykes are folded, deformed and partly amphibolitized. A zone of intense deformation about two kilometres broad (zone 2), sandwiched between these zones, is characterised by intensely folded, boudinaged and amphibolitized dykes which rarely retain discordant relationships; this zone of high Laxfordian deformation occupies a position of great structural significance.

A) ZONE I~ THE ZONE OF LEAST DEFORMATION The acid gneisses of this zone often contain large brown orthopyroxenes that give the rock a rusty colour due to retrogression of the orthopyroxene to biotite and other minerals, and with the development of brown hematitic rims around all grains. Within the gneisses, fragments and blocks of earlier material are often caught up in an agmatitic structure. Basic blocks frequently show an early banding transected by their margins and unrelated to the foliation of the acid gneiss,

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I66 P. W. Francis

The youngest set is granodioritic or granitic with abundant quartz and oligo- clase with lesser microcline and some orthoclase. Biotite is the chief mafic mineral, together with a little partially retrogressed orthopyroxene. These dykes are not common. At two localities they cut dykes of the intermediate suite. This period of dyke intrusion was followed by the local production and intrusion of irregular granitic sheets that invade and replace rocks of earlier ages, particularly in the Skallary area where they form inclusion-complexes filled with rounded enclaves of banded and dioritic gneisses. Dykes of intermediate composition are often recognisable as rafts in these granitic bodies, which are never more than ten metres thick. A period of pegmatite intrusion then followed with numerous discordant pink potash feldspar quartz bodies. It is not always possible to distinguish between these early pegmatites and the almost identical post Scourie dyke pegmatites but a particularly large pegmatite, first described by Hopgood (I 964) at Leenish Point is clearly cut by a thick Scourie dyke. This body and some smaller pegmatites have been dated by Rb-Sr methods at nearly 2,600 m.y. (Francis et al. I97I ). The emplacement of the basic Scourie dykes marked the last major intrusive episode in the area. Dykes over ~o metres thick are common, and the largest is traceable for over 2 km. All dykes tend to have amphibolitic margins, but the centres of the larger dykes are usually granulitic (c.f. Jehu & Craig I925). The following assemblages were observed" (I) Plag-hbde-opaque (2) Plag-hbde-clpx-opaque (3) Plag-hbde-clpx (4) Plag-hbde-clpx-garnet-opaque (5) Plag-hbde-clpx-orpx-garnet-opaque (6) Plag-hbde-clpx-orpx-opaque (7) Plag-hbde-clpx-orpx (8) Plag-clpx-orpx-opaque (9) Plag-clpx-orpx-garnet-opaque Of these, 6 is by far the most abundant. Types I, 2 and 3 occur at the centres ot narrow dykes, and in the marginal zones of larger bodies. Assemblages containing garnet seem to be distributed erratically. The mineral textures are uniformly simple--all the minerals form equant grains of roughly the same size, are perfectly fresh and have regular boundaries between one another, while triple points between like minerals commonly show I2o ° interfaces. Some of the larger dykes show a distinctive maculose texture, with the same minerals, which probably mimics an original ophitic texture.

(B) zoN~ 2, T~. ZONE OF INT~NS~ D~.FORMATIO~ In this zone, boudinaged, folded and migmatized Scourie dykes occur in a band of rather uniform amphibolite facies acid gneisses. There is a sharp transition west- ward from zone I to zone 2, the change from discordant granulites to concordant amphibolite dykes taking place within I5o metres (P1. 2). Within this zone no intrusive bodies older than the Scourie dykes have been identified; it will be shown

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles x67

later that this zone is a tightly infolded synform of Western gneiss type within the Eastern gneisses.

(C) ZONE 3~ THE ZONE OF MODERATE DEFORMATION Within this zone, Scourie dykes are sometimes folded, but their discordant relationships are also commonly preserved (Fig. 3A). The dykes, though predominantly of granulite facies mineralogy, are often foliated and partially amphibolitized. Recognisable earlier dykes have not been found, but larger bodies of dioritic to tonalitic composition, identical in mineralogy with the intermediate dykes of Zone I, are considered to be equivalent to the intermediate dykes. Relatively large bodies of pink foliated granite are interleaved with these dioritic rocks. Biotite-rich gneisses containing orthopyroxene have been observed in one locality.

(D) STRUCTURAL HISTORY OF THE EASTERN GNEISSE$ Evidence of deformation both before and after emplacement of the Scourie dyke is seen in the Eastern Gneisses. Evidence relating to early phases is naturally clearest in zone I where disrupted layering is seen in agmatites and where many tight minor folds are cut by Scourie dykes. A fold of this type figured by Hopgood (I97I , Photo I3) is assigned by him to a phase designated F3 and correlated with structures regarded by the present author as being of a much younger generation in the Western Gneisses. No evidence of deformation between the times of intrusion of the early dyke suites and the Scourie dykes has been noted other than that associated with the invasion of granitic sheets. Laxfordian deformation in the Eastern Gneisses appears to have been related mainly to one early phase in which the overall structure of'the unit was established. A later Laxfordian phase is marked only by minor folds with axes roughly east- west which do not seriously modify the general structure. In zone I, the banding of the gneiss outlines a broad asymmetrical antiform on a northwest axial plane (Fig. 2). The closure of this structure is complex but the plunge appears to be northwestward. The northeastern limb is irregular and gently dipping. The southwestern limb, overturned relative to the hinge, forms a large area of regular north-south foliation. No new fabric and few minor folds or boudins are associated with the antiform in most of zone i, but earlier planar elements and folds may have suffered flattening and realignment. Although dyke discordances are numerous, the actual angular discordance is usually small. In zone 2, tight folds are common, affecting the Scourie dykes (P1. 2) and both dykes and gneisses dip uniformly at about 6o ° FN~.. Hornblende and biotite define planar fabrics parallel to the axial planes of" folds in the gneisses, but the amphibolitized basic rocks generally have isotropic (post kinematic) textures. It is suggested that the strongly deformed rocks of zone 2 occupy the hinge zone of an inclined isoclinal synform complementary to the antiform of zone I (Fig. 2). Zone 3, which on this interpretation represents part of the southwest limb of this fold, has lesser dips and less intense Laxfordian deformation. The Scouric dykes in this zone frequently retain discordant relationships but are also commonly folded (Fig. 3A) or boudinaged. Their marginal parts are amphibohtized and

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I68 P. W. Francis

\~".

<~- c;-'s~J" C~,~.~-

N 0 Scurrival Antiform

LEGEND

Outer Hebrides Thrust

Generalized I "h,, foliation trend D % Antiformal i "IK axis

Synformal '~ axis

t~ Scourie d~ke

"3. Area affected by psucdotachylyte SCALE 0 I 2 3 4 5 6 7 8 9 IO , • . kil ome tres

FIG, 2. Large scale structures in the Barra area.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles I69

often show axial planar fabrics, while their centres may retain two pyroxene assemblages; a new fabric is developed only locally in the gneisses. The overall structure of the Eastern Gneisses, then, is interpreted in terms of a broad antiform in the east flanked by an isoclinal synform. As a synformal closure has not been identified in zone 2, this part of the interpretation remains speculative. It is, however, clear that the three zones show very great differences in the development of minor structures post dating Scourie dykes and, consequently in the relationships of the dykes. These variations were apparently not noted by Hopgood (197 I) instead a time-distinction was drawn between little-deformed and strongly- deformed intrusive suites which appears to be contradicted by the evidence.

E) METAMORPHIC HISTORY A summary of the metamorphic history of the rocks of the Eastern Gneisses is presented in Table 2 which gives a simplified impression of a complex situation.

4/// I I It I.....

