J. geol. Soc. London, Vol. 142, 1985, pp. 663-675, 7 figs. Printed in Northern Ireland

Structural setting and geochronological significance of the West Granitic Gneiss, a deformed early granite within Proterozoic, Moine rocks of NW Scotland

D. Barr, A. M. Roberts,” A. J. Highton,? L. M. Parson$ & A. L. Harrisg British Geological Survey, 19 Grange Terrace, Edinburgh EH9 2LF; * Britoil plc, Exploration Department, Grosvenor Centre, 6C-80 GordonStreet, Glasgow; t British GeologicalSurvey, Murchison House, West Mains Road, Edinburgh EH93LA; $ Institute of Oceanographic Sciences, Brook Road, Wormley, Godalming,Surrey GU85UB; 3 JaneHerdman Laboratories of Geology, University of Liverpool, Brownlow Street, Liverpool L69 3BX; UK.

SUMMARY: Are-investigation of the six major outcrops of granitic gneiss within the and divisions of the Moine has shown that they are a suite of deformed and metamorphosed granite intrusions. They record all the tectonic events recognized in their host metasediments, while the absence of thermal aureoles suggests that their emplacement was synmetamorphic, i.e. syn-D, rather than pre-D,. Their dominant gneissic fabric is of S2 age, but in low-strain augen an earlier S, fabric is preserved which is cut by MPI leucocratic segregations contemporaneous with, but compositionally distinct from, those in the pelitic regional migmatites of the Moine. The deformed granites underwent local MP2 remelting and have been widely intruded by later, Caledonian pegmatites. The gneiss has a uniformly granitic (sensu stricto) bulk composition, regardless of country-rock lithology or location within the Glenfinnan and Loch Eil divisions. It has strongly S-type chemical affinities, being reduced and peraluminous with a high initial 87Sr/86Srratio. It was probably produced at least in part by partial melting of Moine-likerocks at depth. A published Rb/Sr whole-rock isochron of c. 1028Ma probably dates intrusion of the Ardgour body and confirms the reality of a ‘Grenvillian’ event in the Glenfinnan and Loch Eil divisions of the Moine.

One of themore controversial issues in geological geological interpretation is often less ambiguous than interpretations of the Moine of NW Scotland is the that of dates from metasediments. Several deformed distinction betweenPrecambrian and Caledonian intrusions have been used as geochronological markers orogenicevents. Radiometric dates obtained from within theMoine, for example theCarn Chuinneag metasediments, and frompegmatites within them, granite (550 f 20 Ma; Pidgeon & Johnson 1974), the convincingly demonstrate an important Caledonian Glen Dessary syenite (456 * 5 Ma; van Breemen et metamorphicevent (e.g. van Breemen et al. 1974, al. 19796) and the Loch Borrolan syenite (430 f 4 Ma; 1978;Brewer et al. 1979). Olderdates suggest that van Breemen et al. 1979~).The Ardgour granitic ‘Morarian’ (70&800 Ma) or ‘Grenvillian’ (c. 1000 Ma) gneiss (Fig. 1) is the only syntectonic ‘igneous’ body events arerepresented (e.g. Lambert 1969; van within the Moine to have provided convincing Gren- Breemen et al. 1974, 1978; Brook et al. 1977; Piasecki villian radiometric dates (1028 f 43 Ma; Brook et al. & van Breemen 1983). With respect to the southern 1976, recalculated with A s7Rb = 1.42 X 10-” a-l by part of the N Highland Moine, extreme viewpoints are Brewer et al. 1979;Aftalion & van Breemen 1980). provided by Brewer et al. (1979) and Powell et al. The Quoich granitic gneiss is probably also Grenvillian (1983), who arguethat all major units haveexperi- (Piasecki & van Breemen 1979). The usefulness of enced Grenvillian orogenesis, the Morarian represent- these dates has been limited by uncertainty over the ing an episode of non-orogenicpegmatite intrusion, nature and origin of the granitic gneiss. If it represents and Piasecki & van Breemen (1979,1983), who argue pre-existing granitic basement, then the associated for widespread Morarian tectonism but would restrict Moine is post-Grenvillian. If it was produced in situ by the Grenvillian event to one tectonostratigraphic unit, metasomatism, then significant Grenvillian orogenic the Glenfinnan Division, which by inferenceforms activity is implied. If it is intrusive, then the Moine basement to the remainder of the Moine. sediments are older than 1028 Ma, but the case for In general, radiometric dates obtained from igneous Grenvillian orogenic activity is notproven if the bodies can only provide upper and lower limits to the granite can plausibly be interpreted asentirely pre- age of tectonometamorphicevents. However, their orogenic. We have studied in detail three of the major

