The Influence of Country Rock Structural Architecture During Pluton

Journal of the Geological Society, London, Vol. 156, 1999, pp. 163–175. Printed in Great Britain.

The inﬂuence of country rock structural architecture during pluton emplacement: the Loch Loyal syenites, Scotland

R. E. HOLDSWORTH1, M. A. McERLEAN1 & R. A. STRACHAN2 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK (e-mail: [email protected]) 2Department of Geology, Oxford Brookes University, Headington, Oxford OX3 0BP, UK

Abstract: The late Caledonian Loch Loyal syenites comprise three related, but geographically separate, composite intrusions: the Ben Loyal, Ben Stumanadh and Cnoc nan Cuilean bodies. They were emplaced (c. 426 Ma) into Moine and Lewisian rocks of the Moine and Naver thrust sheets previously deformed by Caledonian NW-directed folding and thrusting at amphibolite facies metamorphic conditions. The intrusions lie mainly within a zone of large-scale cross-folds in which the regional foliation trends NW–SE at high angles to regional orogenic strike (NNE–SSW). The cross folds are underlain by a broad, ESE-dipping zone of high strain known as the Ben Blandy shear zone. The NW basal contact of the Ben Loyal body dips generally SE and strikes sub-parallel to the foliation in the underlying Ben Blandy shear zone. The higher parts of this syenite, together with those of the Ben Stumanadh and Cnoc nan Cuilean intrusions, form a series of variably coalesced dykes trending NW–SE sub-parallel to the foliation in the pre-existing zone of cross-folds. There is little evidence for significant country-rock deformation during emplacement of the syenites. Minor components of top-to-the-SE extension occurred along the NW boundary of the Ben Loyal body, whilst minor dextral shearing is associated with the emplacement of the NW–SE, vertical sheet-like units that are thought to form the SE part of this intrusion and the other Loch Loyal syenites. Generally contact-parallel magmatic-state deformation fabrics are dominant in most of the intrusions and are thought to reflect limited magma pressure-driven, internal deformation of the igneous bodies during and immediately after emplacement. The geometry of these fabrics and the Loch Loyal syenites as a whole has been directly controlled by the pre-existing structural architecture of the surrounding country rocks. This appears to be a previously overlooked feature of many plutons emplaced at mid- to upper crustal depths below the region influenced by the free surface. The siting and emplacement of the Loch Loyal syenites may have been initially facilitated by late Caledonian gravity-driven extension of the Moine Thrust sheets. Following the initial phase of intrusion, it is also possible that the presence of magma actually promoted further nappe collapse in the adjacent country rocks. There is no compelling reason to link the upper crustal extension observed during emplacement to the genesis of the alkaline magmas in the underlying mantle. The siting and ascent of the syenites at depth may be linked to the presence of steeply-dipping structures in the Lewisian autochthon buried beneath the Moine Nappe.

Keywords: plutons, emplacement, syenites, alkaline composition, intrusions.

Many recent studies have suggested that active tectonic struc- models may be applicable to equivalent plutons in many other tures are directly associated with the emplacement of granitic regions. plutons (e.g. see Hutton 1997 and references therein). Syn- The Loch Loyal syenites lie in an impressive area of moun- tectonic emplacement of sheeted granites and plutons tainous terrain, approximately 9 km south of Tongue, north- emplaced into localized dilational zones along faults and shear ern Sutherland, Scotland (Fig. 1). The intrusions comprise zones have proved to be particularly popular models (e.g. three geographically distinct masses, the Ben Loyal, Ben McCaffrey 1992; D’Lemos et al. 1992). However, these models Stumanadh and the Cnoc nan Cuilean bodies (Fig. 1; King do not adequately explain the apparently late- to post-tectonic 1942; Robertson & Parsons 1974; Holdsworth et al. 1997) that character and complex geometries of many mid- to upper- collectively occupy an area of approximately 24 km2 forming crustal plutons that have been emplaced into orogenic belts at the largest group of alkaline plutons in the British Isles. The depths below the influence of the free-surface (e.g. 4–8 km; syenites lie approximately 12 km east of the Moine Thrust, Hutton 1997). This is well illustrated by the study of the Loch intruding Moine and Lewisian rocks that had previously been Loyal syenites presented here. Based on detailed remapping deformed and metamorphosed in the amphibolite facies during and structural analysis of the entire pluton and the surround- Caledonian NW–directed thrusting at c. 470–430 Ma (Barr ing country rocks (Holdsworth 1987; McErlean 1993; et al. 1986; Holdsworth 1989a). Halliday et al. (1987) obtained Holdsworth et al. 1998), we demonstrate that much of the a U–Pb zircon date of 426&9 Ma from the Cnoc nan Cuilean pluton shape and internal fabric pattern mimics the geometry syenite which they interpreted as an intrusion age. of earlier folds and shear zones in the country rocks. This There are relatively few published accounts of the Loch emphasizes the less well recognized importance of country- Loyal syenites since the initial mapping by the Geological rock structures in controlling the final shape and emplacement Survey at the end of the nineteenth century (e.g. Peach et al. processes of plutons at intermediate crustal depths. Similar 1889). Most early accounts (e.g. Read 1931, 1934; Phemister

163 164 R. E. HOLDSWORTH ET AL.

normative mineralogy of the Loch Loyal syenites to the alkaline intrusions in Assynt, and showed that there are significant chemical differences. Robertson & Parsons (1974) presented a detailed account of the structure and petrology of the Ben Loyal syenite, with brief discussion of the petrographical and chemical variation exhibited by the Ben Stumanadh and Cnoc nan Cuilean bodies. Previous workers have proposed a number of intrusion geometries for the Loch Loyal syenites. They were originally mapped as a single body by Horne during the first geological survey of the region (summarized by Geikie 1888). Phemister (1948) and King (1942) recognized that the three bodies were separate entities, but proposed differing models for the intrusion shape. Phemister suggested that the Ben Loyal body has a sheet-like or laccolith form, whereas the Ben Stumanadh syenite was described as a series of irregular sheets trending E–W, sub-parallel to the banding in the adjacent country rocks; the Cnoc nan Cuilean body was described as a ‘stock’. King (1942) suggested an alternative, proposing that both the Cnoc nan Cuilean and Ben Loyal bodies had a conical form, with the apices pointing downwards and to the southeast. The most recent studies by Robertson & Parsons (1974) broadly concurred with the models suggested by Phemister, suggesting that the Ben Loyal intrusion has a wedge shape with a shallow-dipping roof to the southeast and steep, outward- dipping contacts to the north and west. They suggested that forceful emplacement of the syenite distorted and folded the regional foliation on the northern and western sides of the pluton.

