Structural, Petrological, and Tectonic Constraints on the Loch Borralan and Loch Ailsh Alkaline Intrusions, Moine Thrust Zone, GEOSPHERE, V

Home , Ben More (Crianlarich), Ben More (Mull), Foinaven, Lewisian complex, Moine Supergroup, Moine Thrust Belt

Research Paper

GEOSPHERE Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, GEOSPHERE, v. 17, no. 4 northwestern Scotland https://doi.org/10.1130/GES02330.1 Robert Fox and Michael P. Searle 24 figures Department of Earth Sciences, Oxford University, South Parks Road, Oxford OX1 3AN, UK

CORRESPONDENCE: [email protected] ABSTRACT during ductile shearing. The minerals pseudomor- Grit Members of the An t-Sron Formation) and the phing leucites show signs of ductile deformation Ordovician Durness Group dolomites and lime- CITATION: Fox, R., and Searle, M.P., 2021, Structural, petrological, and tectonic constraints on the Loch Bor‑ During the Caledonian orogeny, the Moine indicating that high-temperature (~500 °C) defor- stones (Woodcock and Strachan, 2000; Strachan ralan and Loch Ailsh alkaline intrusions, Moine thrust thrust zone in northwestern Scotland (UK) mation acted upon pseudomorphed leucite crystals et al., 2010; British Geological Survey, 2007). zone, northwestern Scotland: Geosphere, v. 17, no. 4, emplaced Neoproterozoic Moine Supergroup rocks, that had previously undergone subsolidus break- The hinterland of the Caledonian orogenic p. 1126–1150, https://doi.org /10.1130 /GES02330.1 . metamorphosed during the Ordovician (Grampian) down. New detailed field mapping and structural wedge comprises schists of the Moine Super- and Silurian (Scandian) orogenic periods, westward and petrological observations are used to constrain group with structural inliers of Lewisian basement Science Editor: Andrea Hampel Associate Editor: Robert S. Hildebrand over the Laurentian passive margin in the north- the geological evolution of both the Loch Ailsh gneisses and intrusive Caledonian granites (Geikie, ern highlands of Scotland. The Laurentian margin and the Loch Borralan intrusions and the chronol- 1884; Dewey and Shackleton, 1984; Holdsworth et Received 13 August 2020 comprises Archean–Paleoproterozoic granulite and ogy of the Moine thrust zone. The data supports al., 2001, 2006; Strachan et al., 2002, 2010, 2020; Revision received 13 November 2020 amphibolite facies basement (Scourian and Lax- the interpretation that both syenite bodies were Mendum et al., 2009; Thigpen et al., 2013; Ashley Accepted 17 February 2021 fordian complexes, Lewisian gneiss), Proterozoic intruded immediately prior to thrusting along the et al., 2015; Mako et al., 2019). Two major orogenic sedimentary rocks (Stoer and Torridon Groups), and Moine, Ben More, and Borralan thrusts. events have been interpreted as (1) the Grampian Published online 14 May 2021 Cambrian–Ordovician passive-margin sediments. event, between 475 and 460 Ma, resulting from the Four major thrusts, the Moine, Ben More, Glencoul, collision of the Laurentian passive margin with an and Sole thrusts, are well exposed in the Assynt ■■ INTRODUCTION oceanic arc (Dewey and Strachan, 2003; Bird et window. Two highly alkaline syenite intrusions crop al., 2013), and (2) the Scandian event, between out within the Moine thrust zone in the southern The west-vergent Moine thrust zone of north- 435 and 415 Ma, with crustal thickening resulting Assynt window. The Loch Ailsh and Loch Borralan western Scotland (UK) (Fig. 1) marks the Caledonian from the continental collision of Laurentia with intrusions range from ultramafic melanite-biotite orogenic front along which Neoproterozoic dom- Baltica and Avalonia (Friend et al., 2000; Streule pyroxenite and pseudoleucite-bearing biotite inantly psammitic metamorphic rocks (Moine et al., 2010; Bird et al., 2013; Mako et al., 2019). nepheline syenite (borolanite) to alkali-feldspar– Supergroup, colloquially termed Moine schists) of Whereas Grampian-age metamorphic events are bearing and quartz-bearing syenites. Within the central and eastern Scotland were emplaced over evident from both the Northern Highland terrane thrust zone, syenites intrude up to the Ordovician the eastern Laurentian passive-margin foreland in northwest of the Great Glen fault and the Gram- Durness Group limestones and dolomites, forming western Scotland (Nicol, 1861; Lapworth, 1885a, pian terrane southeast of the Great Glen, younger a high-temperature contact metamorphic aure- 1885b; Peach et al., 1907; Strachan et al., 2002, 2020; Scandian ages are present only in the Northern ole with diopside-forsterite-phlogopite-brucite Mendum et al., 2009). The foreland comprises Highland terrane (Bird et al., 2013; Mako et al., 2019). marbles exposed at Ledbeg quarry. Controversy Archean to Paleoproterozoic basement gneisses The 50–60 m.y. span of metamorphic ages in the remains as to whether the Loch Ailsh and Loch (Lewisian complex) unconformably overlain by Northern Highland terrane may, however, represent Borralan syenites were intruded prior to thrusting Meso- to Neoproterozoic clastic sediments (Stoer, a continuum of crustal thickening, deformation, and or intruded syn- or post-thrusting. Borolanites con- Sleat, and Torridon Groups, collectively known metamorphism from arc accretion to continental tain large white leucite crystals pseudomorphed by as “Torridonian”). Both Lewisian and Torridonian collision, rather than two short-lived thermal events alkali feldspar, muscovite, and nepheline (pseudo- units are unconformably overlain by a Cambrian– (Oliver et al., 2000; Viete et al., 2013). leucite) that have been flattened and elongated Ordovician passive-margin sedimentary sequence Detailed mapping in the Northern Highlands This paper is published under the terms of the comprising the Cambrian Ardvreck Group (Eriboll of Scotland, combined with thermobarometry and CC‑BY-NC license. Michael Searle https://orcid.org/0000-0001-6904-6398 Formation quartzites, Fucoid Beds and Salterella geochronology, shows that four major east-dipping,