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 664 D.Burr et al. graphic units: from west to east, the , Glenfin- nan and Loch Eil divisions. The Glenfinnan Division comprises interbanded pelites and psammites and passes up stratigraphically intothe dominantly psammitlc Loch Eil Division (Roberts & Harris 1983), but has beentectonically emplaced onto thedominant- ly psammitic Morar Division by the Sgurr Beag slide, an early Caledonian ductile thrust (Tanner et al. 1970; Rathbone et al. 1983). Six major bodies of granitic gneiss have been recognized in the Glenfinnan and Loch Eil divisions (Fig. l), along with anumber of smaller satellite intrusions. The six named bodies of granitic gneiss and their surrounding metasediments carry three well-developed sets of structures, D1-D3. The earliest, D1 structures are represented by sparse, intrafolial F1 isoclines, and a bedding-parallel S1 fabric which is crenulated in F2 fold cores. D2 structures are widespread on all scales and were originally sub-recumbent. Where subsequent strain is low, F2 folds are observed to be tight to isoclinal and intensely curvilinear abouta N-S- trendingextension lineation (Holdsworth & Roberts 1984). Exceptin F2 fold cores, S2 is subparallel to banding, and the mesoscopic 'foliation' mapped in the metasediments is compositea So/S11S2 fabric. D3 structures are upright with NNE-SSW-trending axial planes. In the area (Fig. l),Roberts et al. (1984) have demonstrated that D1 and D2 pre-date the c. 456 Ma Glen Dessary syenite (van Breemen et al. 19796), while D3 is later than c. 456 Ma, i.e. Caledo- nian. The Loch Quoich line (Fig. 1; Clifford 1957) has been shown by Roberts & Harris (1983) to separate two domains of contrasting D3 strain. To the west, in the 'steep belt', F3 folds are tight and steeply plunging Loch EilDivision with a strong axial planar S3 fabric; only in low-strain GlenfinnanDivision F3 fold cores is the pre-D3 disposition of stratigraphy .. and structure preserved. To the east, in the 'flat belt', ... Morar Division(including Lewisian rocks) D3 ductile strain is low,and sub-recumbent D2 structures dominate. The Loch Quoich line lies close FIG.1. Distribution of the West Highland granitic to, but does not precisely coincide with, the lithostra- gneiss (modified from Johnstone (1975) fig. 6). tigraphic boundary between the Glenfinnan and Loch Eil divisions. bodies of granitic gneiss shownin Fig. 1 (Ardgour, Metamorphic grade inthe Glenfinnan Division is high Loch Quoich,and Fort Augustus) and visited three everywhere:kyanite and sillimanite are present in others(Gulvain, Loch Arkaig,and Glen Doe). In suitable lithologies and calc-silicates are pyroxene- combination with existing data, our results confirm the andlorbytownite-bearing (Winchester 1974; Fettes igneous origin of the granitic gneiss, and enable the 1979). Sillimanite + K-feldspar assemblages are, radiometric dates to be placed in a definite geological however,restricted to a small area inSW Ardgour context. (Dalziel & Brown 1965; Tyler & Ashworth 1982). The Loch Eil Division appears to be of somewhat lower Regional setting metamorphic grade (Winchester 1974), although this is due, atleast in part, to Caledonianretrogression of The N Highland Moine comprises thatpart of the calc-silicate mineral assemblages (Johnstone et al. Caledonian orogenic belt lying between the marginal 1969; Tanner 1976). The high-grade mineral assemb- Moinethrust belt andthe transcurrent lages are uniformly early (MS1-MP1: syn- to post-D1, fault; only the southern portion will be discussed in sensu Sturt & Harris 1961), and broadly coeval with this paper (Fig. 1; Johnstone 1975). It was subdivided the regional migmatization of pelites and, less com- by Johnstone et al. (1969) into three majortectonostrati- monly, other lithologies. Most pelites in the Glen-