Regional geology The rocks east of the Moine Thrust in Sutherland are divisible into two major metamorphic thrust sheets bounded by major WNW- to NW-directed Caledonian ductile thrusts, the Moine and Naver nappes (Fig. 1; Moorhouse & Moorhouse 1983; Barr et al. 1986; Moorhouse et al. 1988; Holdsworth 1989a). Neoproterozoic Moine metasedimentary rocks, predominantly psammite, are interleaved with Archaean–Palaeoproterozoic Lewisian orthogneisses that are thought to have formed the high-grade metamorphic basement upon which the Moine sediments were originally deposited (Peach et al. 1907; Holdsworth 1989a; Holdsworth et al. 1994). Tectonic inter- leaving of basement and cover rocks occurred during the main phase of Caledonian ductile thrusting and folding that Fig. 1. Regional geological map of the Moine and Naver nappes in intensely deformed the Moine and Naver nappes under Sutherland, showing the country rock geology and the three syenite amphibolite- to greenschist facies metamorphic conditions. bodies (based on Holdsworth et al. 1997 and Holdsworth & A regional metamorphic inversion is recognized across Strachan unpubl. mapping). Box shows location of Fig. 3. MT, Sutherland so that high-grade rocks now lie at higher struc- Moine Thrust; NT, Naver Thrust; C, Coldbackie; S, Sleiteil; CC, tural levels in the regional thrust pile (i.e. to the east; Soper & Cnoc Craggie ORS outlier; BB, Beinn Bhreac ORS outlier. Lewisian et al. inliers named in text: RWI, Ribigill West inlier; REI, Ribigill East Brown 1971; Barr 1986). Larger Lewisian units, such as inlier; BI, Borgie inlier. Loch Loyal syenite: BL, Ben Loyal; BS, Ben the Borgie and Ribigill inliers (Fig. 1) appear to lie in the cores Stumanadh; CnC, Cnoc nan Cuilean. Inset map shows NW Scotland of highly sheared, major antiformal folds (e.g. Holdsworth and major thrusts. CSZ, Canisp shear zone; A, Assynt syenites; 1988, 1989a). Other, more deformed basement units in the LSL, Loch Shin Line; R, Ratagain complex; SF-Strathconon Fault; region are carried in the hanging walls of ductile thrusts. C, Great Glen Fault ‘carbonatites’. Fabrics and folds directly related to the main phase of Caledonian ductile thrusting usually form the second and 1948) focused on the obvious petrographical similarity of the dominant phase of structures (D2). This deformation phase is Loch Loyal intrusions with the Caledonian alkaline igneous associated with ubiquitous development of a gently ESE- complexes in Assynt (Loch Ailsh, Loch Borralan; Assynt dipping composite foliation (Sn/S1/S2; Fig. 2). In micaceous syenites in Fig. 1 inset). King (1942) carried out the ﬁrst and hornblendic rock types, this fabric forms a tight crenu- detailed petrographical and geochemical study, concentrating lation cleavage that becomes transposed in areas of intense on the Cnoc nan Cuilean body. Parsons (1972) compared the strain. The foliation intensiﬁes into broad zones of high strain COUNTRY ROCK STRUCTURE AND PLUTON EMPLACEMENT 165

Fig. 2. Country rock foliation form line map showing major Caledonian ductile thrusts and major F3 folds (based on Alsop et al. 1996, Holdsworth et al. 1997 and Holdsworth & Strachan unpubl. mapping). BBSZ, Ben Blandy Shear Zone. platy mylonites, up to 80 m thick, associated with ductile to differential displacements along D2 ductile thrusts thrusts. An ESE- to SE-plunging D2 mineral lineation is (Holdsworth 1990; Alsop & Holdsworth 1993; Alsop et al. developed throughout the region and is most strongly devel- 1996). Many F3 folds are cross-folds that plunge SE, at low oped in the mylonite zones. The lineation is thought to lie angles to Caledonian tectonic transport direction and on a sub-parallel to the Caledonian thrust transport direction and it large scale these folds cause very pronounced swings in shows a well-defined swing in orientation from gently ESE- strike (Fig. 2). plunging in the west adjacent to the Moine thrust to SSE- Two distinct groups of post-thrusting structures are recog- plunging in the east adjacent to the Naver Thrust (Barr et al. nized in the region, although they are only widely exposed in 1986; Alsop et al. 1996). Shear sense indicators (mica fish, coastal sections, e.g. Coldbackie–Sleiteil area (O’Reilly 1971; shear bands, etc.) suggest overthrusting towards the WNW to Holdsworth 1989b; Fig. 1). The first group comprise generally NNW (Holdsworth & Grant 1990). Mainly tight-to-isoclinal E- to SE-dipping low angle detachment faults and locally F2 folds are preserved on all scales, the S2 schistosity being developed brittle kink and box fold arrays. Brittle shear axial planar to these structures. Detailed structural analyses criteria and associated fold vergence patterns suggest an ESE- (e.g. Holdsworth 1988, 1989a) indicate that these folds display transport direction which led Holdsworth (1989b)to tight sheath fold geometries due to intense ductile strains so suggest an extensional origin for these structures during that the majority of F2 hinges now plunge sub-parallel to the late Caledonian thinning of the Moine Thrust sheet. The Caledonian mineral lineation. detachment faults are cross-cut by later linked systems of Although regionally less significant than F2 folds, open-to- vertical or steeply-dipping extensional faults of various orien- tight F3 folds are developed on all scales in the Moine and tations. On the north coast some of these faults form the Naver nappes of Sutherland where they appear to be related bounding structures to small graben and half graben filled 166 R. E. HOLDSWORTH ET AL.