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west-vergent ductile shear zones and thrusts affect the metamorphic rocks (from structurally highest in the east to lowest in the west, the Skinsdale, Naver– Sgurr Beag, Ben Hope, and Moine thrusts). The highest-grade rocks in the upper part of the Naver thrust sheet are sillimanite-bearing migmatites above the Naver thrust, with pressure-tempera-

ture (P-T) conditions showing a westward structural N decrease in metamorphic grade (from sillimanite through kyanite, staurolite, garnet, to biotite grade) down section to the Moine thrust (Thigpen et al., 2010a, 2010b, 2013; Ashley et al., 2015; Mako et al., 2019). P-T data show an inverted metamorphic profile from ~700 °C and 8–9 kbar in the hanging wall of the Naver thrust (Ashley et al., 2015; Mako et al., Figure 1. Geological map of north- 2019), down to 450 °C and 5 kbar in the hanging wall western Scotland, modified after of the Moine thrust (Thigpen et al., 2013; Ashley et Peach et al. (1907), Krabbendam et al., 2015). This inverted metamorphic sequence has al. (2008), and Thigpen et al. (2010a, some similarities to the Main Central thrust zone 2010b). in the Himalaya (Searle, 2015; Searle et al., 2008). Figure 2 Originally thought to be Neoproterozoic, the age of metamorphism is now known to be latest Gram- pian or Scandian (445–420 Ma; Mako et al., 2019). The full age range of ages of Moine metamorphism, from U-Pb zircon, monazite, and titanite dating and Lu-Hf and Sm-Nd garnet ages, reviewed by Mako et al. (2019), is 465–415 Ma. It seems likely that this represents one evolving metamorphic sequence lasting ~50 m.y., similar in range to Himalayan metamorphism, rather than separate events. Granites in the Scottish Highlands have a range of U-Pb ages from Neoproterozoic to Silurian (Oli- ver et al., 2008), but the majority of calc-alkaline granites have U-Pb zircon ages spanning 430– 415 Ma, constraining the age of the Scandian event (Kinny et al., 2003; Alsop et al., 2010). These gran- a continuous thickening, regional metamorphism, The Moine thrust zone is well exposed from Loch ites are widely thought to have been generated and deformation evolving from the Grampian Eriboll on Scotland’s northern coast south to the Isle above a northwest-directed subduction system through to the Scandian “events” in the Northern of Skye (Nicol, 1861; Lapworth, 1883–1884, 1885; (e.g., Dewey and Shackleton, 1984; Strachan et al., Highlands. In the Himalaya, the largest Phanero- Peach and Horne, 1884; Peach et al., 1892, 1907; 2002, 2010), which would make the Scandian oro- zoic continental collision belt known on Earth, the Elliott and Johnson, 1980; Coward, 1983, 1984; But- genic wedge a back-thrust belt. However, Searle et back-thrust zone is only 15–20 km wide and charac- ler, 1982, 1987, 2010b; Holdsworth et al., 2007; Law et al. (2019) noted that the Scandian orogenic wedge terized by steep upright folds and thrusts along the al., 2010; Searle et al., 2010, 2019). The thrust zone is and the Moine thrust zone represent a classic fold- suture zone and north Indian margin (Searle, 2015), widest at the Assynt culmination (Fig. 1), and since thrust belt tapering to the west, with a hinterland whereas in Scotland, the observed orogenic wedge the initial detailed mapping of the northwestern structure and metamorphism similar to those of is 100–140 km across, probably extending further Highlands by Peach et al. (1892, 1907), the Assynt the Himalaya. They proposed a southeast-dipping east beneath the Orcadian post-orogenic basin, and window has been renowned for the excellent, com- subduction zone along the Iapetus suture zone, with has a typical foreland-tapering wedge geometry. plete exposures across this classic fold-thrust belt

(Elliott and Johnson, 1980; Coward, 1983, 1984; But- describe the contact metamorphic aureole, and dis- to youngest, and structurally highest to lowest). ler, 1982, 1987, 2010a, 2010b; Law et al., 2010; British cuss the structural relationships of the intrusions These major thrusts have transported allochthonous Geological Survey, 2007). This paper describes the to the thrust structures across the Assynt window. Cambrian–Ordovician rocks from the hinterland structure of the southern part of the Assynt window (east) onto the autochthonous Lewisian basement, around two large alkaline syenite intrusions of Loch Torridonian clastic sediments, and unconform- Ailsh and Borralan (Horne and Teall, 1895; Shand, ■■ STRUCTURE OF THE ASSYNT WINDOW ably overlying Cambrian–Ordovician sedimentary 1910, 1939; Phemister, 1926; Sabine, 1953; Parsons, sequence in the foreland (west). All of these rock 1965a, 1965b; Woolley, 1970, 1973; Goodenough et The Assynt window exposes three or four major units are also exposed in allochthonous thrust al., 2004, 2011; Searle et al., 2010). We describe the west-vergent thrust sheets, including the Moine, sheets within the Moine thrust zone. Figure 2 shows field relationships and petrology of both intrusions, Ben More, Glencoul, and Sole thrust sheets (oldest a geological map of the southern Assynt window

Assynt Klippen B H’blende microdiorite Sole North-West thrust Porphyritic trachyte Highlands minor Loyne Mass Rhyolite intrusions ?

λ λ Canisp porphyry sills

λ Four Burns HMD Mylonite

C λ λ Quartz-syenite Background Topo Loch Ailsh λ λ C intrusion begins am L λ Background Topo λ Ledbeg Alkali f ’spar-syenite och K quarry A lippen Ledmorite Loch Borralan Borolanite complex

26 28

Loch Borralan Intrusion Carbonatite C’ Upper Durness Bad na Achlaise λ Durness limestone B’ An T-Sron formation Parsons and McKirdy (1983) Pipe Rock Carbonatite Allt nan Assynt Excavation site Streach 30 Ben More thrust Basal quartzite window stream Loch Urigill stratigraphy Moine Mylonite Torridonian Aultivullin quarry sandstones

© Crown Copyright and Database Right 2015. Ordnance Survey (Digimap Licence). FOR EDUCATIONAL USE ONLY. quarry Scale 1:10000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 km Lewisian basement N Moine Moine Supergroup thrust