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finnan Division, and somein the Loch Eil Division distinct kind of magma emplacedamong high-grade (e.g. in the Garry Banded Unit of Roberts & Harris metamorphic rocks’. Pidgeon & Aftalion (1978) 1983), are represented by lit-par-lit migmatitic gneisses demonstrated thatinherited, ‘detrital’ zircons are with trondhjemitoid(K-feldspar-free) leucosomes. common in other, undoubtedly intrusive Scottish These formed isochemically, by subsolidus segregation granites. Gould (1966) concluded, on the basis of over within rocks having favourable bulk chemistry (Barr 100 whole-rock analyses, that the Ardgour gneiss has a 1985), particularly plagioclase-rich pelites (sensu Zato) uniform granitic (sensu stricto) composition and is of ‘metagreywacke’ composition (Butler 1965). Litho- quite distinct from any Moinemetasediment. How- logies suitable for migmatization are rare in the Loch ever,he accepted Dalziel’s (1966) field andpetro- Eil Division, hence the lithostratigraphicboundary graphic evidence, and postulated pervasive injection of with the GlenfinnanDivision, andthe subparallel an unusually AI-undersaturated magma in orderto Loch Quoichline, appearto control the onset of retain a significant metasedimentary component in the migmatization. bulk gneiss. Because the Quoich gneiss appeared to crop out in an area of low metamorphic grade, Harris Previous theories of genesis (in discussion of Winchester 1974) suggested that it could be a slice of pre-existing graniticbasement Although Bailey & Maufe (1916, p. 88) originally tectonically emplaced into the Moine. described the augen-gneiss facies of the Ardgour body More recently, Johnstone et al. (1979) have referred as a deformed pre-metamorphic intrusion, subsequent to the ‘foliated Ardgour granite’ and Roberts & Harris workers interpreted it as ametasomatized Moine (1983) described the Quoich body as ‘granitic ortho- sediment. For example, Harry (1953), working in the gneiss’, althoughneither paper broughtforward SgurrDomhnuill area, concluded that agradation specific evidence. In contrast, Brewer et al. (1979) and existed between non-migmatitic pelites, migmatitic pe- Piasecki (1980) haveaccepted Dalziel’s (1966) argu- lites,and thesouthern part of the Ardgour gneiss, ments for a metasomatic origin. reflecting progressive Na- and K-metasomatism of the original sediments.Butler (1965) and Dalziel (1966) Ardgour concluded thatthe migmatitic pelites of Ardnamur- chan andArdgour hadformed isochemically, a An excellently exposed section through the Ardgour conclusion confirmed forthe Loch Eilt area by granitic gneiss is foundin road cuttings along the Steveson (1971), and regionally by Barr (1985). A830, between Glenfinnan and Kinlocheil (Figs 1,2; Dalziel (1966), however, notedthe presence of Barr 1983; Roberts 1984). The gneiss is amedium- rounded (‘sedimentary’) zircons in the Ardgour grani- grained (1-2 mm), equigranular rock containing sub- tic gneiss and traced an apparent lateral passage into equal amounts of quartz, oligoclase, and K-feldspar. metasediments around his Meall Mor fold (Dalziel Biotite forms 515% of the rock, and minor musco- 1966, p. 148). He maintained that the granitic gneiss vite, garnet,hornblende, ilmenite,pyrite, epidote, had been produced by K-metasomatism, prior to and zircon and monazite have been recorded. In places it during D2, in a major shear zone corresponding to the is homogeneous, but for a pervasive fabric defined by Loch Quoich line (Clifford 1957). This was considered thin, impersistent biotite laminae. Elsewhere, concor- by Dalziel (1966) to separatea mobile infrastructure of dant pegmatitic lits comprising quartz, K-feldspar, and migmatitic pelites,subsequently named the Glenfin- oligoclase liesubparallel to this fabric. The granitic nan Division by Johnstone et al. (1969), from gneiss forms a sheet-like body lying within the Moine overlying non-migmatitic psammites,subsequently succession, between the Beinn an Tuim Striped Schists named the Loch Eil Division. Dalziel’s conclusions and Druim na Saille Pelite of Dalziel (1966). This were modified by Johnstone et al. (1969), who showed pelite displays trondhjemitoid migmatization and rep- that the granitic gneiss is not entirely restricted to the resents the highest unit of the Glenfinnan Division vicinity of the LochQuoich line, andthat marked sensu Johnstone et al. (1969). Furthersouth, the divergence occurs from Loch Quoich to Fort Augustus granitic gneiss lies entirely within the Druim na Saille (Fig. 1). Theynevertheless favoured a metasomatic Pelite (Fig. 2). Thispelite passes stratigraphically origin,and suggested (Johnstone et al. 1969, p. 176) upwards intothe Loch Eil Division (GlenGarvan that the folded Ardgour and Loch Arkaig bodies had Psammite of Dalziel 1966). A distinctive quartzite unit formedprior toD2, parallel tothe limbs or axial forms the lower part of the Loch Eil Division around planes of F1 folds, but that the sheet-like Gulvain and the head of Loch Eil(Strachan 1985) and south of Quoich bodies had formed subsequently, as a result of Glenfinnan (Roberts 1984), but has not been mapped renewed or continuing metasomatism along zones of regionally so is not differentiated in Fig. 2. Metamor- shear separating the folded bodies. phic grade, as indicated by fibrolite in semipelites and Mercy (1963) first noted the ‘remarkable magmatic pyroxene in calc-silicates, is everywhere high (Dalziel features’ displayed by six chemical analyses of the & Brown 1965, Strachan 1985). Ardgour gneiss, and postulated thatit could be ‘a The Loch Quoich line (sensu Roberts & Harris

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 666 D. Barr et al. The distribution of D3strain is similar tothat reported by Roberts & Harris (1983) from the type locality of the Loch Quoich line. West of the Callop synform (Figs 2,3), upright F3 folds are tight to isoclinal, with steeply plunging axes. Intense D3 strain precludes systematic mapping ofF3 fold vergence between Glenfinnan and the axial trace of the Callop synform, but south of (Roberts 1984) the Loch Eil Division crops out in a complex D3 synform, and the complementary Meall na Cuartaige antiform lies between it andthe granitic gneiss, within the Beinn an Tuim Striped Schists (Figs 2,3). This broad synform is a locus of low D3 strain and is analogous to the Glen Dessary synform further north (Roberts et al. 1984). East of the Callop synforrn, upright major and minor F3 folds are generally open and gently plunging. From [NM 924 7931 to [NM 9168021, a 1 km-wide transition is seen across the Loch Quoich line, whereby individual F3 folds become tighter westwards and develop platy steep limbs. Within the flat belt the S2 gneissic foliation is folded by upright F3 structures,and is axial planar to sub-recumbent, intrafolial folds of quartzofeldspathic lits. Locally, e.g. [NM 9227961, the flat-lying S2 gneissic foliation is deformed by WNW-ESE-trending shear zones with normaldisplacement of several centimetres or tens of centimetres. In the centreof the granitic gneiss, at [NM 919 8001, the S2 fabric is less .::;::? Ardgourgranitic gneiss penetrativeand is axial planar toopen folds of mLoch Eil Division (undivided) quartzofeldspathic lits and of an earlier planar location fabric (S1). D2 strainincreases again towards the GlenfinnanDivision ~ western margin of the granitic gneiss, but the effects Druimna Saille Mite are obscured by increasing D3 strain as the steep belt Beinnan Tuim Striped Schists is entered. Beinn Gaire Rammite The eastern and western (upper and lower) parts of the granitic gneiss sheet record high D2 strain while FIG. 2. Outline structural map of the Glenfinnan area (modifiedfrom Dalziel (1966)). SGF, Sgurr Ghiubhsachain fold,D3; CGA, Coire Ghiubh- sachain antiform,D3; MBS, Meall a Bhainne synform, D3 (Dz, Dalziel1966); MCA, Meall na Cuartaige antiform, D3 (? = Meall Mor fold; D2, Dalziel 1966); CS,Callop synform, D3 (Loch Quoichline); GDA, Glen Dubh Lighe antiform, D3; DFM, Druim Fearna monocline, ?D2; GGS, Glen Garvan synform, D2 (?D3 component in the southwest). Not recognized: Cona Glen and Meall nan Damh folds (Dzr Dalziel 1966). A-A': line of section, Fig. 3. A

1983) here coincides with the axial trace of the Callop synform(Fig. 2). West of this line, in the regional steepbelt, the dominant structures and fabrics are upright and of D3 age. To the east, in the flat belt, sub-recumbent D2 structurespredominate. Clarifica- tion of thenature and origin of the granitic gneiss 0 km 2 depends upon the recognition of windows through the generally high tectonic strain, within which primary FIG. 3. W-E sketch-sectionthrough the Glenfin- textures and fabrics are preserved. nan area. Symbols and labels as in Fig. 2.