Fig. 3. Geological map of the Loch Loyal syenites and surrounding rocks showing magmatic-state deformation fabric trajectories in the plutons and major F3 axial traces. Major F3 folds: BA, Borgie antiform; BSS, Ben Stumandh synform; IA, Inchkinloch antiform. Localities mentioned in text: LnaB, Loch na Beiste; AC, Allt Chaonasaidhe; ACC, Allt a’Chalbhach Coire; SF, Sgor Fhionnaich; AF, Allt Fhionnaich; ATT, Allt Torr an Tairbh. The hitherto unrecognized outlier of ORS breccia resting unconformably upon the Ben Stumanadh syenite is also highlighted. with undeformed red breccias, conglomerates and sandstones and pelite (Fig. 3). The fine- to medium-grained Moine rocks (e.g. Coldbackie; Fig. 1; Peach & Horne in Crampton & of the Moine Nappe lack gneissic fabrics and pelites typically Carruthers 1914). The age of these unfossiliferous deposits is carry a mineral assemblage indicative of low- to mid- controversial, but most field-based studies favour a Devonian amphibolite facies regional metamorphism (e.g. quartz+ age (e.g. O’Reilly 1983; Blackbourn 1981a, b; Holdsworth muscovite+biotite+albite-oligoclase+garnet&staurolite&K- et al. 1998). Syenite boulders thought to be derived from feldspar&kyanite). In contrast, the Moine rocks of the erosion of the Loch Loyal syenites form a major clast type Naver Nappe adjacent to the eastern part of the Ben found in the upper parts of conglomerate sequences exposed Stumanadh syenite are medium- to coarse-grained, gneissic northwards from Cnoc Craggie to Coldbackie on the coast and pelites carry assemblages indicative of mid- to upper- (Fig. 1). amphibolite facies (quartz+biotite+oligoclase-andesine+ garnet&staurolite& fibrolite&K-feldspar) (Soper & Brown 1971; Burns 1994; Holdsworth et al. 1998). The country rocks around the Loch Loyal syenites In the Moine Nappe south of Borgie Forest, around Loch Loyal and north of Cnoc nan Cuilean (Figs 1 & 3), syenite is Lithologies also intruded into a distinct, lithologically diverse group of The Loch Loyal syenites cross-cut the upper parts of the Lewisian country rocks comprising interbanded units of acid, Moine Nappe and the lower part of the Naver Nappe in intermediate and basic gneiss, together with subordinate Sutherland (Figs 2 and 3). Well-exposed regions of country bodies of amphibolite. These units are assigned to the Lewisian rocks are mainly restricted to the western part of Ben Loyal Complex based on lithological and geochemical evidence (see and eastern flanks of Ben Stumanadh; rocks in the region SE Moorhouse & Moorhouse 1988; Holdsworth 1989a and refer- of Loch Loyal are largely unexposed. Moine rocks in the ences therein). They are thought to represent the basement region around the syenites comprise mainly lithologically upon which the Moine sediments were originally deposited monotonous psammites, with subordinate units of semipelite because, in regions of low strain, the gneisses preserve higher COUNTRY ROCK STRUCTURE AND PLUTON EMPLACEMENT 167

Fig. 4. Equal-area stereonets of country rock structures: (a) poles to foliation, Ben Blandy shear zone (BBSZ) hanging wall, east of Ben Loyal; (b) poles to foliation, BBSZ and footwall region, west of Ben Loyal; (c) F3 minor fold axes; (d) Poles to S3 surfaces. Note that F3 folds are largely absent in the footwall of the BBSZ (Alsop et al. 1996). grade (mid- to upper-amphibolite facies) metamorphic assemblages compared to the adjacent psammites and pelites in the Moine Nappe. However, many of these Lewisian rocks are now banded schists due to the eﬀects of intense deformation and associated retrogression. Previous studies (e.g. O’Reilly 1971) suggested that the nearest outlier of red breccia and conglomerates occurs at Cnoc Craggie about 2 km N of the Ben Loyal body (Figs 1 & 3). Our remapping of the area revealed a small, previously unrecognized outlier of red breccias resting unconformably upon part of the Ben Stumanadh syenite, at 475 m  near to the summit of Ben Stumanadh [NC 6376 4960] (labelled ‘ORS’ on Fig. 3).