/ I metre ___...__-

...-....~,-.:..:,...... • \ ~ ~

!-.-.....---.~;~...-~:~..~:--~:i~ ..... i :

i m , "'~:-~::

F I o. 3 A. Partially amphibolitized granite dyke cutting Scourian granite, Vatersay. Note the folding of the thicker layers, while the thinner apophyses are apparently undeformed. B. Thin pegmatite cutting amphibolitized Scourie dyke. The pegmatite is not folded in the acid gneisses but is buckled within the dyke. (Leenish Point.)

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 17o P. W. Francis

The most difficult problem lies in interpreting the pyroxene granulite facies mineral assemblages in the Scourie Dykes; were these produced by a regional metamorphic event (cf. Dearnley t962 ) or by some other process? Since the country rock acid gneisses also carry orthopyroxene, one might be justified in thinking that both dykes and acid gneisses acquired their assemblages during a single Laxfordian phase of metamorphism. If there had been a regional metamorphism later than the Scourie dyke intrusion, then one would expect to find evidence for it in all earlier rocks. The pre- dyke granites, however, are not pyroxene bearing even in areas where they are cut by Scourie dykes with granulite facies assemblages, and where other, earlier, rocks also contain orthopyroxene. Further, the orthopyroxenes of the gneisses are invariably retrogressed, while those in the Scourie dykes in the same localities are perfectly fresh. Since orthopyroxene is found in rocks as young as the granodioritic dykes, but not in the granites which followed them (Table I), it is suggested that the pyroxene in the acid gneisses was produced by a pre-Scourian dyke granulite facies metamorphism, and that this was followed by a period of retrogression under amphibolite facies, when the granites were produced. This of course, implies that the Scourie dykes did not acquire their granulite facies mineral assemblages as a result of a regional metamorphism. A growing body of evidence suggests that intrusive dykes may crystallize with conventionally non-igneous assemblages when they are intruded into rocks under abnormal conditions of temperatures and pressures (O'Hara 196 I, Dawes 1968). It is suggested that the Scourie dykes in the Eastern Gneisses acquired their present high grade assemblages as a result of intrusion into particularly dry country rock gneisses, and that there was no regional Laxfordian granulite-facies metamorphism

TAB L E 2" Metamorphic history of the Eastern Gneisses

Event Metamorphic conditions Metamorphic effects

Laxfordian phases of Amphibolite facies Folding, boudinage and deformation amphibolitization of Scourie dykes in zones 2 & 3 Intrusion of Abnormally hot and dry ? Dykes crystallise with Scourie dykes granulite facies assemblages, in amphibolite facies country rocks. Intrusion of early Amphibolite facies Presence of granites and granites and pegmatites general retrogression of orthopyroxene in gneisses. Granulite facies Granulite facies Presence of abundant ortho- metamorphism pyroxene in gneisses of Zone x. Intrusion of early dykes Not known No evidence preserved. Intrusion of early Not known intrusive bodies Formation of original Amphibolite facies Banded and agmatitic gneisses gneiss complex with quartzofeldspathic veins and patches.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles I7 x

in Barra. This contrasts with Dearnley's interpretation based on evidence further north (Dearnley i962 ). The effects of undoubted Laxfordian metamorphism are best displayed in zones 2 and 3, where progressive amphibolitization and agma- tization of Scourie dykes may be observed.

3. The Oitir Mhor Zone The rocks of the Oitir Mhor zone are closely similar to the Eastern Gneisses. Acid gneisses containing retrogressed orthopyroxenes are abundant, particularly on the island of Hellisay, and they are cut by intermediate dykes exactly comparable with those of the Eastern Gneisses, by pegmatites older than Scourie dykes, and by Scourie dykes with granulite facies assemblages. No representatives of the other early dyke suites or the Scourian granites were found. The degree of Laxfordian deformation is low. Scourie dykes are commonly discordant, and neither folded nor boudinaged. Locally, as in the eastern parts of Hellisay, the dykes are rather more strongly deformed, mostly in restricted shear zones, and are partially amphibolitized; as a whole however, the zone stands in strong contrast with the surrounding Western Gneisses where the degree of deformation is high. Since the rocks of the Oitir Mhor zone are exposed only on small islands, it is not possible to define accurately its margins, but on the islands of Orosay and Fuday the remarkably sharp transition between Western Gneisses and the Oitir Mhor zone may be observed. On the south coast of Fuday, for example, splendidly discordant Scourie dykes cut orthopyroxene bearing gneisses, while on the north coast, a kilometre distant, dykes of the same suite are concordant and amphibolitized. Through most of the Oitir Mhor zone the gneisses dip gently northward, distorted only by a few asymmetrical warps and by some conspicuous Laxfordian shear zones.

4. The Western Gneisses The Western Gneisses (P1. 3) comprise a monotonous series of hornblende biotite acid gneisses (the 'Grey Gneisses' of Dearnley). The grain size is generally rather coarse and patches and streaks of pegmatitic material are abundant. Three important minor rock types occur in the Western Gneisses" metasedimentary gneisses, amphibolites associated with these gneisses and amphibolites assigned to the Scourie dyke suite. (i) Recognisable metasediments are found at only one locality in the present area, although they are more common in other parts of the Outer Hebrides. At Scurrival Point on Barra, rusty-weathering garnet biotite gneisses are exposed, which are closely comparable with the semipelitic metasedimentary gneisses of other areas (Coward et al. 1969). These metasediments exhibit a characteristic of many other metasediments~they preserve in highly deformed quartzofeldspathic stringers a record of phases of folding that cannot be identified in the adjacent acid gneisses. (ii) Early amphibolites are exposed on the small island of Orosay on South Uist, and at Scurrival Point on Barra. They are typically rather coarse rocks consisting

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I72 P. W. Francis

of hornblende, clinopyroxene and plagioclase in varying proportions. Garnet occurs sporabically, sometimes in great abundance and in very large crystals. The Orosay body, small though it is, is particularly interesting in that it shows a marked layering of different units" (I) A hornblende plagioclase rock, containing occasional quartz0feldspathic stringers; this is the most common type. (2) Garnet hornblende plagioclase rock in which garnets are very large and abundant; a few beach cobbles approach garnetites. (3) An uniform, sugary textured rock consisting entirely of plagioclase. This body could have been an originally layered or banded igneous rock, such as a gabbro or anorthosite, but it is considered more probable that the layering was produced by metamorphic segregation. Perhaps the most interesting aspect of these early amphibolites is their intimate relationships with metasediments. At Scurrival Point, they are tightly interfolded with metasediment bands, and at Orosay small quantities of biotite rich gneiss of similar type are found immediately adjacent to the amphibolite body. This relationship between the two rocl~types is commonly observed in the Outer Isles and is discussed elsewhere (Coward et al. 1969). (iii) Clearly defined concordant sheets of amphibolite folded and boudinaged during several phases of deformation and sometimes veined by pegmatite, are common throughout the Western Gneisses. The majority of these are regarded by the author as strongly deformed and metamorphosed Scourie dykes. They differ from the early amphibolites in mineralogy, in being less extensively streaked with quartzofeldspathic material and in having well defined boundaries. They strongly resemble the amphibolitized Scourie dykes of zone 2 of the Eastern Gneisses and the concordant basic bodies in gneisses adjacent to the Oitir Mhor zone. These in turn are closely similar to the obviously discordant dykes within the zone. The amphibolitization of pyroxene bearing assemblages which has been shown above, accompanies the change from discordant to concordant habit, and is dependent to some extent on dyke size, so that wholly amphibolitized thin dykes may be found associated with thick dykes showing only marginal amphibolitization. Although most of the basic sheets in the Western Gneisses are simple amphibolites, relict two pyroxene assemblages survive in the cores of some large sheets to provide a further connecting link. Finally, a strongly discordant amphibolitic dyke cutting gneisses on the east side of Orosay, South Uist, closely resembles the common concordant amphibolites; such discordant amphibolites, some with pyroxene bearing cores, occur at several localities in the Uists and are always (Coward et al. 1971) most common where Laxfordian minor structures are weakly developed.