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the central portion records low D2 strain. We attribute Meall na Cuartaige antiform, which may be the this to a ductility contrastbetween the massive, northerlycontinuation of his Meall Morfold. Inter- feldspar-rich granitic gneiss andthe surrounding pretation of the granitic gneiss as an originally quartz- and mica-rich metasediments, rather than to discordant sheet removes the need for a D2 slide. the presence of a fold core within the granitic gneiss Throughout most of its outcrop, the granitic gneiss and a slide at its western margin (Dalziel 1966). Our carries a composite S1-S2 or S& fabric composed of remapping of some of the ground in the Glenfinnan orientated micas, quartzofeldspathic lits andbiotite- area (Barr 1983; Roberts 1984) shows that there is no rich laminae. Where D2 and D3 strains are low, an S2 convincing opposed sense of F2 vergence on eitherside tectonic fabric defined by orientated biotite flakes is of the granitic gneiss, and that vergence indicated axial planar to folds of pre-D2 quartzofeldspathic lits within the gneiss by foldedquartzofeldspathic lits is (Fig. 4a), and of an earlier S1 fabric which is also unreliable since they originally lay at a high angle to defined by flakes and discontinuous laminae of biotite the S1 fabric. Figures 2 and 3 show the stratigraphic with no associated segregation of veining. SI is cut by repetition recognized by Dalziel to be produced by a the quartzofeldspathic veins and theirprominent major F3 antiform west of the granitic gneiss, the biotite-rich selvedges (Fig. 4b), which are therefore of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 668 D.Barr et al. MP1 age and coeval with regional migmatization inthe metasediments and are undoubtedly Caledonian (van pelites (Barr 1983, 1985). Where least deformed,these Breemen et al. 1974); these pegmatites are in turn cut earlypegmatitic Zits contain very coarse (several by both foliated and non-foliated microdiorites (Smith centimetres)quartz, plagioclase (similar in composi- 1979). tion tothat of the hostrock) and K-feldspar, and Loch Quoich have textures indicative of subsolidussegregation (Barr 1983). Any possibility thatthe veins were Three concordant sheets of granitic gneiss crop out in produced by injection or metasomatism, with the the vicinity of the Loch Quoich dam, within the steep selvedges being relict metasediment, is clearly ruled belt of the N Highland Moine (i.e. west of the Loch out by these relationships. Rather,the selvedge Quoich line), but stratigraphically within the lower representsrestite after segregation of vein material part of the Loch Eil Division (Figs 1, 5; Roberts & from a homogeneous, biotite-bearing, foliated granite. Harris 1983, fig. 2). The dam spillway [NH 071 0231 The granitic gneiss contains a number of amphibo- containsa completely exposed section through the lite sheets up to l m thick. They are clearly deformed northernmost body. Here the granitic gneiss is 70 m by D2, but convincing evidence for their relationship thick and hassubvertical contacts which strike at to D1 could not be found in the D2 low-strain zone. 020°-040", parallel to bandingin the metasediments Thesesheets are cut by coarse, ramifying pink or and to a strong planar fabric in the granitic gneiss. white pegmatites (e.g. at [NM 921 7991) which post- The Quoich gneiss is a medium-grained (c. 2 mm) date most of the ductile strain in the gneiss and its host equigranular rock and is similar in field appearance

07 09

FIG.5. The Quoich granitic gneiss (after Roberts (1984)).