Thrust fault

Scale 1:10000 Jan 16, 2015 14:21 Robert Fox Oxford 1 km 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 km A’ Strike slip fault Moine Syncline structure axis mylonites Contour Interval: 10m Anticline structure axis

Figure 2. Geological map of the Moine thrust zone in the Loch Borralan region, southern Assynt window, northwestern Scotland (see Fig. 1 for location). Cross-sections A-A′, B-B′, and C-C′ are shown in Figures 5, 6, and 3, respectively. Base map is © Crown copyright and database rights 2020 Ordnance Survey, UK.

in the Loch Borralan region, with a cross-section of opening-angle thermometry estimates deformation with late-stage out-of-sequence motion along the the Loch Borralan profile and a panorama shown temperatures exceeding 500 °C (Law et al., 1984, uppermost parts. The Moine thrust wraps around in Figures 3 and 4. Another cross-section of the 1986, 2010; Thigpen et al., 2010a, 2010b) during the large-scale Assynt window culmination so that Loch Urigill profile immediately south of Loch Bor- ductile deformation in the Moine thrust mylonites. it rests directly on the foreland Durness limestones ralan is shown in Figure 5, and a lateral section The precise location of the Moine thrust itself has in the south at Knockan Crag. in Figure 6 shows the three-dimensional structural been critically debated, with some authors placing In Assynt, the Moine thrust largely places relationships. it along the top of the mylonites, some mapping hanging-wall mylonites on footwall rocks that are The structurally highest, and therefore oldest, it dividing Moine-derived mylonites above from at anchizone conditions, suggesting that at the thrust within the Moine thrust zone is the ductile foreland-derived mylonites below (Law, 1987, 2014; present-day level of exposure, there is a brittle fault, Moine thrust, which is associated with mylonites Law et al., 1986), and others placing it along the particularly in southern Assynt, from Knockan Crag derived from the foreland succession below the base of the mylonite sequence (Johnson, 1965, to Loch Ailsh. Footwall mylonites are barely recrys- thrust (quartz mylonites, calc-mylonites “oyster- 1967). In reality, the Moine thrust is a large-scale tallized by grain-boundary bulging, suggesting shell rocks”; Lewisian-derived mylonites) as well as thrust fault, diachronous in space and time, that deformation conditions of little more than 300 °C, the overlying Moine Supergroup psammitic schists shows the full exposure from early, deep, ductile whereas in the hanging wall, deformation tempera- (White, 2010; White et al., 1982). Quartz c-axis fabric mylonites up to a later, shallow, brittle thrust fault, ture was 500–575 °C, and thus a large thermal break

CAM LOCH KLIPPEN CAM LOCH A - A’ LOCH MOINE ALLT NAN MOINE THRUST AILSH MYLONITES STREACH STREAM WNW [2180 1357] ESE C MOINE THRUST [3314 1107] C’ 800 BEN MORE-CAM LOCH THRUST 700 800 600 BORRALAN Ledbeg quarry 700 500 CAM LOCH THRUST and Pre-Borralan thrust 600 400 SOLE THRUSTTHRUST Four Burns 500 300 400 200 300 100 200 0 MOINE THRUST 100

Elevation (m) Elevation -100 0

-200 -100 (m) Elevation -300 LOCH BORRALAN -200 -400 SYENITE COMPLEX -300 -400 BEN MORE THRUST ?

Equal horizontal and vertical Scale SOLE THRUST 1 km BORRALAN THRUST

Approximate thermal aureole

Coordinate reference system: EPSG: 32630 WGS84 / UTM 30N

Legend

Figure 3. Cross-section of the Borralan region, C-C′; line of section is shown on Figure 2. Coordinate system: British Geological Survey (2007).

Meall Diamhain Cnoc an Stroine Benn Fhuarain Loyne Mass Ledbeg quarry Ledmore Loch Borralan Loch Awe Quartz syenite Ben More Thrust Loch Urigill

Alkali feldspar syenite

Cromaltite ultrama c Ledmorite syenite

Figure 4. Photo panorama across the Moine thrust zone; view is to the southeast toward the Borralan massif from Knockan. Borralan intrusion is approximately 5 km in length.

CAM LEDMORE LOCH LOCH RIVER B - B’ URIGILL

NW SE [221 125] [304 075] A A’ m m 800 MOINE THRUST 800 700 700 600 600 CAM LOCH THRUST 500 500 400 400 SOLE THRUST Carbonatite 300 MYLONITES 300 200 200 100 CAM LOCH 100

0 KLIPPEN ? 0 (m) Elevation Elevation (m) Elevation -100 MOINE THRUST -100 -200 -200 Simpli ed-multiple thrusts in upper -300 -300 limestone poorly exposed -400 SOLE THRUST -400

Multiple Imbricates of Durness Group Equal Horizontal and Vertical Scale 1 km Coordinate reference system: EPSG: 32630 WGS84 / UTM 30N Legend For line location, colours and legend, please refer to gure 2.

Figure 5. Cross section across the Moine thrust zone through Loch Urigill, south of the Borralan complex, A-A′; line of section is shown on Figure 2. Coordinate system: British Geo- logical Survey (2007).

CAM LOCH LOCH LEDMORE LOYNE SYENITE ASSYNT KLIPPEN URIGILL KLIPPEN B - B’ C - C’ RIVER BODY (BEN MORE THRUST)

SOUTH NORTH B B’ MOINE THRUST [2315 1050] [2516 1511] m m Cam Loch normal faults-late 800 800 reactivation of Borralan lateral Pre-Borralan 700 700 ramp (reverse faults) BEN MORE THRUST thrust 600 600 500 BEN MORE - CAM LOCH 500 Ledbeg Quarry L 400 THRUST 400 T 300 300 200 200 ? 100 100 BaQ ? 0 L ? 0 LOCH BORRALAN -100 (m) Elevation

Elevation (m) Elevation -100 ? ? SYENITE COMPLEX -200 -200 Multiple Durness -300 -300 ? Group imbricates -400 -400 Multiple Durness BORRALAN THRUST Group imbricates Parsons and McKirdy (1983) SOLE THRUST Borralan lateral Excavation site Pre-Borralan ramp thrust Cambrian-Ordovician imbricates above Sole Thrust (see g. 35) Thermal aureole