(A) STRUCTURAL HISTORY OF THE WESTERN GNEISSES Evidence of Scourian deformation is rarely preserved in the Western Gneisses, due to the intensity of Laxfordian deformation and recrystallization. At the locality mentioned above on the island of Orosay, however, Scouric dykes may be observed cutting already gneissosc rocks showing fold interference patterns, indicating a complex history of pre-dyke deformation. Four phases of Laxfordian deformation have been rccogniscd. The F1 phase is poorly documented; it may have produced local folding and boudinagc, and,

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationshi#s in the Barra Isles 173

more important, a new planar fabric in the gneisses, which is the present gneiss foliation. The existence of folds of this phase is best demonstrated at those few localities where F~ folds in Scourie dykes refold earlier folds. The F2 phase produced some very conspicuous folding and boudinage of Scourie dykes. Folds are now often isoclinal, but it is difficult to determine to what extent fold shapes have been modified by the more intense F 3 phase. No new fabric was developed in the acid gneisses in this phase, and none is preserved in the amphibolite dykes, though it is probable that tectonite fabrics have been obliterated by later static recrystallization. Much the most important phase was the F 3 phase which was responsible for the regional structure (Coward et al. 1971 ). In the Barra Islands, two major asymmetrical folds are present, the Scurrival antiform to the northeast and a complementary synform (Fig. 2). The style of these folds is distinctive and accords with the pattern found in the islands to the north where broad rounded antiforms separate tight 'pinched' synforms. The Scurrival antiform is well exposed on Scurrival Point where the axis, as noted by Hopgood (I97I , Fig. 29) plunges northwest. The northeastern limb, showing moderate or low northeastward dips occupies Fuday, Eriskay and southern South Uist. It is important to note that the Oitir Mhor zone occupies the core of the antiform (cf. Fig. I and 2, Fig. 4). The overturned limb shared with the complementary synform is much narrower and forms part of a zone of intense deformation striking about I7 o° and dipping 65-70°F.NE. This regularly-foliated zone, partly covered by the sands of Traigh Mhor, extends into Ard Veenish, Ard Mhor and the small islands of Orosay northeast of Traigh Mhor. The profiles of minor asymmetrical folds change from 'S' northeast of the beach to 'Z' southwest of it, suggesting that a flattened synformal closure underlies the sands. To the southwest, the dips become gentle on the normal fold limb which occupies most of southwest Barra and the adjacent islands. The dominance of the two gently dipping limbs of the fold pair accounts for the general regularity of the orientation of the foliation as emphasized by Hopgood. The strong deformation in and near the steep common limb resulted in flattening of pre-existing folds and in the development of tight F 3 folds some of which have been correlated with Scourian folds in the Eastern Gneisses by Hop- good.

Oitir Mhor Zone I • iw.

t -1- t t + ~ t I+ + + + + + +"~,, FIG. 4. Section to illustrate the nature of \ ~+ ++ + + + + + # + + + + + + + the postulated major structures N~,++ ++ ++++++++++ of the present area, showing the , ' + ++++++++-+++ i relationship between the Wes- \\\',,~\+~ ++++++++ + +++ N~ +++ + + + + , tern Gneisses and the Oitir + + + + + + .L Mhor Zone.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 174 P. W. Francis

The F 4 phase produced no major structures, but abundant minor folds, commonly about o. 5 metres in amplitude with axes trending roughly IOO°. These folds are the most easily recognised structures in the area. Thin pegmatites are abundant in the axial planes of folds of this generation, especially in Vatersay.

(B) METAMORPHIC HISTORY The metamorphic history of the Western Gneisses was characterised by migmatization, amphibolitization, and recrystallization which accompanied the later Laxfordian folding, and reached a climax after folding had ceased. Migmatization was most intense in the south, where F 4 folds are sometimes swamped by granite material. Since metamorphism continued after deformation had ceased, the amphibolites identified as Scourie dykes are almost always texturally homo- genous. This makes it difficult to comment on the detailed relationship between deformational and metamorphic episodes. A further problem concerns the early metamorphic history of the Western Gneisses. Some large Scourie dykes near the Oitir Mhor zone have centres with granulite facies assemblages" does this imply that all the dykes were originally of granulite facies, as in the Eastern Gneisses ? Similarly, at one locality in southern Eriskay orthopyroxene bearing gneisses are preserved; does this mean that all the gneisses were originally pyroxene-bearing? These points are developed in subsequent sections.

5. Correlations The major F3 Scurrival antiform in the Western Gneisses has a gently dipping northeastern limb with northwesterly or westerly strike. It is reasonable to suppose that this structure is continued into the Oitir Mhor Zone, and that the northwest trending rocks in that zone lie on the same limb of the same F 3 structure. In the Western Gneisses, conspicuous folds, possibly emphasized by the F3 deformation, are common. In the Oitir Mhor zone, no minor folds post dating Scourie dykes were observed, though pre-F 3 deformation may have produced some re-orientation of dykes. The structural history of the Eastern Gneisses is closely comparable with that of the Oitir Mhor zone. It is suggested that the large open antiform in Zone I of this unit, and the postulated 'pinched' synform in Zone 2, were produced by the same phase of deformation as that which produced the principal structures in the Western Gneisses and Oitir Mhor zone, namely the F3 phase. No post Scourie dyke folding earlier than this phase could be positively identified, though some early re-orientation of dykes may have occurred. Minor folds of broadly the same style and orientation as those of the F 4 phase in the Western Gneisses are observed locally. These relationships are summarized in Table 3. 6. Synthesis The resemblances between the rocks of the Oitir Mhor Zone and the Eastern Gneisses which reflect similarities in the history of intrusive, metamorphic and

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles I75

tectonic events, indicate that these units are parts of the same original assemblage. As the structural evidence shows that the Oitir Mhor Zone lies in the core of the northwesterly plunging Scurrival antiform, rocks of Western Gneiss type are on a regional scale structurally above those of Eastern gneiss type. Using a term first applied by Wegmann (i935) to rocks in Greenland, there is here a supra-structure of highly deformed Laxfordian migmatitic acid gneisses overlying an infra-structure of less deformed gneisses without Laxfordian migmatization. Wegmann was describing the inverse of the present situation where more migmatitic rocks were overlain by less. The terms are useful, nevertheless, since they avoid the use of equivocal terms such as 'basement' and 'cover' which have a stratigraphical implication. Although the infra-structure rocks in the Barra region are only exposed in a small area west of the Outer Hebrides thrust, it appears that the thrust has carried a large mass of these rocks over the Western Gneiss supra-structure, reversing their original positions. The belt of intensely deformed acid gneisses (Zone 2) in the Eastern Gneisses can now be tentatively interpreted as a tight synformal infold of the supra-structure. If the major folds on either side of the thrust are parts of the same structures, the displacement must be of the order of five kilometres and may, as will be suggested elsewhere, have involved a rotation of about 3 °0 clockwise. Isotopic age and geophysical data support the interpretation drawn from field studies. The age data (Francis et al. 197 x) demonstrate that the infra-structure of rocks belong to a relatively little altered Scourian assemblage, the dates of 2600 m.y. obtained from Barra pegmatites according very well with those recorded earlier from the Scourie area itself. Dates from the Western Gneiss supra-structure are wholly Laxfordian (c. 16oo m.y.). A gravity survey by the I.G.S. shows a very high gravity anomaly of +76 milligals centred over the Oitir Mhor zone (Geol. Surv. Report for 1964). This This anomaly clearly reflects the presence at depth of relatively dense rocks and the suggestion has been made that these are pyroxene granulites. Some rough determinations of the specific gravity of some Western gneisses gave values of about 2.6, while the Eastern gneisses were consistently higher at about 2"7 to 2.8. Although the gravity map of the region east of the thrust is not so clear, there is a general trend towards higher gravity anomalies in the east as one would expect from the northwesterly plunge.