Structures The Loch Loyal syenites are intruded mainly into country rocks where the foliation trends generally NW–SE at high angles to the regional strike (Figs 2 & 4a). This trend reflects the presence of at least three major F3 cross-folds (from N Fig. 5. Line drawing showing disharmonic F3 minor folds coaxially to S): the Borgie antiform, Ben Stumanadh synform and refolding F2 isoclines in Moine psammites. Viewed to SE, near the Inchkinloch antiform (Figs 2 and 3; Alsop et al. 1996; summit of Ben Stumanadh, [NC 6436 4964]. Holdsworth et al. 1998). Mesoscopic F3 closures are also exposed on the north-facing slopes SW of Loch Craggie [NC many exposures, they coaxially refold tight-to-isoclinal F2 60 51] and NW of Loch na Beiste, e.g. at [NC 583 466] (Fig. 3). minor folds (e.g. Fig. 5). Although many F3 minor folds The Naver Thrust and a structurally higher ductile disconti- plunge sub-parallel to the L2 mineral lineation, refolding of nuity, the Torrisdale Thrust (Holdsworth et al. 1997, 1998), are this fabric is common where significant differences in orien- both folded by the large-scale F3 cross-folds in the region east tation exist, e.g. at [NC 6602 5202]. Most F3 structures are of Loch Loyal. The western limit of cross-folding coincides disharmonic in style with irregular, NW–SE striking axial with a NE–SW-trending zone of strongly deformed platy rocks surfaces that are curviplanar on all scales about an axis up to 200 m thick which runs parallel and immediately ad- sub-parallel to the mean L2 mineral lineation and F3 fold axis jacent to the eastern contact of the Ribigill East Inlier and also orientations (Fig. 4d; see also Alsop et al. 1996). Overall, to the western contact of the Ben Loyal body (Figs 2 & 3). This moderately to steeply SW-dipping surfaces are dominant in the belt of high strain, the Ben Blandy shear zone (Alsop et al. region. Irregular S3 crenulation fabrics are only weakly to 1996), forms a marked structural boundary since F3 folds are moderately developed in most areas and are best preserved in generally absent in its footwall region to the west where the more micaceous units such as Moine pelite. country-rock foliation has a NE–SW strike (Fig. 4b; contrast with a). Alsop et al. (1996) suggest that differential displacements along the shear zone may have generated the large-scale The Loch Loyal syenites F3 cross-folds in its hanging wall. F3 folds form the dominant minor structures on all scales in Shape and contact relationships the country rocks adjacent to most of the Loch Loyal syenites. The present study confirms that the syenites cross-cut all folds They are typically open-to-tight buckle folds, plunging gener- and associated ductile deformation fabrics in the country ally to the southeast (Figs 4c & 5), although locally, e.g. at [NC rocks, as first noted by Read (1931). Contacts between syenite 6602 5202], they are markedly variable in their orientation. In and Moine or Lewisian country rocks are always sharp, with 168 R. E. HOLDSWORTH ET AL. much veining of the country rocks adjacent to mapped contacts (Fig. 3). There is little evidence for significant defor- ation of the country rocks synchronous with emplacement of the complex. The Ben Loyal body has an approximately semicircular map pattern, truncated to the southeast by the Loch Loyal Fault (Fig. 3). The northern contact is only exposed in the Allt Chaonasaide stream section [NC 588 506] where a sharp junction between strongly foliated syenite and Moine psammite dips moderately southwards. Similar, sharp contacts between strongly foliated syenite and platy Moine psammites and Lewisian gneisses are exposed at three localities around the north-west and west faces of Ben Loyal: in a stream section at [NC 573 501]; in the Allt a’Chalbhach Coire stream section [NC 568 495]; and in the steep west face of Sgor Fhionnaich [NC 561 482]. At these localities, the compositional layering in the host rocks dips gently to the south-east and east beneath the boundary of the intrusion which dips moderately to steeply inwards towards the centre of the pluton. No support has been found for the view of Robertson & Parsons (1974) that on the northwest face of Ben Loyal, this boundary dips steeply outwards away from the centre of the pluton, and that in the region of Sgor Fhionnaich, it is subvertical. New exposures in Fig. 6. Minor structures associated with syenite emplacement and the Allt Fhionnaich section [NC 5633 4760] show that the deformation. Syenite is shown using the crosses (a, b, c)orgrey(d) outward dipping contact referred to here by Robertson & ornament. Where appropriate, offsets of layers in the country rocks Parsons (1974) is a WNW–ESE-trending fault. Narrow sheets are indicated. (a) Line drawing of syenite sheet emplaced into platey of syenite intruded into the country rocks close to the N and W Moine psammites showing evidence of top-to-the-SE extensional margins of the Ben Loyal syenite, e.g. at [NC 57495012], lie shear synchronous with intrusion along the NW contact of the Ben sub-parallel to the main contacts and offset more shallowly- Loyal pluton. Coille na Cuile on lower slopes of Sgor Chaonasaid dipping marker layers in the country rocks in a manner [NC 5752 5017]. (b) Line drawing of sub-vertical, NW–SE-trending suggesting small components of top-to-the-SE shear during syenite dykelets intruded along F3 axial surfaces in folded Moine emplacement (Fig. 6a). Limited exposures of the south and psammites, south of Borgie Forest, [NC 6605 5117]. (c) Line east contacts of the pluton, e.g. in the region of Creag nan drawing showing plan view of minor dextral drag folds developed in Speirig at [NC 58 46], suggest that the syenite in this region Moine psammites intruded by sub-vertical, NNW-trending syenite comprises a series of steeply NE-dipping coalesced dykes (Fig. dyke. West side of Creag Bhreag, [NC 6486 5020]. (d) Schematic diagram of the Allt Chaonasaide stream section exposures of the 3) with a form analogous to the Cnoc nan Cuilean and Ben northern contact of the Ben Loyal pluton showing the fabric Stumanadh bodies (see below). In general the contacts follow geometries in the syenite and their orientation relative to the country the strike of the country rock foliation and/or the planes of rock foliation in Moine psammites [NC 5867 5057]. local F3 axial surfaces. A number of inclusions of Moine psammite occur within the adjacent Moine psammites and Lewisian gneisses in which Ben Loyal body (Fig. 3). The largest inclusion, at the south- there are numerous smaller sheets of syenite lying sub-parallel west end of Bealach Clais nan Ceap at [NC 59 49], is c. to the larger intrusions. The easternmost contact exposed at 800#300 m in map dimensions. Other mappable inclusions the base of the waterfalls at [NC 612 472] is a sub-vertical fault. outcrop in the Allt na Creige Riabhaich at [NC 585 488] and The subvertical N- to NW-trending contact between the west- on the north slopes of Ben Hiel at [NC 595 502; 599 503]. ern side of the syenite and Moine psammites is also exposed in Planar and linear fabrics within these inclusions are irregularly two localities at [NC 5985 4547; 5985 4503] within Bealach na orientated suggesting that the inclusions are rafts rather than Beiste. On the basis of the observed contact relationships, roof pendants or remnants of the floor to the intrusion. McErlean (1993) interprets the Cnoc nan Cuilean body as Evidence for thermal metamorphism in the psammitic country a series of coalesced, steeply dipping, NW-trending dykes, rocks is mainly restricted to minor hornfelsing represented by although this remains a tentative proposal given the rather recrystallization of regional metamorphic fabrics into equi- poor exposure of the pluton and its margins. granular quartz and feldspar mosaics. Subordinate units of The preservation of widespread amphibole and pyroxene in Moine garnetiferous semipelite have locally developed fibrolite the country rocks adjacent to the syenites was thought by King in regions adjacent to the intrusion, e.g. at [NC 5909 4922]. (1942) to indicate widespread metasomatism of Moine psam- The contacts of the Cnoc nan Cuilean body are unexposed mitic rocks. Our observations suggest, however, that most of except on the steep NW slopes of Cnoc nan Cuilean at [NC 59 the supposedly altered country rocks are typical banded 46] and in the Allt Torr an Tairbh stream section (King 1942) Lewisian gneisses in which the preservation of amphibole- [NC 61 47]. In these areas, contacts are sharp and well-defined. and pyroxene-bearing lithologies is common (Moorhouse & On the mountain slopes, concordant, steeply dipping, NW– Moorhouse 1977; Holdsworth 1989a). Some hornfelsing of the SE-striking sheets of syenite project outwards from the main country rocks is evident adjacent to most exposed contacts, but intrusion into the Lewisian hornblende–biotite gneisses (Fig. apart from some pyroxene growth in Moine psammites 3). In the stream section, which has previously been described exposed immediately adjacent to the syenite at one locality at in some detail by King (1942), the syenites form a series of [NC 5985 4547], the effects of metasomatism are apparently steeply WSW-dipping dykes concordant to the foliation in the very much more limited than has previously been suggested. COUNTRY ROCK STRUCTURE AND PLUTON EMPLACEMENT 169