Legend Equal Horizontal and Vertical Scale 1 Km

Coordinate reference system: EPSG: 32630 WGS84 / UTM 30N

BaQ = Basal quartzite L = Lewisian basement

Figure 6. Lateral (north-south) section near the western end of the Borralan complex, B-B′; line of section is shown on Figure 2. Coordinate system: British Geological Survey (2007).

was produced by major slip along a brittle thrust overlying Torridonian and Cambrian–Ordovician thrust sheets (Elliott and Johnson, 1980; Butler fault (Thigpen et al., 2010a, 2010b). These condi- sedimentary rocks; these include the Ben More and Coward, 1984; Searle et al., 2010). In south- tions transition to the north at the Stack of Glencoul thrust in central and southern Assynt and the ern Assynt, the overall “piggy-back” sequence of and in northern Assynt, where the footwall mylon- Glencoul thrust in northern Assynt. The structur- thrusting has been complicated by late out-of-se- ites have a greater thickness and the thermal break ally lowest and latest thrust is the Sole thrust, which quence motion along the Moine thrust (Butler and between the footwall and hanging wall is reduced commonly carries numerous imbricate thrust slices Coward, 1984; Searle et al., 2010). Coward (1985) (100–150 °C), but the break between the footwall of Cambrian–Ordovician sediments (Coward, 1985). estimated a total shortening across the southern mylonites and the underlying brittle Assynt rocks is Several klippen of Ben More thrust sheet rocks Assynt window of at least 54 km. quite large (Law et al., 1986; Law, 1987, 2014; Thig- lie above the Sole thrust sheet (e.g., Cam Loch A number of minor intrusions composed of a pen et al., 2010a, 2010b, 2013). klippe). Thrusts developed in sequence from hin- wide variety of alkaline igneous rocks has been Structurally beneath the Moine thrust, several terland to foreland, with early thrust sheets riding mapped across the Assynt window (Goodenough thrusts exhume Lewisian basement rocks and “piggy-back” (i.e., passively) on subsequent lower et al., 2004, 2006; Searle et al., 2010; Wood, 2015).

We now describe these intrusions before describing of Borralan syenite (430.7 ± 0.5 Ma) as indicating Goodenough et al., 2004, 2011). It has a mean the petrology and microstructure of the Loch Ailsh emplacement before or during thrusting, and the 206Pb/238U age of 430.4 ± 0.4 Ma (Goodenough et and Loch Borralan rocks as well as the contact met- late suite of Borralan (429.2 ± 0.5 Ma) as post-dating al., 2011). An abundant suite of hornblende-phyric amorphic aureole in detail. The field relations and thrusting. Searle et al. (2010) showed that the flat- lamprophyres (“vogesites”; Horne and Teall, 1895; fabrics in these intrusions are important because tened pseudoleucites within the late Borralan suite Teall, 1900; Shand, 1910, 1913, 1939) and microdio- they can be used for determining whether they were deformed by high-temperature ductile shear- rites intrude the Cambrian–Ordovician sedimentary were intruded before or after thrusting in the Moine ing, and therefore the crystallization ages gave a rocks in the Sole thrust sheet but only rarely occur thrust zone. maximum age constraint on the timing of motion in the foreland (British Geological Survey, 2007). along the Moine, Ben More, and Borralan thrusts. Similarly, a suite of peralkaline rhyolite (“grorudite” The relationship between these petrologically com- of Sabine [1953]) sills and dikes intrude the Moine ■■ ASSYNT ALKALI INTRUSIVES plex alkaline intrusions and thrust movements can thrust zone, mainly at higher structural levels, but be used to constrain the magnitude, timing, and are clearly pre-thrusting given that they are affected A range of alkaline intrusive rocks occur sequence of thrusting in the Assynt window, given by folding and imbrication in the thrust zone. throughout the region, intruding the foreland suc- restored cross-sections and radiometric dating. Finally, a suite of porphyritic quartz microsyenites cession, within the Moine thrust zone, and less Two other large syenite intrusions have been (“nordmarkites” of Sabine [1953]) intrude immedi- commonly in the hinterland schists of the Moine mapped intruding Moine Supergroup schists in ately beneath the Moine thrust. Sabine (1953), Law Supergroup. Two syenite intrusions, the Loch Bor- the hinterland above the Moine thrust. The Glen et al. (1986) and Law (1998) reported that one such ralan complex and Loch Ailsh complex, crop out Dessary pluton east of Knoydart (445.3 ± 1.9 Ma; dike intruded across the Moine-derived mylonites within the thrust zone in northwestern Scotland. Goodenough et al., 2011) intrudes the Sgurr Beag and quartz mylonites at the Allt nan Sleach stream, Both contain the full range of alkaline igneous rocks thrust sheet in the eastern part of the Moine Super- east of Loch Ailsh (Fig. 2). This dike definitively cuts from ultramafic pyroxenites to melanite garnet + group, and the Loch Loyal syenites (with one the mylonite fabrics (Law et al., 1986), but it is not pseudoleucite–bearing syenites to alkali feldspar concordant zircon grain ca. 425 Ma; Goodenough clear from the poor exposure in the locality whether (± quartz) syenites (Shand, 1910, 1939; Phemister, et al., 2011) intrude the Moine Supergroup schist it intrudes higher up across the mylonites into the 1926; Parsons, 1965a, 1965b, 1968, 1972). Dates from ~15 km east of the Moine thrust in the far north of non-mylonitized Moine Supergroup above. Most of the Loch Ailsh syenite from 206Pb/238U zircon ages Sutherland (Fig. 1). The Loch Loyal syenite intru- this area is now covered by extensive forestry land are 430.6 ± 0.3 Ma, and for the Borralan early suite sion post-dates regional Scandian metamorphism and is poorly exposed. Elsewhere along the Moine are 429.2 ± 0.5 Ma (Goodenough et al., 2011). The and ductile folds and fabrics (deformation events thrust zone, there are no such intrusions mapped origin, emplacement mechanism, and structural D2, D3; Holdsworth et al., 1999, 2001), so is clearly except for the large Loch Loyal syenite 15 km east relationships with the surrounding rocks of the post–Scandian metamorphism at this locality. Ages of the Moine thrust at Loch Eriboll (Fig. 1). Loch Ailsh and Loch Borralan intrusions are con- of prograde garnet growth from Moine rocks in the troversial. Debate remains as to whether they were Naver thrust sheet from Lu-Hf and Sm-Nd dating intruded (1) pre-thrusting (Coward, 1985; Searle et range from ca. 475 to 460 Ma, and younger ages ■■ BORRALAN INTRUSION al., 2010), or (2) syn- or post-thrusting with intru- of 450 Ma were determined from the structurally sive margins cutting thrust sheets (Woolley, 1970; lower Moine thrust sheet (Bird et al., 2013). U-Pb The Loch Borralan intrusion, occupying an Parsons, 1965a, 1965b, 1968; Johnson and Parsons monazite geochronology suggests that most of the area of ~26 km2 in the southern part of the Assynt 1979). Parsons and McKirdy (1983) concluded from deformation and metamorphism was younger than window, is the only large, silica-undersaturated field relationships and a shallow borehole drilled 425 Ma (Mako et al., 2019; Spencer et al., 2021). (feldspathoid-bearing) pluton in the British Isles along the southwestern margin flank of the Bor- A range of alkali dikes and sills intrude both and contains the full spectrum of alkaline ultramafic ralan syenite at Bad na Achlaise that the syenite the foreland succession and the Moine thrust zone to quartz-bearing syenites. It was first described by intruded across the thrust sheets and therefore the (Goodenough et al., 2004, 2011; British Geological Horne and Teall (1895) and Teall (1900) and mapped age of syenite might constrain a minimum age of Survey, 2007). The Canisp porphyry (red feld- by Peach et al. (1907). Shand (1910, 1939) and shearing. Searle et al. (2010) interpreted these struc- spar-quartz microsyenite) is part of a series of sills Woolley (1970, 1973) made detailed petrological tural relationships as intrusive but lying above a up to 50 m thick that intrude the Torridonian and descriptions and gave local names to many of the lower thrust, the Borralan thrust, that transported Cambrian–Ordovician sedimentary rocks in the sta- distinctive rock types, such as “borolanite” (nephe- the whole complex westward above the Sole thrust ble foreland below the Sole thrust in Assynt. It does line syenite and pseudoleucite-bearing syenite), sheet. Goodenough et al. (2011) also interpreted the not crop out above the Sole thrust and is thought “cromaltite” (biotite–melanite garnet pyroxenite), ages of the Loch Ailsh pluton and the early suite to pre-date thrusting (Sabine, 1953; Parsons, 1999; “ledmorite” (garnet–aegirine augite–nepheline