TAB r.r'. 3" Comparison of Laxfordian structural histories

Western Gneisses Oitir Mhor Zone Eastern Gneisses

F 4 Abundant minor No folds observed Some minor folds trending

folds trending I oo° C. IO0° F 3 Regional structures, Regional structure, Regional structures, some abundant minor folds. many minor warps, minor folds. Intense no minor folds. deformation in Zone 2. F 2 Abundant minor No folds, possibly No folds, except possibly isoclinal folds. some dyke some coaxial with F 8 reorientation in Zone 2. F 1 Some few minor No evidence No evidence

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 x76 P. W. Francis

Aeromagnetic maps (I.G.S. Sheets I0 and 12) also conform broadly with this general pattern. Since these maps reflect more superficial features, the contrast across the thrust is well defined and a body of strongly magnetic rocks is indicated beneath the Minch immediately east of the Barra Islands where the infra-structure might be expected to crop out.

7- Some aspects of dyke defot'mation Intrusive dykes are perhaps the most widely used time markers in basement rocks. It is therefore of considerable interest to examine how basic dykes have responded to deformation in the present area, bearing in mind the contrast already described between the dykes of the supra-structure and those of the infra-structure. Two relevant points are the distribution of areas of high and low Laxfordian deformation in the Hebrides and the relationship between the dyke mineralogy and their response to deformation. The first point has been discussed elsewhere (Coward et al. I97 I) and it has been suggested that the form of the F 3 Laxfordian structures in the Hebrides, with cuspate synformal zones of high deformation alternating with broad antiformal areas of low deformation, was the result of the deformation of an interface between media of different viscosities. The relatively dense and 'dry' rocks exposed at low structural levels near the eastern sides of the southern islands were interpreted as the more viscous medium, and the acid gneisses and associated metasedimentary relics on the western side as the less viscous. Now while the shape of the major structures is not so clear in Barra as in the areas further north, the contrast between areas of high and low deformation is more marked, and the areas of low deformation are larger. The rocks of the low deformation area in eastern Barra are probably the least modified Scourian assemblages in the Hebrides. Thus the major difference between supra- and infra- structure is one of lithology, and of metamorphic state in particular; the interface between less and more deformed regions is the interface between the 'wet' amphibolite facies rocks of the supra-structure showing strong Laxfordian reworking and the 'dry' granulite facies rocks of the infra-structure. Of interest is the difference in response to deformation between dykes with granulite and amphibolite facies mineral assemblages. Empirical field observations indicate that: (i) Dykes with granulite facies assemblages cutting pyroxene bearing acid gneisses are neither folded nor significantly boudinaged (the Oitir Mhor zone, Zone I in the Eastern Gneisses). (ii) Dykes with granulite facies assemblages in gneisses of amphibolite facies are folded. (southern Eriskay, Zone 3 in the Eastern Gneisses). (iii) Dykes with amphibolite facies assemblages are folded in amphibolite facies gneisses (the Western Gneisses, Zone 2 in the Eastern Gneisses). (iv) Thin amphibolite bodies in amphibolite facies gneisses were less competent than the acid gneisses. These observations have some far-reaching implications, so it is proposed to discuss each one separately. (i) This is merely a statement of fact; not only are the dykes not folded, they are also cross-cutting and branching.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 S¢ourian-Laxfordian relationships in the Barra Isles I77

(ii) In Zone 3 of the Eastern Gneisses, folded granulite dykes occur in amphibolite facies gneiss (Ru-fear-Vatersay) and in Scourian granite (Vatersay). The dykes are partially amphibolitized, but still show discordant relationships. The style of folding suggests that the dykes behaved competently on folding, but this could not be positively confirmed by isogon studies (Ramsay 1967), largely because the bodies seem to have originated as non planar branching and anastomosing bodies. These complex intrusion shapes have only been observed in dykes cutting rocks which were in amphibolite facies at the time of intrusion, and are significant since this association of intrusion shape with condition of country rocks might be usable as a guide to the condition of the gneisses before intrusion of Scourie dykes. (iii) Folded amphibolite sheets in the amphibolite facies gneisses of the supra- structure are abundant. The question is whether these dykes were folded as granulites, and subsequently amphibolitized, or whether they were in an amphibolitic condition before folding. Isogon studies of some typical F, folds have shown that most of the folded layers that are now amphibolites have the geometry of layers which acted incompetently on folding (Fig. 5). Many of these layers however, also show excellent boudinage, and the boudins are sometimes folded, which

0.1m

FIo. 5. Isogon plot oftwo folded amphibolite layers in the Western Gneisses at Ludag. These layers are interpreted as concordant amphibolitized Scourie dykes. Note the upward convergence of isogons in both layers.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I78 P. W. Francis

indicates that they were more competent than the gneisses at some stage of deformation (/71?) before the observed folding. These facts combine to suggest that the dykes of the supra-structure were originally more competent than the gneisses, probably had granulite facies assemblages, and were amphibolitized before F2 folding took place. (iv) It is a conspicuous feature of the infra-structure, and of other areas of low deformation such as Ardivachar point, in South Uist, that narrow apophyses from Scourie dykes tend to be more irregular and more discordant than the dykes themselves; significantly, the dykes have granulite facies assemblages whereas the apophyses are invariably amphibolites. It is suggested that this situation arose when relatively competent granulites and relatively incompetent amphibolites were deformed together. Granulite bodies would either buckle or rotate, depending on their original orientation, while the amphibolite apophyses deformed almost homogeneously with the gneisses, no matter what their original orientation. Other features lead to a similar conclusion. Folded granulite dykes with apparently undeformed apophyses, some of which must have been correctly orientated for buckling have been observed (Fig. 3A). Cuspate structures are often seen at the interface between dykes and gneisses (Figs. 6A & B) where amphibolite dyke material is 'pinched' into the gneisses. This phenomenon is identical with the large scale phenomenon mentioned earlier, but in this context the significance is that the amphibolitic dykes have acted as the less viscous material and the acid gneisses as the more viscous. Quartzofeldspathic Laxfordian pegmatites which cut granulite dykes have not been seen to be folded within them. Where an amphibolite dyke is cut by a pegmatite; the pegmatite may be folded. In the example illustrated in Fig. 3B, the pegmatite has deformed homogeneously within the acid gneiss, but has buckled as a competent layer within the amphibolite. Finally, some amphibolite dykes have a foliation parallel to their margin and independent of any fabric in the surrounding gneiss. This is the result one might expect in an incompetent body on deformation, when shearing parallel to the margins would develop. Thus it is concluded that dykes with different mineral assemblages had different competences relative to the acid gneiss; granulite dykes behaving competently and amphibolite dykes incompetently. Coward (pers. comm.) has reached much the same conclusions from work on the dykes of South Uist, and if this conclusion is generally applicable, it could be of considerable importance in using dykes as time markers in basement complexes. The great variations in style, frequency and tightness of minor folds displayed not only in dyke rocks but also in gneisses make the correlation of fold-sets by Hopgood (I97x) difficult to accept and in many respects this correlation seems to the present author contrary to the evidence.

