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 petrographi- cal 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 intru- sion 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.