syenite), and “perthosite” (K-feldspar syenite), which are used below for convenience. The most- Quartzolite Q evolved quartz syenites, containing as much as 12% quartz, intrude the older suites and form most of the summit region of Cnoc an Stroine, the highest ridge of Borralan (Fig. 4). The late Borralan suite members are mineralogically and composi- Quartz-rich Granodiorite granitoids tionally related to the peralkaline rhyolite dikes of Tonalite the Assynt minor intrusive rocks (Goodeneough Alkali feldspar et al., 2004; Wood, 2015). Shand (1910, 1939) and Quartz monzodiorite granite Phemister (1926) suggested that the Borralan com- Quartz monzogabbro plex was a stratified laccolithic intrusion, with the most fractionated, youngest magmatic unit, the Monzo- Quartz diorite Quartz alkali Syeno- quartz-bearing syenites, forming the highest struc- granite granite Quartz gabbro feldspar syenite Quartz anorthosite tural position. Loch Borralan Quartz Quartz Monzodiorite Compositions of samples from the Loch Bor- Alkali Late Suite syenite monzonite monzogabbro ralan and Loch Ailsh alkaline complexes plotted on feldspar syenite Monzonite a quartz–alkali feldspar–plagioclase–feldspathoid syenite Foid-bearing Foid-bearing Ailsh A P (QAPF) diagram (Fig. 7) show a fractionation trend syenite monzonite Diorite Leucosyenites from mafic rocks toward quartz-bearing syenites. gabbro anorthosite The Borralan early suite includes the ultramafic Foid-bearing Foid Foid alkali feldspar Foid-bearing diorite biotite pyroxenites (cromaltites), which grade into monzosyenite monzodiorite syenite Foid-bearing gabbro feldspathoid-bearing quartz-undersaturated units, Loch Borralan Ledmorite Foid-bearing anorthosite aegirine augite + nepheline syenites (ledmorites), Early Suite and biotite + K-feldspar + nepheline pseudoleucite Foid-bearing monzodiorite syenites (borolanites) that crop out in the eastern Foid syenite Foid-bearing monzogabbro part of the massif (Fig. 2). The Borralan late suite forms a plug or sheet of alkali-feldspar syenites (perthosites) and quartz syenites that form the main hill Borolanite Foidolite Foid diorite at Cnoc san Sroine (Fig. 2). An isolated outcrop of Foid gabbro borolanite, named the Loyne mass, occurs to the northwest of Loch Borralan and appears to be a small thrust-bound klippen structurally above the Sole thrust sheet (Searle et al., 2010). F Figure 7. Compositions of samples from the Assynt alkaline intrusions, plotted on a quartz–alkali feldspar–pla- Ultramafic Pyroxenites (Cromaltites) gioclase–feldspathoid (QAPF) diagram. The range of composition within the Loch Ailsh syenite shows fractionation toward more quartz-rich compositions. The early and late suites of the Borralan syenites are compositionally different. Black arrows show fractionation trends. Several small bodies of ultramafic rock are exposed around the southwestern margin of the Borralan complex (Fig. 2; Woolley, 1970, 1973; Mat- thews and Woolley, 1977). These unusual rocks Melanite is a titanium-rich dark-colored garnet compositions from the borolanites, suggesting they 3+ consist of varying proportions of clinopyroxene, (Ca3[Fe ,Ti]2Si3O22) characteristic of alkaline ultra- were both derived from a common parental magma melanite garnet, amphibole, biotite, and magnetite, mafic rocks. Garnets from the Borralan ultramafic (Fox, 2015). The ultramafic cromaltites represent