8. Discussion It remains to account for the origin of the two major units in the Barra area, taking into account the evidence provided by dyke deformation. The presence of a supra-structure of uniform acid gneisses overlying an infra-structure revealing older structures immediately suggests the possibility of a simple cover of Laxfordian

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles ~79

metasediments overlying a Scourian basement. The remarkably sharp transition between the two units, especially if the interpretation of Zone 2 of the Eastern Gneisses as a synformal wedge of supra-structure acid gneisses is correct, would be consistent with such an interpretation. There is, moreover, a marked difference in rock types; in particular, recognisable metasediments are found only in the supra- structure, while the early intrusive dykes of the infra-structure have not been found in the overlying rocks. This hypothesis would account for the difference in metamorphic state of the two units, if a cover of wet first generation sediments were laid down on top of 'dry' gneisses, then on metamorphism one might expect the cover to produce hydrous amphibolite facies assemblages, and the basement to produce anhydrous granulite facies assemblages. Time relations, however, rule out any simple cover to basement relationship. Whatever differences existed between the two major units were effective before the main phases of Laxfordian deformation, and probably before the intrusion of the Scourie dykes; this follows from the markedly different response to Laxfordian deformation of the rocks of the two units. It is also clear that the rocks of the

--Z; ....

~~~.~ \~, ..... ~-~i~!i~ ~~:::::::::::::::::::::::::::::::::::::::::::::::::::::::::: " 2"" ~-~~ ~---" ~ ~~i??======i:iiiii ~~k-~ ::::::::::::::::::::::: ~~ :!:~:~:~:i:ii:~:i:!:i:i:!:~%i:i:i:i:!:!i:i:i ~-ii:.i:::::::::::::::::::::::: ::::::::::::::::::::::::: ~::::i ...... "...... ~"-~,~...... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::~-~ ::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::...... :u.~i~i:i-!:. ~:.:!~:!:~:i:~:~ii~i~ ======...... ~!~:~x.:.:::::: ..~.~:~::~.::~...... :.~...... ::.::...... :~:!:~:~:~:~:~:~:...... ~:~:~:~i~::~:~::,~:.'..!~i~!i~!!::~i~i~::~

-::.,.-:-....-.~...~.~.i.-~.-Ni~)...... -......

.----- ~i'~::'" ~ i-.:!i!!!!!!!!i!ii!ii!i!!!iiii!i ~ ..::iiii!i!i!!i:ii!!ii ---~~;7 - ~ ~.~!!;:!:!!: ~ ======

t ~:-!".-~ ...... 0~ -~- ":i:!:i:~:i:i:i: ..-.-,..-~. :!:!:i:i:~:i:i - 'i!!iiiiiiii J" iiiiiii~iii!ii j -~!!..'.-. .... -- ::::::::::::::::

/~::'::::: "---~ ].:.i:.i:.:i:i:i:i

L O.3m

FIG. 6. Examples of cuspate structures at dyke margins on different scales. A, Orosay, South Uist; B & C, Bruenish, Barra.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 18o P. IV. Francis

supra-structure were gneissose before the intrusion of the Scourie dykes. This can be seen unequivocally at Orosay in southern South Uist, where Scourie dykes cut highly deformed gneisses and at very many localities further north (cf. Dearnley & Dunning I968 , Coward et al. I97I ). The gneissose banding, often emphasised by pegmatitic layers, is distorted by even the earliest Laxfordian structures and pre- dates Laxfordian deformation. It is possible, however, that a very much older cover to basement relationship may have been preserved. A possible reconstruction of the history of the area is as follows: First a thick sedimentary sequence was deposited on a basement complex, and then the whole was mobilised in a major crustal event, the Scourian, at about 3,ooo m.y., a date suggested by the recent geochronological work on the Lewisian by Moorbath et al. (1969). The original interface might be preserved, though much modified, throughout this mobilisation, and a contrast in water content between the two units would remain. The original sedimentary sequence would become gneissose for the first time, while the underlying gneiss complex would merely be reworked, perhaps with a general migration of water upwards. This upward migration of water in metamorphism seems to be generally acceptable (Verhoogen et al. I97O). After this event, a series of minor intrusions were emplaced, the most widespread of which were of intermediate, dioritic composition. These intrusions may have congregated near the interface between supra and infra-structure, a tendency which has frequently been described from Greenland, but is not well understood. Following the intrusion of the early dykes, an episode of static metamorphism led to the crystallization of orthopyroxene in the 'dry' gneisses of the infra-structure and amphibole in the 'wet' supra-structure, the assemblages in both units being in equilibrium under the prevailing conditions of pH,O. The final stage in this major crustal event was marked by an episode ofgranitiza- tion, migmatization and pegmatite intrusion which affected both units, and brought about a general convergence between them. It is significant that late Scourian granites have been described from South Uist, North Uist and Harris, so this may indeed have been a widespread event; the process of convergence may have been carried to completion in many areas, eastern Barra and some parts of eastern South Uist being least affected. This late Scourian event can be confidently dated from its pegmatites at about 2,6oo m.y. (Francis et al. i971). There is no local evidence for a major late Scourian orogenic episode equivalent to the Inverian of Evans (I965). At a later date, (perhaps about 22oo m.y., el. Evans and Tarney I964), the Scourie Dykes were intruded. It has been suggested that the dykes in the infra- structure crystallized with primary pyroxene granulite facies mineral assemblages or acquired these assemblages very soon after emplacement. It is possible, even probable, that the dykes of the supra-structure may have crystallized with different assemblages, hornblende granulite, amphibolite or normal igneous assemblages. In the early phases of the Laxfordian orogeny, the dykes of the supra-structure were boudinaged, folded, and progressively amphibolitized. During the F3 fold phase, the overall structure of the area was defined, with tight synforms of supra- structure gneisses pinched into the infra-structure, and amphibolite facies

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles I8I

migmatization of the supra-structure took place on a regional scale. Amphibolite facies conditions continued in the supra-structure during and after the minor F 4 fold phase, especially in the extreme south of the area. Three factors arising from this discussion are stressed. First is the preservation of cover to basement relationships through later episodes of deformation and metamorphism. This is a fairly well known geological phenomenon, and is well illus- strated by the preservation of the Moine to Lewisian relationships through the Caledonian orogeny. In this case, some contrast in metamorphic state has also survived. Second, it appears that the major episode of deformation and mobilisation usually termed the 'Scourian' orogeny may be a good deal older than the 2,600 m.y. age usually associated with it. This date is obtained mainly from pegmatites, which, in the present area, are the very latest rocks of a last episode in a long history. This implies that the main phases of the Scourian orogeny could be as old as 3,000 m.y. as inferred by Moorbath et al. (I969). Third, although this hypothesis is primarily directed at the Barra area, it is broadly applicable to other areas, notably South Uist, where Coward has demonstrated the presence of areas having many affinities with the Barra Eastern Gneisses. There are, of course, also analogies with the Scourian block of the mainland, although there are some important differences, particularly in the metamorphic state of the Scourie dykes; those of the Scourie area having igneous rather than metamorphic mineral assemblages.

ACKNOWLEDGEMENTS. This work was carried out during the tenure of a University of London post-graduate Studentship and Scholarship. Part of the fieldwork costs were met by a grant from the University of London Central Research Fund. The assistance of Dr. J. V. Watson both in the field and in criticising the manuscript is gratefully acknowledged.