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and petrography to the Ardgour gneiss, although its set of vertical shear zones, which show features plagioclase is less calcic (An1~15v. AnZc25) and it indicative of partial melting (Barr 1985). These shear contains rather less biotite. Its northwestern boundary zones are reorientated on the eastern limb of the F3 is sharp, the granitic gneiss giving way abruptly to a Beinn Beag synform [NH 093 0251, and are geometri- 1m-thick migmatitic pelite followed by c. 300 m of cally equivalent to the normal, WNW-ESE-trending flaggy psammite (Fig. 5). The southeastern boundary, shear zones in the flat-lying Ardgour gneiss. Certain with pelites of the Garry Banded Unit, is also sharp, pelites within the Garry Banded Unit, unusual for the and although there is someinterleaving of granitic Moine in having K-feldspar-rich initial compositions, gneiss and metasediment,rocks with bulk composi- also underwentpartial melting (Barr 1985). The tions intermediate betweengranitic gneiss and geographical restriction of thesepartial melting metasediment are notdeveloped. Away from the phenomena is probablycontrolled primarily by the spillway, the contact is everywhereconcordant and availability of K-feldspar-rich host rocks. Their post- sharp, although locally interbanded. In poorly exposed D2 age rules out any possibility that they represent a ground south of Loch Quoich, the gneiss appears tocut ‘cryptic aureole’ tothe granite which formed the across stratigraphy at a low angle (Roberts 1984; BGS protolith of the Quoich gneiss. sheet 62W (Loch Quoich)). In the spillway, both the The granitic gneiss and the metasediments contain a granitic gneiss and itshost rocks are moderately to number of folded amphibolite sheets up to 1 m thick. highly strained (planar discordances being rare, folds These carry astrong LS fabric of S2 age, whichis tight to isoclinal), butreadable deformed cross- axial planar to the folds. In the hinge zones of these bedding is preserved within 50 m of the granitic gneiss folds, an earlier, S1 mineral fabric is crenulated. Some [NH07100245],and the contactzone lacks the sheets contain xenoliths of granitic gneiss, confirming high-strain platy fabrics, steep straingradients, and their intrusive relationship. The S2 fabric is continuous blastomylonites which characterize proven allochtho- from the host granitic gneiss, through the amphibolite nous basement slices within the Moine (cf. Rathbone andits xenoliths (Fig. 4fl. All structuresup toand & Harris 1979; Rathbone ef al. 1983). Northeast of including D3 are cut by SE-dipping microdiorite sheets the spillway (Fig. 5), psammites of the Garry Banded (Smith 1979), which carry amphibolite-facies mineral Unitare littledeformed and contain abundant assemblages butretain relict ophitictexture and sedimentarystructures which young southwards,to- amygdaloidal chilled margins. Severalsets of late, wards the grantitic gneiss. weakly deformed pegmatites cut all structures up to Two psammitic enclaves, probably deformed xeno- and including D3but are themselves cut by the liths, crop out in the spillway near the southeastern microdiorites. They are clearly unrelated to the origin margin of the granitic gneiss. The xenoliths have sharp of the granitic gneiss. boundaries concordant with foliation, and one defines an isoclinal fold to which the dominant gneissic fabric Fort Augustus is axial planar. On the southern shoreof Loch Quoich [NH 0624 01101, in an area of unusually low tectonic The Fort Augustus granitic gneiss lies well to the east strain,another xenolithpreserves angular discord- of the Loch Quoich line (Fig. l), within the regional ances with the granitic gneiss (Fig. 4c). The folded flat belt of the Moine. It is flanked to the southeast by psammitic enclave in the spillway contains a concor- theGreat Glen fault, andelsewhere dips beneath dant band of metabasic hornblende-biotite(-andesine- flaggy psammitic and semipelitic units of the Loch Eil sphene-epidote) schist, a widespread minor lithology Division (Fig. 6); its base is not seen. The surrounding in the local Loch Eil Division; this carries an early, metasediments are similar to rocks in the envelope of banding-parallelfoliation which is crenulated in the the Achnacarry dome 20km tothe southwest, and core of the fold. Thus the dominant gneissic fabric is probably lie at a low stratigraphic level within the at least S2 in the metasediments, and indeed identical Loch Eil Division. Foliation within the gneiss and the folds of bedding, migmatitic lits and an S1 fabric are metasediments is approximately concordant and dips seen in the surroundingMoine (Barr 1983; Holds- gently west- or northwestwards, although it steepens worth & Roberts 1984). Although pegmatitic lifs in the locally in the cores of curvilinear F2 folds. Overall, the granitic gneiss lie subparallel to the S2 fabric, this is body defines a broadD3 antiform, but the poorly invariably dueto transposition (Fig. 44, and S2 is exposed southwestern boundary appears tobe control- axial planar to isoclinal folds of these pre-D2 lits. In led by reclined F2 folds (Fig. 6). Where exposed, e.g. one such F2 fold (Fig. 4e), an earlier, S1 gneissic fabric [NH 312 0451, gneiss-metasediment contacts are sharp is preserved. The granitic gneiss outcrop passes and concordant with foliation. around the F3 Beinn Beag synform (Fig. 5), the axial Like the other bodies, the FortAugustus granitic trace of which defines the Loch Quoich line (Roberts gneiss is homogeneousand uniformly granitic, & Harris 1983), confirming thatthe origin of the although dioritic patches occur on an outcrop scale. It granitic gneiss is unrelated to this ‘line’. comprises K-feldspar, quartz, plagioclase andbiotite The S2 fabric of the granitic gneiss is deformed in a with accessory sphene,hornblende, garnet, allanite,

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., : , :..,...... , :. .fl,.. Caledonian lntrusives (:::::I Granodiorite ntrusive breccia Ultrabasic

FIG. 6. The Fort Augustus granitic gneiss (after Parson (1982) and BGS field slips of A. J. Highton).