with almost no feldspar. Biotite pyroxenites (jacu- rocks contain as much as 7% TiO2. Elemental con- the homogeneous cumulus phase, and the boro- pirangites) contain late poikilitic biotites and have centrations of Ca, Fe, Ti, and Mn in the ultramafic lanites are fractioned from these parent magmas. a cumulus origin (Matthews and Woolley, 1977). cromaltite garnets are almost identical to garnet Pyroxenes from the cromaltites are euhedral and

define a cumulus texture, with strong flow banding textures and a shape preferred orientation (SPO) parallel to the contact, which has a strike and dip of Figure 8. Variations in pseudoleucite 101°/45°S. Pyroxenes exhibit an exsolution texture, morphology in borolanites: Equant where diopsides host blebs of exsolved calcium-rich and undeformed pseudoleucite crystals (A) from the Four Burns location, hedenbergite, a consequence of a steep tempera- northeastern Borralan complex, and ture gradient and fast cooling rate (Fox, 2015). flattened and stretched, deformed Numerous shallow (10–40 m) boreholes were pseudoleucites from the Aultivuillin drilled through the ultramafic margin of the Bor- quarry (B). ralan complex for exploration for platinum-group elements (Styles et al., 2004). Gravity and magnetic anomalies suggest that the complex is a shallow body <400 m thick and that the pyroxenites extend

further west than the mapped margin (British Geo- (hence pseudoleucite). Leucite (KAlSi2O6) is a sili- and commonly have large K-feldspar cores and logical Survey, 2007), defining a structural fabric cate member of the feldspathoid group that has a regions of recrystallized nepheline. At the Altivuilin dipping 40°–60°NE (Parsons, 1965b; Styles et al., tetragonal habit (Deer et al., 1966). Leucite is a rel- quarry (Fig. 9), the pseudoleucites have been flat- 2004). Both gradational and intrusive contacts atively rare mineral restricted to low-Si, high-alkali tened and elongated into elliptical streaks that have between the ultramafic and syenitic rocks are parental magmas and forms only under low-pres- a fabric (striking 030°, dipping 15°–20°E) parallel to inferred from the borehole data (Styles et al., 2004). sure, high-temperature conditions. Variations of the Moine thrust. The three-dimensional structure Along the margin of the intrusion, small slivers of textures in borolanites are shown in Figure 8, where of the borolanite is shown in Figure 10, where the serpentinites and phlogopite + brucite marbles are undeformed leucite crystals pseudomorphed to ductile fabric (in blue) consists of aligned mafic present in a few localities. K-feldspar and nepheline show magmatic fabrics minerals, a SPO in biotite, and flattened pseudo- (Fig. 8A) and flattened pseudoleucites from the Aul- leucite ellipsoids; a microfracture network (in red) tivullin quarry show a tectonic overprint (Fig. 8B). is oblique to the shear fabric. The Y-Z plane is cut Early Mafic Undersaturated Syenites Garnets from the pseudoleucite borolanites vertically on a strike of 330°, and thus the fabric is (Pseudoleucite-Melanite Borolanites) are zoned with a slight increase in Fe from core to collinear with the direction of transport on thrusts rim and a concomitant decrease in Ca, Ti, and Mn. in the Assynt window. In thin section (Fig. 11), two Borolanite is a syenite composed of the minerals Pseudoleucite symplectites consist of K-feldspar, distinct microstructural fabrics can be identified: melanite garnet + pyroxene + nepheline containing nepheline, cancrinite, analcite, and muscovite and a major ductile fabric consisting of pseudoleucite large white leucite crystals that are pseudomor- appear chaotic in texture. Symplectites were pro- ellipsoids and biotite, which show a common phed by alkali feldspar, muscovite, and nepheline duced from the subsolidus breakdown of leucite SPO; and a minor, late brittle overprint, which is

Analcite Alkali N feldspar Figure 9. Photo panorama of the disused Aultivuillin quarry (NC 2868 0966, coordinate reference system: EPSG: 32630 WGS84 / UTM 30N), view looking north, Pseudoleucite Biotite showing the orientation of the pseudo- lineations 5cm leucite lineations (red arrows) and the outline of the undeformed pegma- Undeformed Aegerine tites (dashed white line). Inset shows pegmatite the best exposure of the undeformed pegmatitic borolanite, with crystals as Nepheline much as 7 cm in length. Coordinate sys- Pegmatite tem: British Geological Survey (2007). Borolanite sample mineralogy EBSD—electron backscatter diffraction. (EBSD ) 5m photograph Cancrinite

Plane polarized Microstructure fabric Cross polarized light (PPL) interpretation light (XPL) Fractures and microcracks

Y X

Shear Fabric 37 mm Lineation

35 mm

20 mm N 330o NORTH Y X 30 Oriented Sample Y-Plane LINEATION DIRECTION W E Pole to dedding in thrust sheets (Elliot and Johnson, 1980) Z

Borolanite lineations

150o S Figure 10. Thin sections of borolanite cut in three perpendicular directions. Borolanite EBSD sample taken from Aultivullin quarry, from the deformed area highlighted by the blue box in Figure 9. 3D thin section reconstructions from left to right are as follows: plane-polarized light (PPL), microstructure fabric interpretation, cross-polarized light (XPL). On the microstructure fabric interpretation, blue dots indicate intersection of lineation with plane of thin section. Lower hemisphere equal-area projection of Borolanite lineations (black) and pole to bedding of Assynt window area thrust sheets after Elliot and Johnson (1980). The Y-Z plane is oriented parallel to the pseudoleucite ellipsoid stretching lineation. The ductile fabric (blue) is defined by aligned mafic minerals, shape preferred orientation in biotite, and flattened pseudoleucites. The Y-Z plane is cut vertically on a strike of 330° and is colinear with the direction of transport of thrusts in Assynt. Red lines are a late fracture network oblique to the shear fabric.