The Ben Stumanadh body is formed from a series of petro- a hydrous gas phase in the marginal parts of the pluton as it graphically similar, sheet-like intrusions which trend NW–SE was deformed during intrusion. and are truncated to the northwest by the Loch Loyal Fault The Cnoc nan Cuilean syenite has an overall more mafic and (Fig. 3). Most of the intrusions dip steeply to the southwest, heterogeneous composition compared to either the Ben Loyal subparallel to the strikes of the foliation in the country rocks or Ben Stumanadh bodies (King 1942; Robertson & Parsons or to local F3 axial surfaces (Fig. 6b). Steeply-dipping sheets 1974). The marginal parts of the syenite display relatively basic (or dykes) occur on all scales, from tens of millimetres to compositions that grade into a more homogeneous, feldspathic several hundreds of metres thick. Contacts with country rocks central portion. The marginal syenites consist of a two feld- are generally sharp; minor amounts of hybridization have been spar, subsolvus assemblage, analogous to the outer unit of the recorded at Coille na Leitire Bige [NC 62 50] (Robertson & Ben Loyal body, but aegirine-augite is much more abundant Parsons 1974). Hornfelsing of the country rocks is apparent whilst amphibole is absent. The inner syenite is a one feldspar, for up to ten metres from contacts. This is most obvious hypersolvus syenite, with aegirine augite as the dominant mafic in quartzo-feldspathic lithologies (Moine psammites and phase. Accessories in both the marginal and main facies are Lewisian acid gneisses) where regional metamorphic fabrics similar to the Ben Loyal intrusion. are strongly recrystallized to form equigranular quartz and The Ben Stumanadh syenite displays a two-feldspar-bearing feldspar mosaics. In the summit regions and northeast flanks assemblage, with perthitic alkali feldspar, albite, quartz, of Ben Stumanadh, the presence of late fibrolitic sillimanite in aegirine-augite, amphibole (hornblende, arfvedsonite) and outcrops of Moine semipelite is probably due to this thermal minor accessories (biotite, titanite, allanite, opaque oxides and metamorphic overprint, e.g. at [NC 6466 5077]. Small offsets of apatite). Petrographically, most of the Ben Stumanadh intru- layering across syenite dykes, e.g. at [NC 6653 5045] and sions are similar to the subsolvus outer unit of the Ben Loyal occasional minor drag folding adjacent to dyke contacts intrusion in that they carry two feldspars and amphibole is (Fig. 6c) indicate a small component of dyke-parallel dextral always present as a mafic phase. However, some samples, e.g. shear during intrusion. at [NC 6440 5010], more closely resemble the inner hypersolvus In addition to the three main bodies, numerous minor sheets unit of Ben Loyal as they carry perthitic alkali feldspars and dykes of syenite also outcrop in the Moine and Lewisian separated by marginal zones of fine-grained albite and inter- country rocks adjacent to these masses. The majority lie within stitial quartz. These rocks are noticeably more mafic (c. 20% a kilometre of the main plutons (Fig. 3), but a few examples pyroxene and amphibole) and may lie in the central regions of may be found up to 5 km away, e.g. at [NC 575 545]. Minor large sheets. Exposures are generally poor, but as there appear intrusions associated with the Cnoc nan Cuilean, Ben to be no clearly defined contacts it seems likely that any Stumanadh masses and southern parts of the Ben Loyal internal boundaries within the syenites are gradational. syenite, e.g. around [NC 58 46], are typically NW-trending The Ben Stumanadh syenites display two important charac- dykes. In contrast, those associated with the northern and teristic textures that differ from the other members of the Loch western margins of the Ben Loyal body are more moderately Loyal syenites. Firstly, quartz in the Ben Stumanadh intru- dipping sheets. Thus the minor intrusion geometries appear to sions locally occurs with feldspar forming a graphic inter- mimic those of the adjacent plutonic bodies. growth texture suggesting simultaneous crystallization of these mineral phases. Secondly, the northwestern parts of the Ben Stumanadh syenite have a pronounced vuggy texture that, in contrast to Ben Loyal, seems to have two different origins. An Petrography early set of vugs is similar to miarolitic cavities that have been The mineralogy of the Loch Loyal syenites is dominated by described from the Ben Loyal body, containing quartz, apatite feldspar (up to 80%), but all units additionally carry quartz. and zeolites (see Robertson & Parsons 1974). A later set of Aegirine-augite is generally the most abundant mafic mineral, vugs form irregular cavities filled with clay-like minerals, ore with amphibole and biotite also locally common. Most of the and carbonates that may represent weathered-out pseudo- syenites are medium to coarse-grained and carry a variably morphs of mafic minerals. This replacement is thought to be developed foliation defined by alignments of feldspar laths, associated with the effects of late faulting (see below). mafic grains and elongate aggregates of mafic and accessory minerals. The Ben Loyal body is typically composed mainly of feldspar, with quartz, aegirine-augite, amphibole (hornblende, Internal foliations and deformation textures arfvedsonite) and minor accessories (titanite, apatite, allanite Most of the Loch Loyal syenites, with the exception of the and opaque oxides); for a detailed petrographic description see inner unit of the Ben Loyal body, carry weak to moderately Robertson & Parsons (1974). Two distinct units are present strong foliations. These fabrics are sub-vertical and trend within the mass: an outer marginal syenite which is conspicu- NW–SE throughout both the Ben Stumanadh and Cnoc nan ously foliated; and an inner core syenite within which foli- Cuilean bodies, lying at low-angles, or sub-parallel to the ation is moderately to weakly developed or absent (Fig. 3; intrusion margins (Figs3&7a&b).Steep NW–SE trending Robertson & Parsons 1974). The two units are also distin- fabrics also occur throughout most of the Ben Loyal body guishable on petrographical grounds through the presence of a (Figs 3 & 7c), except in the region of the northern and western two feldspar (alkali feldspar+albite), subsolvus assemblage in margins of the pluton where the fabrics swing into approxi- the outer syenite, whereas the inner unit appears to be hyper- mate concordance with the inward dipping contacts (Figs 3 & solvus with only one perthitic feldspar present. The boundary 7d). In the only fully exposed section across this contact, seen between the two units is gradational over several hundred in Allt Chaonasaide [NC 5867 5057], it appears that the dip of metres and is difficult to accurately define on a map. Robertson the foliation within the syenite decreases as the contact with & Parsons (1974) attribute the differences in petrography the underlying psammitic country rocks is approached (see between the marginal and core syenites to the concentration of Fig. 6d). 170 R. E. HOLDSWORTH ET AL.