zircon, apatite and ilmenite. The country-rock psam- lies at no great depth. The granitic gneiss contains a mites and semipelites locally preserve an S1 fabric, number of hornblende schists and amphibolites (Smith which like the gneiss-metasediment contact is folded 1979), someretaining relict pyroxene [NH by recumbent F2 isoclines. TheD2 structures are 3720 08191, and is cut by a variety of late Caledonian associated with a N-S extension lineation,and thus minor intrusions. may becorrelatable with curvilinear folds described further west by Holdsworth & Roberts (1984). Sub-concordant quartzofeldspathic lits with biotite-rich Other bodies selvedges occur throughout the granitic gneiss. Com- monly, these define augened, relict fold cores to which thedominant S2 gneissic fabric is axial planar.As Gulvain elsewhere, they representdeformed andtransposed The Gulvain granitic gneiss (Fig. 1) lies within the pre-D2 segregations within initially homogeneous Druim na Saille Pelite (Dalziel 1966; BGS sheet 62W granitic gneiss. D2 strain is high throughout the Fort (Loch Quoich)),and exposed pelite-gneiss contacts Augustus gneiss, so low-strain augenpreserving S1 are invariably sharp (e.g.in the Allt Coire Reidh fabrics are only rarelypreserved (e.g. [NH2940 [NM 9856 85351 and on the slopes of Na-h-Uamachan 01541). The S2 fabric is deformed by a number of small [NM 9753 84981). Like those of the Ardgour body, the shear zones, similar tothose described at Loch margins of the Gulvain gneiss are strongly foliated and Quoich, but evidence for partial melting is lacking. F, concordant with the S2 fabric in boththe granitic folds are restricted to open flexures. gneiss and the enclosing pelite. In contrast, the centre The granitic gneiss contains a number of agmatitic of the body is weakly strained, and on the slopes of breccias with granitoid matrices (Fig. 6). They contain Sron Liath [NM 97 851, the rock has the appearance of rafts of foliatedgranitic gneiss and country-rock homogeneous,a magmatic granite with weak,a psammite, so cannot be interpretedas xenolithic zones dispersedbiotite fabric. We believe that, as at in the protolith of the granitic gneiss. However, Ardgour,the less-deformed central portion of the breccias exposed in cliff sections at [NH 347 0681 are Gulvain body holds the key to its origin, and that the dominated at low topographic levels by psammite precursor tothe foliatedgranitic gneiss was a clasts, suggesting that the base of the granitic gneiss homogeneous, magmatic granite.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 The West Highland granitic gneiss 67 1 Loch Arkaig Discussion The Loch Arkaig or Sgurr Mhurlaggan body of granitic gneiss (Fig. 1) was briefly discussed by Roberts et al. (1984). It forms a relatively thin sheet, Origin of the granitic gneiss in relation to its broad outcrop resulting from F3 folding. Like the the surrounding metasediments Ardgour and Gulvain bodies, the Loch Arkaig gneiss We believe that the published evidence in favour of is more highly strained at its margins than in its centre ametasomatic origin forthe granitic gneiss can be and has sharp boundaries concordant with the +S2 rejected. Before the fact that a granitic gneiss is more foliation.Nevertheless, it possesses astrong gneissic potassic than a pelite is used to infer metasomatism, a fabric throughout, and only locally (D. I. Smith, pers. lateral passage or transition must be convincingly comm.) are augen of weakly strained, magmatic demonstrated in the field. All the evidence indicates granitepreserved. Roberts et al. (1984) have shown that interbanding of granitic gneiss and metasediment that it cuts from west to east across the stratigraphy is the rule, rather than smooth compositional varia- from Glenfinnan Division pelites and striped schists to tion. Harry’s (1953) chemical arguments are inconclu- Loch Eil Division psammites. Roberts etal. (1984) sive and based on few samples, and Dalziel’s (1966) also reported two small, previously unrecorded bodies zircon evidence has been superseded by the discovery of granitic gneiss which lie east of the Loch Quoich of ‘sedimentary’ zircons in granites of undoubted line, in psammites assigned to the Loch Eil Division. magmatic origin (e.g. Pidgeon & Aftalion 1978). The As at Fort Augustus, the Arkaig gneiss is intruded by outcrop pattern of theArdgour gneiss can be amphibolite dykes and sheets, some containing relict explained by somedegree of discordance in the pyroxene (observations by A.J.H.). original intrusive contacts (Fig. 3). The contact of the Glen Doe granitic gneiss with its country rock is invariably sharp, rather thangradational as would beexpected in a TheGlen Doe granitic gneiss has been partially product of structurally localized metasomatism. The described by Peacock (1977). It lies east of the Loch host metasediment to the granitic gneiss ranges from Quoich line (Fig. l), within the regional flat belt, and pelitic gneiss at Gulvain, through semipelitic schist at like the other bodies described has sharp, concordant Ardgour,to psammite atFort Augustus.At Loch contacts with the adjacent striped schists and pelites. Arkaig, the granitic gneiss transgresses the boundary We have examined the subsidiary bodies of granitic between the Glenfinnan and Loch Eil divisions, from gneiss lying north (Roberts 1984) and south (A.J.H.) pelites and semipelites into psammites,and at Loch of the area described by Peacock, i.e. between Glen Quoich it lies along the boundary between a striped Doe and Glen Affric, and south of the River and a psammitic unit.Nevertheless, no along- or Moriston. The gneiss possesses ubiquitousa S2 across-strike variations were observed which could be foliation, but in the Doe river section rare isoclinal F2 attributed to the initial composition of a sedimentary folds of an earlier S1 fabric are preserved (Peacock precursor. It would be a remarkable coincidence if a 1977, fig. 4b). North of Glen Doe the granitic gneiss subsolidus process, or indeed Gould’s (1966) magma/ formsa concordant sheet within rocks transitional sediment mixing process, had generated such a large, between the Glenfinnan and Loch Eil divisions. South homogeneousbody of rock with asimilar, near- of the RiverMoriston the granitic gneiss is steeply cotectic composition throughout this wide range of dipping, probably as a result of local F3 folding, and lithological units. Equilibration with infiltrating fluids hassharplya interbanded, sheetedcontact with should homogenize mineral compositions rather than psammites of the Loch Eil Division. Within the body, whole-rock compositions, since it is the mineral rafts and xenoliths of psammitic and semipelitic gneiss composition which defines the thermodynamic activity occur,concordant with the S1-S2 foliation. Any of a particular component or ionic species and hence original discordant relationships have been eliminated determines whether or not the rock is in equilibrium by intense D2strain, and the xenoliths have sharp with a specified fluid. boundaries lacking evidence for chemical gradation. Wheredeformation and metamorphicsegregation The gneiss is intruded by a now-foliated suite of are slight, asin parts of theArdgour, Gulvain and metabasic rocks (Peacock 1977), whichmay be Loch Arkaigbodies, the granitic gneiss has the correlatable with the major early amphibolite suite of appearance of a deformed, homogeneous granite. The the Loch Eil Division (Smith 1979). These include parallel striping of micaceous and quartzofeldspathic texturally modified (hornfelsed?) xenoliths of granitic constituents evident in high-strain zones results from gneiss and are back-veined by remelted granitic gneiss the deformation of MP1 segregation veins or lits and (Peacock 1977, p. 5). Subsequent minor intrusions their biotite selvedges: the selvedges do not represent include Caledoniangranite and pegmatite veins, relict metasediment. These early lits and thelater, together with appinites, microdiorites and felsic cross-cutting Caledonian pegmatites are not related to porphyrites. the origin of the granitic gneiss, which had essentially