defined by sheared and cracked melanite garnets eutectic, points, which are all pressure dependent. 1–2 kbar and that the breakdown of leucite occurred and alkali feldspar. The pseudoleucite symplectites At pressures of 1 or 2 kbar, a period of cotectic crys- at ~550–600 °C at pressures of 1–2 kbar. (K-feldspar, nepheline, cancrinite, analcite, musco- tallization of K-feldspar and nepheline would occur The flattened pseudoleucite fabrics have been vite) after leucite appear as vermicular and chaotic during cooling toward the eutectic, where nepheline thought to be magmatic (Parsons, 1965a, 1965b, intergrowths (Fig. 12). and feldspars would then crystallize at fixed compo- 1972, 1999; Parsons and McKirdy, 1983; Halliday et

Ternary (Fig. 13) and binary diagrams (Fig. 14) sitions. Pure leucite (KAlSi2O6) would break down to al., 1987; Goodenough et al., 2011) but were inter-

show that leucite was the first mineral to crystal- kalsilite (KAlSiO4), the potassic equivalent of nephe- preted by Searle et al. (2010) as high-temperature

lize in the Nepheline-Kalsilite-SiO2 system. The next line (NaAlSiO4), and to K-feldspar if cooled in the ductile fabrics associated with shearing along the mineral to crystallize would have depended on the presence of water. We suggest that the P-T condi- Moine thrust after intrusion. Sodium-rich leucite temperature contours and peritectic, cotectic and tions of borolanite crystallization were ~1100 °C and breaks down to nepheline + K-feldspar at subsolidus

40 mm

25 mm Microstructure fabric interpretation Microstructure fabric overlay Plane polarized light (PPL)

330° Oriented Sample Plane

Sample orientation 330o 150o Borolanite W Lineations E True Vertical

True Horizontal

150° S Figure 11. Oriented sample of Aultivullin quarry sample (see Fig. 9 for location; blue box indicates location) pseudoleucite-bearing Borolanite cut in the plane: 330°–150° (± 2o) parallel to the orientation of maximum elongation of pseudoleucites. The stereonet and the stereonet cross section show the orientation of the thin section below. Red lines show the ductile shear fabric imposed after the subsolidus breakdown of leucite to nepheline + K-feldspar + muscovite. Yellow lines are late, minor brittle microfractures. Green oval related to another figure not included here.

temperatures, so the flattened and elongated pseudoleucite underwent breakdown and pseudomorph- fabrics suggest that ductile shearing along the leucites at Borralan must have been deformed at ing to K-feldspar and nepheline at temperatures of Moine thrust zone was contemporaneous with the relatively high temperature (~500–550 °C) during ~650–550 °C, followed by nepheline breakdown to end of alkaline magmatism (Searle et al., 2010). Late late tectonic deformation. Figure 15 shows a crys- analcite, cancrinite, and mica. The original tetragonal nepheline syenite pegmatites cut across deformed tallization and deformation history of the leucite (pseudo-cubic) leucite crystals were then subjected pseudoleucite-bearing syenites and are seen in the within the borolanite, as deduced from scanning to high ductile strain during thrusting along the Altivuilin quarry, but all magmatic phases, includ- electron microscope and electron backscatter diffrac- Moine and Ben More thrusts and flattened and ing the late undeformed pegmatites of the Borralan tion analyses (Fox, 2015). The highest-temperature elongated in the transport direction. This confirms complex, are cut by thrusts above (Ben More thrust) minerals to crystallize were melanite garnet and that the pseudoleucites were affected by deforma- and below (Borralan thrust; Searle et al., 2010) and leucite at temperatures of ~1100 °C. During cooling, tion after crystallization of the borolanite. These do not cut across either thrust.

the Ben More thrust cutting through the syenites Nepheline 200 µm 20 µm Apatite (Parsons, 1972). Deformation affects perthitic intergrowths of plagioclase with K-feldspar that imply K-feldspar high-temperature shearing. The mylonite horizons Cancrinite at the upper structural levels of the quartz syenites Analcite are roughly parallel to the orientation of the mylo- Analcite nites along the Moine thrust and also parallel with the foliated pseudoleucites in the borolanites. All the structural and petrographic evidence points Cancrinite K-feldspar to ductile shear fabrics being superimposed onto Nepheline K-feldspar 50 µm core already crystallized syenites that were still at temperatures of 500–400 °C. Kfs - Ne - Anc - Can - Ms Muscovite symplectite Analcite Loch Urigill Carbonatite

Analcite A single occurrence of carbonatite along the southwestern margin of the Borralan complex on Cancrinite the northern shore of Loch Urigill (Fig. 2) lies ~50– K-feldspar 50 µm 100 m from the ultramafic pyroxenites, adjacent Cancrinite to the main body of syenite (Young et al., 1994). Sharp intrusive contacts and chilled margins Muscovite indicate that the carbonatite intruded into unmet- Recrystallized nepheline amorphosed or slightly altered dolomite (Fig. 16). Barite Kfs = K-feldspar The carbonatite contains xenoliths of pyroxenite, Ne = Nepheline nepheline syenite, and dolomitic limestone. Young Analcite Anc = Analcite Muscovite Can = Cancrinite et al. (1994) described three types of carbonatite: a Apatite Ms = Muscovite porphyritic sovite, a phlogopite-apatite sovite, and a silica carbonatite, with a marginal sovite brec- Figure 12. Scanning electron microscope images of the pseudoleucite symplectite from borolanite cia containing clasts of brown phlogopite, apatite sampled from the Aultivuilin quarry, showing a large K-feldspar core with tangled, chaotic symplectite rims. Symplectites are composed of K-feldspar, nepheline, cancrinite, analcite, and muscovite. En- rosettes, and serpentine pseudomorphs after oliv- larged images (red, blue, yellow boxes) show the detailed structures formed during leucite breakdown. ine. Calcite makes up ~95% of the carbonatite and