Fig. 7. Stereonets of deformation fabrics from the Loch Loyal syenites. (a) Poles to foliations, Ben Stumanadh syenite. (b) Poles to foliations, Cnoc nan Cuilean syenite. (c) Poles to foliations, eastern Ben Loyal syenite. (d) Poles to foliations, western Ben Loyal syenite. (e) Lineations, Loch Loyal Syenite Complex. Dots are Cnoc nan Cuilean; squares, Ben Loyal east; crosses are Ben Stumanadh. (f) Poles to joint planes, Ben Loyal syenite. (g) Poles to joint planes, Ben Stumanadh syenite.

In the Ben Loyal body, a planar fabric is defined by the fabric is associated with subgrain development around feldspar preferred orientation of pyroxene, amphibole and feldspar phenocrysts and minor recrystallization of quartz and feldspar. laths. This is relatively well-developed at the outer margins of These weak solid-state fabrics are similar to those found in the the body, becoming moderate or weakly developed in the core. Ben Loyal body. Mafic enclaves and country rock xenoliths are elongate parallel The Ben Stumanadh intrusions display a well-developed to the foliation, e.g. at [NC 6029 4990]. In many parts of the magmatic-state foliation defined by the subparallel alignment pluton, interstitial quartz displays very minor undulose extinc- of feldspar laths, prismatic mafic minerals and elongate aggre- tion, but is otherwise relatively unaffected by internal plastic gates of mafic minerals. Unlike the Cnoc nan Cuilean and Ben strain. This suggests that the foliation formed due to deforma- Loyal bodies, there is little evidence of an overprinting mod- tion of the syenite whilst it was still in a magmatic state erate temperature solid-state deformation fabric (McErlean (Hutton 1988; Paterson et al. 1989). The weaker development 1993). The only exception is a localized region adjacent to the of the fabric in the core syenite is taken as evidence that, at Loch Loyal Fault in which relatively intense crystal plastic the current level of exposure, this part of the intrusion was deformation textures are preserved including serrated grain emplaced last. boundaries and polygonal recrystallized quartz grains. This The magmatic-state deformation fabric is overprinted co- zone may reflect an early phase of movement along the fault axially by a weak deformation fabric formed after the syenite which has also produced, later low temperature cataclastic had largely crystallized, i.e. a post-rheologically critical melt textures in several parts of the Ben Stumanadh intrusions percentage (RCMP) fabric (Tribe & D’Lemos 1996 and refer- (see below). ences therein). This fabric is most intense in the regions around the north-west margin of the pluton where it is characterized by the limited development of serrated grain boundaries in feldspar and quartz. In a quartzofeldspathic rock, such fea- Low-temperature deformation and the Loch Loyal Fault tures suggest that some grain boundary migration recrystal- The final phase of deformation observed in the Loch Loyal lization has occurred at moderate temperatures (c. 400–550)C; syenites is associated with movement along late brittle faults, White 1976; Lister & Snoke 1984; Simpson 1985). in particular a major NE–SW-striking structure, the Loch The Cnoc nan Cuilean syenite also carries a well-developed Loyal Fault (Fig. 3; Holdsworth et al. 1997). This structure is shape preferred orientation of pyroxene prisms and feldspar thought to separate the Ben Loyal syenite and its associated laths, typical of a magmatic state deformation fabric. A weak, country rocks from those of the remainder of the complex. The coaxially overprinting, moderate-temperature post-RCMP fault forms a broad NE-trending topographic low that can be COUNTRY ROCK STRUCTURE AND PLUTON EMPLACEMENT 171 traced for at least 12 km along strike trending obliquely across few samples analysed by Robertson & Parsons (1974) from the Loch Loyal. It is only exposed as a 150 m wide zone of Ben Stumanadh body are slightly more potassic than those reddened cataclastic breccias and gouge cutting Ben from Ben Loyal, but they are otherwise very similar. The Stumanadh syenite on the northeast shores of Loch Loyal [NC existing chemical and petrographic data suggest, therefore, 62 50]. The largest fault exposed at [NC 6228 5071] dips steeply that the Loch Loyal syenites are likely to be genetically related, NW with a 5 m wide zone of reddened breccia. Fracture although this hypothesis remains necessarily tentative in the surfaces within the fault zone, e.g. at [NC 6227 5070], carry absence of more detailed chemical and isotopic data. slickensides obliquely oriented to the strike, e.g. average pitch of 65)SW. These structures suggest that the Loch Loyal Fault may have an oblique displacement, but the sense and magni- Discussion tude of movement is uncertain. In the central part of the Loch Loyal Fault zone, e.g. at [NC 6228 5071], a 0.5 m wide, white Pluton geometry and mode of magma emplacement carbonate-cemented hydrothermal breccia is preserved. This The remapping of the Loch Loyal syenites has shown that the displays concentric vuggy textures, with small radiating crys- Cnoc nan Cuilean and Ben Stumanadh bodies appear to be tals adjacent to the vein walls becoming coarser-grained formed by variably coalesced, steeply dipping, NW-trending inwards. The pure carbonate cross-cuts an earlier carbonate- sheets or dykes. Similar types of intrusion form the upper, hematite–goethite breccia which in turn post dates fine- southern part of the Ben Loyal syenite and are inferred in the to ultrafine-grained, red-brown cataclasite/breccia rich in poorly exposed eastern parts based on the predominantly hematite-goethite. NW–SE orientations of the internal deformation fabrics in this Brittle deformation is widespread in the western part of the area (Fig. 3). These appear to coalesce to the northwest Ben Stumanadh syenite and probably relates to movements forming a more regular body whose base is defined by the along the Loch Loyal Fault and the NW-trending faults in the moderately S- to E-dipping contacts with host rocks. Coir’ Achadh nan Sac area [NC 63 49]. Irregularly oriented Magmatic-state deformation fabrics are present throughout sets of brittle fractures are common in syenite exposures, often much of the Loch Loyal syenites, apart from the core unit of with an infilling of fine grained brown cataclasite up to 10 mm the Ben Loyal body. In most units, the foliations are sub- thick, e.g. at [NC 6311 5048]. These syenites display a charac- parallel to adjacent pluton contacts (Fig. 3). teristic reddening in fresh surfaces due to pervasive iron oxide The shape of the intrusions and the geometry of their internal staining. They weather deep-red and produce the strikingly fabrics reflects broadly the pre-existing structural architecture coloured rock screes lying on the steep west slopes of Sron of the Moine and Lewisian host rocks. The steeply dipping, Ruadh [NC 62 50] and Coir’ Achadh nan Sac. Carbonate is NW-trending parts of the complex lie parallel to the regional found in numerous small fractures and vugs in the Ben foliation that was deformed into this orientation during earlier Stumanadh syenites adjacent to the Loch Loyal Fault. The F3 cross-folding (for example, compare the foliation data in fault-related movements of hydrothermal fluids inferred to be Figs 4a & 7a–c) whereas the lower contact of the Ben Loyal responsible for the carbonate mineralization may also cause intrusion follows the platy mylonites associated with the Ben the widespread breakdown of mafic minerals forming the Blandy shear zone that lies along the upper contact of the second set of vugs observed in western parts of the Ben Ribigill East Lewisian inlier (compare Figs 4b & 7d). Drag folds Stumanadh syenite. within Moine country rocks and offsets of markers indicate a small component of dextral shear parallel to NW-trending dyke walls during emplacement. Minor sheets of syenite intruded Jointing into the Moine rocks adjacent to the lower, north-west margin of the Ben Loyal mass are associated with minor amounts of All syenites of the Loch Loyal Syenite Complex are well top-to-the-southeast extensional displacement. jointed, with between one and three prominent sets developed Robertson & Parsons (1974) suggest that forceful intrusion of in any single exposure; they show no obvious relationships to the Ben Loyal syenite resulted in significant reorientation of the internal foliations and no systematic pattern was detected country rock structures. This conclusion is based largely on the (Fig. 7f–g). In many exposures, one joint set is finely spaced on apparent bending of the regional strike in the Moine and a scale of millimetres and can be mistaken for a foliation when Lewisian rocks around the NW contact (e.g. Fig. 2) of the viewed from a distance. This morphology is developed in a pluton and the observed concordance of country rock struc- variety of joint sets throughout the complex and shows no tures, contacts and internal fabrics. We have found no consistent orientation. unequivocal field evidence consistent with significant deformation of the country rocks during emplacement, e.g. folding at contacts, anomalous reorientation of regional fabrics. We sug- Genetic relationship of the plutons gest that the open folding of the country rock foliation to the The only significant study of the chemistry of the three syenite NW of Ben Loyal is likely to predate intrusion; a similar feature intrusions was carried out by Robertson & Parsons (1974). is seen, for example to the W of the Beinn Bhreac Old Red Their results show that the Loch Loyal syenites are all of Sandstone outlier (Figs 1 & 2). Thus, the concordant nature of similar alkaline affinities and that the dominant control of rock the Ben Loyal body is thought to reflect exploitation of pre- composition has been magmatic fractionation. The Cnoc nan existing country rock structures rather than forceful intrusion. Cuilean body is more mafic in terms of its chemistry than the The sharply defined, hornfelsed intrusion contacts, with other syenites and this appears to be consistent with its local development of fibrolite in pelites, and the lack of occurrence as a separate, less fractionated intrusive unit. The intrusion-related deformation in the country rocks are consist- local preservation of mafic syenite as enclaves within both ent with emplacement of the Loch Loyal syenites at relatively the Ben Loyal and Ben Stumanadh bodies may indicate that shallow crustal depths (<10 km). The contact-parallel the Cnoc nan Cuilean syenite is the earliest intrusive unit. The magmatic-state deformation fabrics are thought to reflect 172 R. E. HOLDSWORTH ET AL.