9. References BowEs, D. R. & HOPGOOD, A. M. 1969. The Lewisian Gneiss Complex of Mingulay, Outer Hebri- des, Scotland. Mern. geol. Soc. Am. xx5 (Poldervaart vol), 3x7-356. COWARD, M. P., x969. Unpublished Ph.D. Thesis, University of London. ~, FRANCIS, P. W., GRAHAM, R. H., MYERS, J. s. & WATSON, J. V. x969. Remnants of an Early metasedimentary assemblage in the Lewisian complex of the Outer Hebrides. Proe. Geol. Ass. 80, 387-4o8. , FRANCIS, P. W., GRAHAM, R. H. & WATSON,J. V. t97 I. Large scale Laxfordian structures in the Outer Hebrides in relation to those in the Scottish Mainland. Tectonophysies, xo, 425-35. DAWES, P. R. I968. Contrasted types of metamorphism of basic intrusions in the Pre-Cambrian basement of the Tassiussaq area, South Greenland. Meddr Gronland Bd. 185 Nr. 4- D~T EY, R. 1962. An outline of the Lewisian Complex of the Outer Hebrides in relation to that of the Scottish Mainland, Q. Jl geol. Soc. Lond. ,88, x43-x76. , & DtmmNO, F. W. x968. Metamorphosed and deformed pegmatites and basic dykes in the Lewlsian complex of the Outer Hebrides and their geological significance Q. dl geol. Soc. Lond. xu3, 335-378. EVANS, C. R. & TARNEY,J. i964. Isotopic ages of Assynt dykes. Nature (Lond.) 2o4, 638. x965 . Geochronology of the Lewisian basement near Lochinver, Scotland. Nature (Lond.) 2o7, 54-56 • FwANCXS, P. W., MOORBATH, S. & W~LXE, H. x97x. Isotopic age dates from the Isle of Barra, Outer Hebrides. Geol. Mag. xoS, x3-22.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I82 P. W. Francis

GEOLOGICAL SURVEY OF GREAT BRrrA~, Report on Progress for year x964. See. 5 Geophysical Department. 84 . HOPGOOD, A. M. 1964. Unpublished Ph.D. Thesis, University of St. Andrews. x965 . Theoretical consideration of the mechanics of tectonic re-orientation of dykes (with particular reference to the Lewisian dykes of the Isle of Barra). Tectonophysics. 3~ x7-28. x97 x. Structure and tectonic history of Lewisian Gneiss, Isle of Barra, Scotland. Kristalinikum 7, 27-60. J~HU, T. J. & CRAXG, R. M. I925. Geology of the Outer Hebrides. Part x. The Barra Isles and Part 2. South Uist and Eriskay. Trans. R. Soc. Edinb. 53 Part. 3 (No. 3o). KURSTEN, M. ! 957. The metamorphic and tectonic history of parts of the Outer Hebrides. Trans. Edinb. geol. Soc. x7~ i-3I. O'HARA, M. H. x96x. Petrology of the Scourie dyke, Sutherland. Min. Mag. 32~ 848-865. MOORBATH, S. WELKE, H. & GALE, N. H. 1969. The significance of lead isotope studies in high- grade metamorphic basement complexes, as exemplified by the Scourian rocks of North- West Scotland. Earth and Planetary Science Letters. 6, 245-256. RAMSAY, J. G. I967. Folding and fracturing of rocks, McGraw-Hill, New York. SUTTON, J. & WATSON, J. V. 195I. The pre-Torridonian metamorphic history of the Loch Torri- don and Scourie areas in the north-west Highlands of Scotland, and its bearing on the chronological classification of the Lewisian. Q. Jl geol. Soc. Lond. x~ 24I-3o7 . VERHOOGEN, J., TURNER, P. J., WEISS, L. E., WAHRHAFTIG, C. & FYFE, W. S., I97 o. The Earth, Holt, Rinehard & Winston, New York, 594- WEOMANN, C. E. I935. Zur deutung der migmatite. Geol. Rdsch. 26, 305.

Received 9 April x97i; read 26 January I972.

Peter William Francis, Department of Earth Science, Open University, Walton Hall, Walton, Bletchley, Buckinghamshire.

DISCUSSION OF BOTH PAPERS BY FRANCIS ~ CO~VARD DR. R. G. PARK congratulated both authors on their interesting accounts of the very complex geology of the southern Outer Hebrides. Lacking first-hand know- ledge of that area, he was tempted, despite his sceptical approach to structure correlation in general, to try to match the structural sequence proposed by the authors with the sequence obtained by the speaker and others in their recent work on the southern Laxfordian belt of the Scottish mainland, but found considerable difficulty in doing so. A superficial similarity existed between the f3 folds of the authors and the large-scale second Laxfordian folds on the mainland with a NW-SE trend and steep axial planes. The distribution of these structures showed a marked heterogeneity of deformation with narrow intensely deformed synformal belts (e.g. the Gairloch schist belt) separated by wide, relatively little-deformed zones. A characteristic feature of the Scourie dyke margins in the highly deformed zones was the presence of tight small-scale folds with relatively sharp antiformal and synformal hinges suggesting fairly ductile behaviour of dykes and gneiss, whereas no small-scale folding of dyke margins has been observed in the antiformal zones. In contrast, during the earlier Laxfordian deformation, small-scale folding of dyke margins is apparently absent throughout the region investigated and the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles I83

dykes have developed a foliation only which is absent from the enclosing gneisses. In the light of these observations, could Dr. Coward comment on the role of small-scale oft and f2 folding of the dyke margins in his area and on the physical conditions under which he envisaged the deformation in the dykes taking place. The speaker also wished to ask Dr. Francis whether there was any evidence in the Barra area of the Late Scourian structural-metamorphic event widely represented on the mainland and referred to there as the Inverian. It would seem that this event ought to fall between the two sets of basic dykes. It was, of course, possible that the heterogeneous nature of the Inverian deformation, obvious on the mainland, resulted in the Outer Hebrides in the preservation of enclaves where the Inverian was not represented.

Replying jointly to Dr. Park, the AUTHORS thanked him for his comments relating to the mainland of Scotland. They have examined some of the structures in the southern Laxfordian belt of the Scottish mainland and found a close (but possibly superficial) resemblance between the first and second deformation phases there and the f2 and f3 phases in the Outer Hebrides. They agree with Dr. Park that if structures are to be correlated across the Minch, this would be the best match on present evidence. Also in reply to Dr. Park, DR. COWARD said that the presence or absence of folds in dykes, dyke margins and dyke apophyses depended on the relative orientation of dyke or apophysis to the direction of maximum shortening and the relative competence of dyke and matrix. Dyke apophyses may be folded while parent dykes show only a foliation parallel to their margins, or vice-versa. Deformation took place under amphibolite facies conditions, hornblende and/or biotite mineral fabrics were produced during each phase. Dykes with such mineral assemblages often developed a cuspate form at their edge, the sharp cusps pointing towards the gneiss matrix. These dykes often show a Laxfordian mineral fabric which is absent from the enclosing gneiss.

DR. D. R. Bow~s, commenting on Dr. Coward's paper, pointed out that the author's interpretation was based on the existence of four phases of deformation whereas six phases of deformation had been demonstrated for the Lewisian of nearby Barra (Krystalinikum I97i , 7, 27) and for Mingulay (Mem. geol. So¢. Am. I969, xxS, 3x7). Evidence for the corresponding six deformational phases in South Uist as well as in the Lewisian of the islands of the Outer Hebrides to the north was also available (Bull. geol. Soc. Am. 1972, 83, 107). The coaxial attitude of some fold axes made separation of successive deformational phases difficult. However, mapping of considerable areas had confirmed evidence, based on attitudes and mutual relations of fold axes, axial planes and metamorphic mineral growths, of deformation including the formation of large folds, between the author's f3 and f4 phases and after the author's f4 phase. The gneisses of South Uist had been referred to in the lecture both as products of the Scourian episode and as the result of regeneration during the Laxfordian episode. Was there isotopic evidence from the gneisses themselves to indicate formation during both these widely separated episodes? And was SrsT/SrSe data

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I84 P. W. Francis

available ? It seems possible that proposals relating to the time(s) of gneiss formation were linked to an assumption that basic minor intrusions, whatever their structural relations within the gneissose foliation, were all 'Scourie dykes'. Any such assumption appeared to be unwarranted in view of the available evidence relating to a series of basic minor intrusions in the Lewisian, with at least five sets of basic minor intrusions present in one area, spaced through a long (isotopically dated) time span. The granitic activity and evidence of a thermal high related to the f4 deformation in South Uist had much in common with the injection complex of Harris which had been dated at I75O-I7i 5 m.y. (Scott. 3". Geol. I97I , 7, I39) and shown to have been emplaced in large part, between the fifth and sixth deformational phases recognised in the Lewisian of Harris. Did the author consider the granitic activity in South Uist and Harris to be coeval ?