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 672 D. Barr et al. its present bulk composition and texture by the end of D1. The granitic gneiss records all the tectonic and metamorphic events recognized in its host metasedi- PH20=7 kbar . Ardgwr gneiss ments, it locally has interbanded contact relationships, (Gould 1966) it containsxenoliths of psammite andinterbanded Cotectic data A Quoich gneiss hornblende schist identical to local Loch Eil Division from Winkler (1979) lithologies, andit is cut by pre-metamorphic basic gnelss dykes which also cut the surrounding Moine.These observations demonstratethat the protolith of the granitic gneiss was intrudedinto Moine metasedi- ments, and that it has occupied its present structural position throughout their recorded tectonic history. Its location within rather than at the base of the Moine succession rules outan interpretation asautochtho- nous, pre-Moinegranitic basement, and gneiss- metasediment contacts lack the zones of extremely high ductilestrain which characterizeallochthonous Lewisian basement slices within the Moine (Rathbone & Harris 1979). Although strain increases towards the FIG.7. Quartzofeldspathic mesonorm components margins of the larger bodies, it does not decline again ofgranitic gneiss, projected from An onto the in the metasediments-these high strains are typical of Q-Or-Ab face. For most samples, Q + Or + Ab thesurrounding Moine, and it is thecentre of the + An make up 85-95% of the total. Projections of the Q + P1 + Kfs + L + V cotectic line and of the granitic gneiss which displays anomalously low strain. Kfs + P1 + L + V and Q + P1 + L + V cotectic sur- Thegranite which formed the protolith of the faces are also indicated. Those for PH2~= 5 kbar granitic gneiss was intruded either before or during the would be geometrically similar, but with tempera- first phase of Moine deformation.There is some tures c. 15 “C higher. evidence to support a synorogenic origin: in particular the lack of a thermalaureole. In this the granitic compositions of Anlo toAn3O); thosethat scatter away gneiss contrasts with theCarn Chuinneaggranite from the cotectic line lie close to the Q + P1 + L + V (Wilson & Shepherd 1979), which was intrudedat or Kfs + P1 + L + V cotectic surfaces. Gould (1966) c. 550Mainto Moine metasediments which had detectedan igneousdifferentiation trend within the alreadyexperienced garnet-grade metamorphism Ardgour body,whereby A1203, Na20,CaO, MgO, (Long & Lambert 1963). Despite Caledonian kyanite- P205 andTiOz decreasedsouthwards but Si02and grademetamorphism, the thermal aureole at Carn K20 increased. The Quoich sample lies on this trend, Chuinneag remained resistant to Caledonian deforma- butthe Fort Augustussamples have slightly lower tion and sedimentary structures and pseudomorphs of A1203and K20 and higher CaO fora given Si02 contact-metamorphicminerals arepreserved. The content. absence of hornfels textures in the vicinity of igneous In Chappell & White’s (1974) classification the rocks is normally considered to reflect intrusion into granitic gneiss is essentially S-type. It is relatively rocks which werealready undergoing high-grade small in volume, has a restrictedrange of truly granitic metamorphism (e.g. Watson 1964), while the absence bulk compositions with high Si02 contents and low of low-pressure minerals implies intrusion at consider- Na20/K20and Fe203/Fe0 ratios, is corundum- abledepth (15 + km).Thus the protolith of the normative (c. 1%)and shows very limited differentia- granitic gneiss was probably intruded during the early tion. It lacks genetically associated basic to intermedi- metamorphism of the Moine, i.e. syn-D1 rather than ate dykes and xenoliths (the amphibolites are part of a pre-DI. regional suite,and the dioriticpatches at Fort Augustus may represent partially absorbed xenoliths), Petrogenesis of the granitic gneiss protolith but contains metasedimentary xenoliths and biotitic wisps. Minor hornblende is occasionally present, but Gould’s (1966) chemical datafrom the Ardgour the gneiss contains two micas (although muscovite is gneiss are summarized in Fig. 7, with one sample from subordinate)and has accessory garnet, monazite, the Quoich gneiss and three from the Fort Augustus ilmenite, and pyrite, with ‘sedimentary’ zircons retain- body. Virtually all are granites (sensu stricto) in the ing an old crustalisotopic component.The ratio Streckeisen (1976) classification. Mostpoints cluster (molecular A1203/Na20+ K20 + 0.5Ca0) is 1.15 for around the Q + P1 + Kfs + L + V cotectic line for Gould’s (1966) average Ardgour gneiss, 1.10 for the PHz0= 5-7 kbar and an anorthite content in the melt Quoich sample and 1.14 for the FortAugustus samples of 5-12 wt% (corresponding to normative plagioclase (cf. Pitcher’s (1983) suggestion of 1.05 forthe