Fig. 8. Three dimensional diagram showing proposed geometry for the Ben Loyal–Ben Stumanadh intrusion. Movement along the Loch Loyal Fault has subsequently juxtaposed different structural levels of the same composite magma body. deformation of the magma synchronous with emplacement, very minor deformation of largely crystallized magma during but these fabrics are significantly less intense than those the emplacement of final magma pulses such as, for example, typically found in most forcefully intruded plutons (e.g. the core syenite of Ben Loyal. The development of such solid Molyneux & Hutton 1998). It is suggested that the magma state fabrics may have been enhanced by hydrolytic weakening driving-forces arising due to density-driven buoyancy contrast effects if the hydrous gas phase suggested by Robertson & and other factors acted against the intrusion walls to produce Parsons (1974) was present. a modest amount of coaxial magmatic strain in which the axis In summary, although there is evidence of limited defor- of shortening is aligned normal to the pluton walls (McErlean mation of the magmatic bodies during and immediately fol- 1993). Non-coaxial shearing parallel to pluton margins will lowing intrusion, emplacement of the Loch Loyal syenites was produce magmatic fabric asymmetries (e.g. Blumenfeld & a relatively passive process in which the pluton exploited the Bouchez 1988; Fernandez & Laporte 1991; McErlean 1992), pre-existing fold and shear zone architecture of the Moine and but the often poor exposure of the Loch Loyal syenites Lewisian country rocks. This led to the development of an precludes a detailed analysis of such features. However, locally internal deformation fabric pattern that mimics these country preserved magmatic fabrics running obliquely across NW- rock structures. However, in order to fully understand the trending syenite dykes, e.g. at [NC 6650 5050] and locally likely original form of the entire igneous complex, it is neces- developed, shallowly NW-plunging linear fabrics in all three sary to speculate concerning the nature of the offset along the plutons (Fig. 7e) suggest minor components of dyke-parallel Loch Loyal Fault and the possible original relationship of dextral shear consistent with those proposed based on features the Ben Loyal and Ben Stumanadh bodies. associated with intrusion contacts. The shallowing of the foliation adjacent to the NW contact of the Ben Loyal pluton, e.g. at Allt Chaonasaide [NC 5867 5057] (Fig. 6d), suggests a The pre-faulting form of the plutons top-to-the-northwest sense of shear opposite to that given by Petrographically, the Ben Loyal and Ben Stumanadh syenites offsets of markers across syenite sheets in the same region. The are similar, but there are significant textural differences shallowing of the fabrics may be a localized feature caused by between the two plutons. The occurrence of graphic inter- minor top-to-the-NW shearing of the magma against the lower growth textures in the Ben Stumanadh syenite and the absence margin during the emplacement of the slightly later core of the moderate temperature solid state foliation seen in the syenite unit (McErlean 1993). other syenite bodies indicates that the Ben Stumanadh intru- Weak, moderate-temperature (c. 400–550)C) post-RCMP sions may have been emplaced at shallower crustal depths. deformation fabrics are developed in the NW parts of the Ben This suggests that a component of SE-side-down motion along Loyal syenite and throughout the Cnoc nan Cuilean body. the Loch Loyal Fault has juxtaposed two different structural These fabrics formed at temperatures that are likely to be levels of the Loch Loyal syenites at the present-day surface. On significantly higher than the host rock ambient conditions at the basis of their close petrographic similarity, we further the time of intrusion (<300)C?) based on the observation that propose that the overlying, eroded parts of the Ben Loyal extensional structures in the country rocks that are possibly syenite could have been formed by a series of steeply dipping contemporaneous with emplacement (see below) are rather sheets that linked directly to those forming the Ben Stumanadh brittle in style. Thus, we propose that the post-RCMP fabrics body, i.e. the two bodies are different levels of a single pluton. formed during pluton cooling synchronous with or immedi- This proposal is consistent with the coalesced dyke geometry ately following the final stages of emplacement (McErlean preserved in the upper, SE parts of the Ben Loyal body 1993; Tribe & D’Lemos 1996). These weak fabrics are coaxial (Fig. 3). The proposed geometry of the original pluton is with the preceding magmatic-state foliations and may reflect shown in Fig. 8. COUNTRY ROCK STRUCTURE AND PLUTON EMPLACEMENT 173