Commenting on the paper by Dr. Francis, DR. D. R. BowEs said that much of the information given, particularly relating to the sequence of igneous intrusions had already been published by Dr. A. M. Hopgood (Krystalinikum I97I, 7, 27). The structural sequence demonstrated by Dr. Hopgood for the rocks of Barra, which corresponded with the structural sequence for the Lewisian of the other islands of the Outer Hebrides (Bull. geol. Soc. Am. I972 , 83, Io7) , was much more extensive than the sequence demonstrated by the author. There is also correspondence of structural sequence in the gneisses now disposed above and below the Hebridean Thrust. These factors and the detailed structural evidence of Dr. Hopgood would need to be taken into consideration by the author before setting out large scale tectonic reconstructions. The author appears to assume that there is only one set of basic minor intrusions, referred to as the 'Scourie Dyke suite'. However, the published evidence, including photographs, clearly indicates more than one phase of emplacement with some basic masses metamorphosed to amphibolite or epidiorite and deformed by F, folds with others showing pyroxene granulite facies mineral assemblages which cross cut F8 folds. In view of this, the presence of amphibolites without cross cutting relationships west of the thrust, and of basic masses with granulite facies assemblages cross cutting the foliation (and F3 folds) east of the thrust, cannot be used as evidence for the existence of two different structural regimes. If these basic igneous suites are being equated and used as a single time datum ('the Scourie Dyke suite') then the significance of the term 'Scourian' and 'Laxfordian' varies from place to place and the resultant interpretations must be treated with great caution.

DR. COWARD and Dr. FRANCIS replied jointly to Dr. Bowes. Dr. Francis observed that he had carefully studied both Dr. Hopgood's thesis and his publications and had pointed out the significant observations made by him. He did not wish to detract from Dr. Hopgood's valuable contribution, but he could not agree with Dr. Hopgood's structural correlations and did not consider that the work had thrown definitive light on the overall structural setting of the Barra area. One source of difference between Dr. Bowes and the authors is that Dr. Bowes recognises six deformational phases whereas the authors consider there to be only

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles I8 5

four prominent phases that post date the intrusion of the Scourie dykes suite. The other difference of interpretation is that Dr. Bowes believes that there is evidence for five sets of basic minor intrusions on the mainland whereas the authors have not yet been able to accept the evidence as being at all conclusive. Dr. Bowes does not perhaps agree with the emphasis laid on variations in the intensity of deformation and in style and orientation of folds. A large part of Dr. Coward's paper is centred on this subject. To correlate fold episodes in such areas of varying intensity of deformation it is necessary to build up a complete picture of the major structures. The examination of a small area or a series of small areas may give an unrepresentative picture. Nevertheless, Dr. Bowes explains the structural geology of South Uist from a structural history of three widely separate localities: Kilbride, Trollaskeir and Howmore (Bull. geol. Soc. Am. 1972 , 83, 112). In areas of non-coaxial folding, the authors have often found it difficult to distinguish adequately between the orientations of different sets of minor fold axes from small areas on one stereogram. Yet Dr. Bowes (Bull. geol. Soc. Am. 1972, 83, Fig. 9) presents a single stereographic plot to represent the whole structural history of the Outer Hebrides. When it is borne in mind that this plot is compiled from two hundred observations over the entire area north of Barra, a ratio of one observation per I o sq. km, the authors doubt that Bowes and Hopgood are justified in some of their correlations. Dr. Coward has at no time suggested that the gneisses of South Uist formed at widely separated episodes. He described the effects of modification during Scourian and Laxfordian episodes. Regarding the 'Scourie dykes' of Barra, Dr. Francis does not agree that any basic dykes cross cut F 3 folds. In his view this is a case where Dr. Hopgood's fold correlation sequence is demonstrably wrong; the folds Dr. Hopgood describes as cut by dykes with granulite facies mineral assemblages are wholly unrelated to other major folds in the west of Barra described as 'F3'. Dr. Francis does not consider that the criteria of fold orientation and style provide a sufficient basis for structural correlations of the type proposed by Dr. Hopgood. Dr. Bowes' remarks concerning the 'Scourie Dykes' are hard to understand, since the author has been at pains to demonstrate that in the Oitir Mhor Zone, west of the thrust, a transition occurs over a very short distance between discordant granulite dykes and concordant amphibiolite dykes. A similar rapid transition was described within the Eastern Gneisses of Barra, and Dr. Coward has described examples from South Uist. To suggest that the discordant basic masses which occur within such low intensity deformation areas are of a different age from the concordant bodies outside them would require special pleading of a rather extraordinary kind. Concerning the late-stage granite activity, Dr. Coward considered that this seemed to have been a fairly widespread event, with a series of widely spaced thermal 'highs', and that it was likely that this event may be broadly contempor- aneous over the length of the Hebrides.

MR. F. W. DUNNING sought further information from both authors on two aspects of local geology concerning which there appeared to be either real differences in the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I86 P. W. Francis

sequence of tectonothermal events between Barra and South Uist, or differences of interpretation between the two authors. These were (a) the pre-Scourie Dyke pegmatites and (b) the granulite-facies assemblages in the Scourie Dykes. Dr. R. Dearnley and the speaker had mapped in great detail several very small areas in Benbecula and South Uist. At Ardivachar Point in South Uist, deformed augen- pegmatites were traversed by Scourie Dykes which cut through the augen of deformed graphic microcline. Dr. Francis had illustrated Scourian pegmatites on Barra which had been dated at 2600 million years. Were these in any way deformed ? If not, the Ardivachar pegmatites were either older (pre-Scourian according to Dearnley and Dunning, this journal 1968) or an important deformation, equal to or older than the widespread agmatisation of the complex, occurred in South Uist but not in Barra. How did Dr. Coward regard these pegmatites and their deformation ? The two authors apparently differed in their explanation of the origin of the granulite-facies mineralogy of the Scourie Dykes. Dr. Francis spoke of dyke emplacement in a hot environment leading presumably to crystallization directly into the granulite-facies, while Dr. Coward seemed to prefer a single post-dyke metamorphism which simultaneously produced amphibolite margins in the dykes and dry garnet-pyroxene assemblages in the dyke centres. The speaker thought that both explanations encountered certain difficulties. It would not be unreason- able to assume that Scourie Dykes in the Uists and Barra had acquired their present mineralogy in the same way. Therefore one at least of the above theories was incorrect. One of the problems with direct crystallization in the granulite-facies was that the garnet-pyroxene cores of the dykes in Benbecula and South Uist displayed a conspicuous linear fabric, showing that the minerals crystallized under stress. Such stress and the tensional regime of basic dyke intrusion are quite incompatible. The second theory of simultaneous development of garnet-pyroxene cores and amphibolite selvedges ran into the difficulty that blue-green amphibolite-facies hornblende can be seen to replace the olive-green granulite-facies hornblende and clinopyroxene. The widths of the granulite-facies cores vary inversely with the intensity of local Laxfordian deformation, the dykes being totally amphibolitized in the areas of most intense deformation, such as Ballivanish in Benbecula. It might be argued that this deformation permitted the entry of larger amounts of water during metamorphism, but the theory also has to explain the Scourie Dykes in Barra that have no amphibolite selvedges. Was this apparently dry metamorphism in Barra also amphibolite-facies? What does Dr. Coward think of Dr. Francis' explanation ? And why do both authors reject the possibility of post-dyke granulite-facies metamorphism later retrograded in the amphibolite- facies ?