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I-typelS-type boundary).Given the known meta- scale concordance of the contacts is a result of igneous composition of the Lewisian foreland, and of deformation,and the Ardgour, Lock Arkaig, and Lewisian inliers within the Moine (Watson & Dunning Loch Quoich bodies are demonstrably cross-cutting on 1979), Moine metasediments are the only supracrustal a regional scale. Although broadly contemporaneous, rocks which could plasuibly berepresented in the they represent distinct intrusions,and may have source area of the granitic gneiss protolith. differed slightly in timing and source area. Brook et al. (1976) reported an Rb/Sr isochron of (2) These granites were intruded while their coun- 1028 f 43 Mafor the Ardgour gneiss. The initial try rocks were undergoing metamorphism during the 87Sr186Srratio was 0.709, close to the 87Sr/86Srratio of earliest (D1) tectonic event. They are typical S-type Moinemetasediments at c. 1000Ma (Brewer et al. granites (sensu Chappell & White 1974), andwere 1979). Pidgeon & Aftalion (1978) found upperand probably derived at least in part by partial melting of lower intercepts to concordia at 1556 and 574 Ma in a Moinemetasediments at depth. Following D, de- U/Pb study of the Ardgour gneiss. However, Aftalion formation,they developed pegmatitic segregations & van Breemen (1980), using Rb/Sr data independent- which are broadly contemporaneous with the climax of ly todate the metamorphicepisodes, showed that regional migmatization in the metasediments (MP,), zircons in boththe granitic gneiss and nearby and then were intensely deformed during D2 and metasediments could beexplained by athree-stage underwent localized MP2 remelting. model involving initial homogenization at c. 1750 Ma (3) The 1028 f 43 Ma Rb/Sr isochron onthe followed by partialresetting at 1030-1100Ma and Ardgour gneiss (Brook et al. 1976) dates its emplace- c. 490 Ma. The Grenvillian component was dominant ment or metamorphism, and in view of (2) above in the granitic gneiss, but in the metasediment, almost certainly dates the Precambrian metamorphism Grenvillian and ‘Caledonian’ events were responsible of the Moine as Grenvillian.This conclusion is for similar degrees of disturbance. Both the gneiss and supported by U/Pb zircon results(Aftalion & van the metasediments thus can be interpreted as having Breemen 1980), which yielded evidence for a c. 1000 an ultimate Laxfordian (c. 1700Ma) source, but it is Ma event affecting the Ardgour gneiss, an MP1 inferred thatthe gneiss was more fundamentally pegmatite and a Moine metasediment. The D2tectonic modified during the Grenvillian event, melted rather event is probablyPrecambrian (cf. Holdsworth & than merely metamorphosed.The granitic gneiss Roberts 1984) but could conceivably be younger than protolith could perhaps have been derived by Grenvil- 1000 Ma. lian partial melting of Lewisian basement, but the (4) The hornblende schist which occurs in a xeno- ‘sedimentary’ zircons and other S-type characteristics lith in the Quoich gneiss andthe later amphibolite favouran input from Lewisian-derived sediment- sheets which cut all the granitic gneiss bodies carry SI presumably the Moine. In that case, Moine rocks at fabrics and so are of Grenvillian age. Both may belong depth must have attained temperatures sufficiently to a long-lasting igneousevent which overlapped elevated to permit partial melting, and it can reason- graniteintrusion, butthe hornblende schist could ably be inferred that those presently exposed under- represent lava or tuff contemporaneous with Moine went significant metamorphism at c. 1028Ma. Thus sedimentation. the granitic gneiss protolith is unlikely tobe a (5) The granitic gneiss bodies of Ardgour, Gulvain, pre-orogenic granite, and thesimplest interpretation is Loch Arkaig, and Loch Quoich are folded by the D3 that this Grenvillian event corresponds to D1. An upright structures which define the Loch Quoich line early pegmatite in the granitic gneiss, one of the MP1 (Roberts & Harris 1983). At Loch Arkaig, these lits, gave identical U/Pb results to its host rock deform the c. 456 Ma Glen Dessary syenite and are (Aftalion & van Breemen 1980), confirming the certainlyCaledonian (Roberts et al. 1984). The Grenvillianage of thesesegregations and of the granitic gneiss did not originate along the Loch Quoich regional migmatization. Powell et al. (1983) have line, and the two are unrelated except spatially. The confirmed the reality of a Morarian intrusive event in gneiss outcrop follows this line because the granite the Moine, but it is worth emphasizing that Aftalion & bodies werepreferentially intrudedat or nearthe van Breemen (1980) found no evidence for Morarian Glenfinnan-Loch Eil division boundary, which runs (c. 750 Ma) metamorphismin the Ardgour gneiss, parallel to the Loch Quoich line within the regional which is 15 km east of the Loch Eilt pegmatite dated steep belt. as Morarian by van Breemen et al. (1974). (6) Moinemetasediments which contain granitic gneiss must be older than c. 1028 Ma. Hence the Loch Conclusions Eil Division rocks between Loch Quoichand Fort Augustus cannot be part of a post-Grenvillian cover (1) The protolith of the West Highland granitic sequence as suggested by Lambert et al. (1979) and gneiss was intruded as a magmatic granite into Moine Piasecki & van Breemen (1983). If, as suggested by metasediments: it is nota metasomatized sediment, Smith (1979), the hornblende schists and amphibolites nor does it represent pre-Moine basement. Outcrop- of the Glenfinnan and Loch Eil divisions form part of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 674 D. Burr et al. a regional suite, the whole of the Loch Eil Division University of Liverpool. The contributions of D. B. and A. J. must be Grenvillian. This is in contrast to the views of H. appear with the approval of the Director of the British Piasecki et al. (1981) and Talbot (1984). GeologicalSurvey (NERC).Our understanding of the granitic gneiss hasbenefitted from discussions with Mike Atherton,Bob Holdsworth, Frank May, Ian Miller, Doug ACKNOWLEDGEMENTS.D. B. andA. M. R. acknowledgePeacock, RobStrachan, and Donald Smith, whom we thank. receipt of Natural Environment Research Council (NERC) We would also like to thank David Gould for permitting us Research Studentships and L. M.P. an Institute of toquote data from his thesis andJoe Lynch for draughting Geological Sciences Research Assistantship, held at the the figures. References

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Received 20 December 1984; revised typescript accepted 5 March 1985.

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