but no examples are seen to cut or be cross-cut by syenite veins. Continued influx of magma led to dyke coalescence at depth in the pluton and ended with the final emplacement of the core syenite of Ben Loyal and minor deformation of the marginal phases. Once pluton emplacement had begun, it is also quite possible that the presence of mechanically weak magma in the Loch Loyal region actually enhanced the extensional collapse of the northern part of the Moine Thrust sheet. The origins and deep crustal controls on the ascent of the Loch Loyal syenite magmas are a matter of speculation. Geochemical and isotopic data suggest that the alkaline complexes of NW Scotland, including the Loch Loyal syenites formed in the mantle either in the lithosphere (e.g. Halliday et al. 1987) or asthenosphere (e.g. Thompson & Fowler 1986). Alsop & Holdsworth (1993) have suggested that regional zones of F3 cross-folds like those surrounding the Loch Loyal syenites may have formed due to interference between Caledonian thrusting and older, NW-trending sub-vertical Laxfordian shear zone fabrics in the underlying Lewisian basement. If this hypothesis is correct, it is conceivable that one of these old shear zones has acted as an ascent channel for the sub-crustal magma in the lower to mid-crust below the Fig. 9. Schematic diagram showing how the pre-existing zone of F3 Moine Nappe. Further speculation concerning the generation cross-folds may have focused differential dextral shear during late and ascent of the Loyal Loyal syenites is beyond the scope of regional extension leading to initial emplacement of the Loch Loyal the present paper, but we see no compelling reason to link syenites in a series of en echelon dykes above an extensionally- reactivated Ben Blandy shear zone. directly the proposed extension in the upper crust during emplacement to the genesis of the alkaline magmas in the mantle. In our view, the siting and ascent of most Caledonian alkaline complexes are more obviously associated with steeply Regional significance dipping strike-slip/transtensional faults or shear zones The generally passive emplacement of the Loch Loyal syenites, exposed either at the surface (e.g. Ratagain Complex along with only fairly weak syn-magmatic strains preserved in the Strathconon Fault; Hutton & McErlean 1991; Great Glen plutons and adjacent wall-rocks suggests that external tectonic Fault ‘carbonatites’; Garson et al. 1984; Fig. 1 inset) or buried forces may have played some role in generating initial space for at depth forming clusters or linear zones of sub-crustal mag- the intruding magmas (Hutton 1988, 1997). Top-to-the-ESE, matism (e.g. Ach’uaine hybrids, Loch Shin Line; Watson 1984; low-angle extensional detachment faulting occurs in a belt of Assynt syenites, Canisp Shear Zone; Peach et al. 1907; Fig. 1 Moine and Lewisian rocks exposed along the north Sutherland inset; Loch Loyal syenites, this study). coast along strike from the pluton between Coldbackie and Torrisdale Bay (Fig. 9; Holdsworth 1989b). Holdsworth (1989b) has suggested that this deformation occurred late in the Caledonian orogeny and that it was driven by the diffusely Conclusions distributed gravitational collapse of the Moine thrust sheet. The extension occurs by low-angle detachment faulting at low This study shows that the emplacement geometries of the Loch angles or sub-parallel to the foliation planes in the metamor- Loyal plutons and their internal, magma pressure-driven phic rocks and is likely to be favoured by the general NNE– fabrics are controlled by the exploitation of essentially in- SSW strike of the regional foliation in the pre-existing thrust active pre-existing fold and shear zone structures in the sur- sheets. It is conceivable that the anomalous zone of NW–SE- rounding metamorphic rocks. Similar processes may account oriented foliation produced by F3 cross-folding in the hanging for the complex forms of many other plutons in equiva- wall of the Ben Blandy shear zone was much less favourably lent settings. More speculatively, the emplacement of the oriented for ESE-directed extension and that this zone formed Loch Loyal syenites may reflect the interaction of magma- a belt of resistance or ‘sticking’ (Fig. 9). This generated an tism and gravity-driven extension in the Moine Thrust small dextral shear couple which, combined with the effect sheet late in the Caledonian cycle. This example illustrates, of magma overpressure, allowed initial intrusion of syenites therefore, the opportunistic nature of pluton emplacement at into a series of en-echelon tensile dykes that reactivated intermediate crustal depths (cf. the models of Hutton 1997) appropriately oriented structural anisotropies in the country and emphasizes the important role played by country rock rocks (foliation planes and/or S3 surfaces) (Fig. 9). Some structures other than faults during granitoid intrusion. Such extensional opening may also have occurred along the under- emplacement processes are local solutions to the magmatic lying segment of the Ben Blandy shear zone to produce the space problem and may often obscure direct evidence for the early dip-slip components of extensional shear observed as- controls of magma siting and ascent at deeper levels. In the sociated with the NW contacts of the Ben Loyal pluton. Loch Loyal syenite example, crustal scale, steeply-dipping Small-scale top-to-the-SE detachment faults and associated shear zones buried beneath the Moine Nappe are tentatively brittle minor folds are locally developed in the Ben Blandy proposed as conduits for the ascent of sub-crustal alkaline shear zone rocks adjacent to the pluton, e.g. at [NC 573 500], magmas. 174 R. E. HOLDSWORTH ET AL.

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Received 19 February 1998; revised typescript accepted 3 July 1998. Scientiﬁc editing by Hugh Rollinson.