In reply to Dr. Dunning, Dr. Coward said that at least three phases of pre- Scourie dyke deformation (fsl to fs3) have been recognised at Ardivochar Point. Much of the migmatite veining in the gneisses and early amphibolites may be of fs~ to pre-fsl age and where fsz folds are clearly seen it is evident that the rocks were gneissose before deformation. There may have been a considerable amount

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian-Laxfordian relationships in the Barra Isles ~87

of deformation and migmatisation prior to the development of the first recognisable structures. The pegmatites and granites of Ardivachar post-date this fsl deformation but pre-date fs~ and fs3 deformation. The pegmatites carry an fs2 blasto-mylonitic fabric; augen of potash feldspar with deformed graphic texture are enclosed by granulitised quartz and feldspar. Thefs3 phase folded the early fabrics into large 'S'-shaped folds, but thefs3 fabric is weak to non-existent. The pre-Scourie type folds were not originally co-axial; closed interference patterns were produced. Later deformation however, has brought the earlier fold axes and lineations into parallelism, and closed interference patterns resemble elongate "dunces-cap" structures. This parallelism also relates to early Laxfordian fold fabrics and is presumably due to Laxfordian deformation. A minimum age of 256o 4- 80 m.y. has been obtained from the pegmatites at Ardivachar (Lambert et al., Scott J. Geol., 6, 1970). As augen in the pegmatite still carry a graphic texture and have not suffered much recrystallisation since formation, the minimum age of 256o 4- 8o m.y. may represent an only slightly modified original age. Similar ages have been obtained from a late Scourian pegmatite on eastern Barra (2462 4- 53 m.y., Francis et al., Geol. Mag., 1971, xo8) and a late Scourian pegmatite from Badcall on the Scottish mainland (254 o m.y., Lambert et al., Scott. J. Geol., ~97o, 6). Thefs, andfs3 deformations at Ardivachar may represent late-Scourian episodes of deformation which are absent in eastern Barra and may be coeval with events which produced 22oo-23oo m.y. "Inverian" dates on the Scottish mainland. Park (Scott. J. Geol., 197 o, 6) has pointed out that late Scourian or Inverian events on the mainland seem to be best developed away from what are now areas showing granulite facies metamorphism. A similar distinction may exist in the Hebrides. As discussed in the paper, there was an increase in metamorphic grade from west to east across South Uist during the early Laxfordian history. There is no evidence for unity of metamorphic grade throughout the Hebrides and the author can see no reason why the metamorphic grade in Barra should be the same as that at Ardivachar. The author agreed with Dr. Dunning that the degree of amphio bolitisation of Scourie dykes is directly related to the intensity of deformation. In the gneisses of the western part of South Uist there is no evidence for a granulite facies metamorphism during any of the Laxfordian deformation phases (Coward et al., Proc. Geol. Ass., 1969) and the same can be said for Barra. In reply to Dr. Dunning, DR. FRANCIS said that he had little to add to what Dr. Coward had already said, save that the granulite facies dykes on Barra do in fact almost always have narrow amphibolite selvedges (this paper, p. 167) which, as far as mineral textures are concerned, appear to have formed simultaneously with the granulite cores in some cases, but subsequent Laxfordian metamorphism has also produced similar effects. As far as the possibility of a post-dyke granulite facies metamorphism subsequently retrograded to amphibolite facies is concerned, Dr. Francis considered that the chief obstacles to this hypothesis were the generally fresh textures in the dykes, while the pyroxenes were heavily retrogressed, and the absence of pyroxenes in small late-Scourian granite bodies which cut pyroxene- bearing acid gneisses.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 I88 P. W. Francis

PROFESSOR J. G. RAMSAY supported Dr. Coward's conclusions on the nature of the cusp-like downfolds of amphibolite facies into the more competent granulite facies rocks. This type of structural style was common in many areas of deformed crystalline basement rocks and seemed to occur on many scales. The speaker had noted different types of cuspate forms at the contacts of the deformed Scourie dykes and the surrounding gneisses. At some localities the sharp points of the cusps pointed towards the dyke interior and seemed to be best interpreted in terms of the dyke being more competent than its matrix, while at other localities the cusp points were directed towards the gneissic matrix and seemed to imply a reversal in competence contrast. The change in relative competence of the dyke and its matrix seemed to be correlated with the metamorphic transformations suffered by the dyke. Where the dyke was made up predominantly of pyroxene it behaved more competently than its matrix, but where it had been amphibolised and contained a significant proportion of biotite it behaved in less competent manner.

DR. COWARD thanked Professor Ramsay for his comments. He agreed that changes in relative competence of the dykes and matrix seem to be related to metamorphic mineral changes suffered by the dykes. Using methods of fold-wavelength--layer thickness relationships, viscosity ratios between dykes and adjacent gneiss have been obtained from folded dykes. These ratios are variable, ranging from over 65: I for dykes containing garnet-pyroxene bearing assemblages to less than one for dykes with biotite-hornblende assemblages.

DR. D. I. SMITH, Commenting on the paper by Dr. Francis, informed the meeting that he and Dr. D. J. Fettes had spent four weeks in 1971 mapping on Barra and the adjacent islands as part of an I.G.S. project covering the whole of the Outer Hebrides. They were in general agreement with the author's main conclusions but found some difficulty in accepting his interpretation of the 'pre-Scourie dyke' metamorphic and tectonic history, particularly with reference to the early dyke suites in the Eastern Gneiss and Oitir Mhor Zone. According to Dr. Francis these dyke suites preceded a phase of granulite facies metamorphism and although orthopyroxene is recorded in the numerically insignificant oldest (basic) and youngest (granodiorite) members could he offer an explanation for its absence in the abundant and widespread intermediate dyke suite ? A characteristic feature of the intermediate dykes is that they show well-developed internal planar fabrics which in some cases are cut by apparently undeformed Scourie dykes. In addition a number of examples had been mapped by the speaker of folded or boudinaged intermediate dykes again cut by undeformed Scourie dykes. Although it may be possible to explain some of these relationships in terms of competence differences and variation in orientation of the dykes prior to a 'post-Scourie dyke' deformation, would the author admit the possibility of a tectonic episode separating the early dykes and the Scourie dykes ?

DR. FRANCIS thanked Dr. Smith for his remarks, and for bringing up some of the most problematical aspects of this area. The distribution of orthopyroxene in the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/161/4884620/gsjgs.129.2.0161.pdf by guest on 25 September 2021 Scourian.Laxfordian relationships in the Barra Isles I8 9

Eastern Gneiss was not easy to explain and particularly its general absence from the intermediate dyke suite. It is possible that this may reflect some original difference in chemistry between the dyke suites, intermediate dykes may have had a higher water content. It is worth noting, however, that at one locality on Ru-fear-Vatersay, abundant orthopyroxene occurs in biotite rich rocks related to the intermediate suite. On the second point, which is broadly similar to that raised by Dr. Park, the possibility of late-Scourian or 'Inverian' deformation, Dr. Francis said that he admitted at least the possibility of a phase of deformation between the early dykes and the Scourie dykes. He emphasized however, that this could not by any means have been an important phase, and was confident that most of the cases of apparently folded early dykes cut by Scourie dykes could be explained by original differences in competence and orientation. It is important to note in this context that although Zone I of the Eastern Gneisses has been spoken of as a "low deformation zone", this is only a relative term and much Laxfordian deformation had in fact occurred. For introductory and concluding remarks by Dr. Janet Watson, see the discussion of the preceding paper by Coward.