is nearly pure CaCo3 with <1 wt% MgCO3 and as

much as 2 wt% SrCO3, suggesting an igneous origin (Young et al., 1994). High Sr, Rb, and Ba contents, Late K-Feldspar–Quartz Syenites structurally upwards. Parsons (1972) proposed that rare earth element patterns, and C and O isotopes the Borralan leucosyenites could not have been are typical of continental magmatism and are not Feldspathic and quartz syenites that are largely derived by fractional crystallization from a single compatible with a metasomatic assimilation of undeformed form the structurally highest and magmatic body, and instead proposed the possi- the country rock, the Durness limestone-dolomite youngest units of the Borralan complex. Perthosites bility of a composite intrusion. He stressed that the (Shand, 1910). Carbonatites are commonly associ- are composed mainly of alkali feldspar with minor Borralan syenites were also chemically and petro- ated with alkaline igneous complexes (Le Bas, 1987), melanite garnet. The most fractionated quartz graphically different from the Loch Ailsh and Loch but their origins remain obscure. They could be syenites (nordmarkites) have minor clinopyrox- Loyal syenites, although undoubtedly they were all differentiated products of a carbonated silicate liq- ene (aegirine augite), amphibole (kaersutite), and broadly contemporaneous. uid or related to carbonate-silicate immiscibility. melanite garnet in addition to alkali feldspar and There is evidence of cataclasis in many of the It is possible that the carbonatite at Loch Urigill as much as 12% quartz. The leucosyenites on Cnoc- quartz syenites along the upper horizons of the Bor- is somehow related to the metamorphic aureole na-Sroine become more albitic and quartz-rich ralan pluton, with mylonite horizons associated with around the western margin of the Borralan complex

SiO2 SiO2 Cristobalite 10 90 10 90 1 atm = 0.001 kbar 1 Kbar

20 80 20 80 Tridymite 1300 Quartz 30 70 30 70 1100

40 1060 60 40 60 1040 M Sanidine 870 50 (Na, K) 50 50 50 NaAlSi3O8 NaAlSi3O8 950 Feldspar 865 890 (Albite) KAlSi3O8 (Albite) KAlSi3O8 1060 40 60 890 40 60 1100 (Orthoclase) 880 865 850 (Orthoclase) 1500 840 Figure 13. Quartz-nepheline-kalsilite ter- 1020 1300 800 750 nary diagram at a range of pressures: 70 1100 30 70 30 1100 Leucite 0.001 kbar (1 atm), 1 kbar (after Schairer, Leucite 850 Leucite Leucite 1950; Scarfe et al., 1966), 2 kbar (Ham- 20 80 80 M 890 20 ilton and McKenzie, 1965), and 5 kbar (Roux and Hamilton, 1976). Contours 1300 Nepheline 950 90 (Na, K) Nepheline Orthorhombic 10 90 1000 10 show lines of constant temperature 1100 Kalsilite Carnegieite [(K, Na)AlSiO4] 1200 (°C). The leucite stability field is shown 1500 in blue. The light red area is the range of 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 bulk compositions of borolanite calcu- NaAlSiO4 4 NaAlSiO4 KAlSiO4 lated by Woolley (1973), and the dark red (wt%) KAlSiO (wt%) area is the composition of pseudoleucite calculated from scanning electron

2 microscope analyses. Red arrow shows SiO2 SiO path taken by borolanite sample. As- suming isobaric crystallization, leucite 10 90 in the borolanite formed at 1 kbar and 10 90 2 kbar 5 kbar 1100 °C, given that a cotectic between nepheline and feldspar is required to 20 80 20 80 explain the observed symplectites and Quartz the red field must lie within the leucite 30 70 30 70 field. For the 5 kbar diagram, an inset enlargement of the low-T region shows the eutectic is Analcite + Liquid + Gas 40 60 40 60 (635 °C ± 39 °C). 700740 720 Feldspar 50 50 50 50 NaAlSi3O8 NaAlSi3O8 KAlSi3O8 KAlSi3O8 (Albite) 860 (Albite) 840 60 40 60 820 40 (Orthoclase) K-Feldspar (Orthoclase) 800 820 780 Na-Feldspar 800 760 70 70 760 30 680 720 30 Leucite 740 Leucite Leucite 5 10 80 20 80 20 Nepheline Nepheline 70 665 640 E 10 90 10 90 75

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 NaAlSiO4 KAlSiO4 NaAlSiO4 KAlSiO4 (wt%) (wt%)

Average pseudoleucite high-temperature thermal metamorphism of sili- Section through the isobaric model along the nepheline-kalsilite join at 1 kbar bulk composition K0.82 Na0.18 ceous dolomite, occurs in late calc-silicate veins 1400 1400 close to the borolanite, and formed from the reaction: 1300 1300

CaCO32+→SiOCaSiO3 + CO2. 1200 1200 Devolatization of the dolomite occurred due to Liquid heating from the injected syenite, forming periclase, 1100 1100 calcite, and CO2. Cubic periclase breaks down to pseudomorphs of brucite, talc, and serpentinite at 1000 1000 temperatures of ~580–600 °C at ~1–1.5 kbar (Turner,

1965). Brucite (Mg[OH]2), talc, and serpentine are pale yellow to dark green (Fig. 17C). The existence 900 Leucite + Liquid

Temperature (°C) Temperature 900 of both SiO2-free minerals (brucite, periclase) Nepheline + Liquid Leucite P requiring low XCO2 and SiO2-rich minerals (talc, E Leucite + Nepheline + Liquid 800 800 diopside) requiring high XCO2 in close proximity is unusual and attests to the role of metasomatic Leucite + Nepheline fluids and the heterogeneous nature of contact 700 Nepheline + Feldspar + Liquid + Feldspar 700 metamorphism. Brucite, talc, and antigorite ser- Nepheline + Feldspar pentine occur in metasomatic veins and give the 600 600 marble its unique pale green-yellow color. The con- 90 80 70 60 50 40 30 20 10 NaAlSiO4 KAlSiO4 trasting range of silica content in the calc-silicate (Nepheline) (wt%) (Kalsilite) minerals can be explained by the devolatization of a wide range of protoliths (Durness Group dolomites, Figure 14. Nepheline-kalsilite binary phase diagram at K Na showing the average bulk composi- 0.82 0.18 Cambrian quartzites, etc.) in the country rock. tion of pseudoleucites within the borolanite. KxNay notation: This represents the percentages of the composition at this point along the scale, i.e., the weighted composition (weight scale runs along the Field mapping shows that the high-tempera-

x-axis and is labeled at the bottom of the page). K0.82Na0.18 indicates 82% K and 18% Na in the example ture metamorphic aureole is exposed only around given. The crystallization path (red line) shows that leucite crystallizes first, followed by a peritectic Ledbeg quarry along the western margin of the reaction point (P) where the last leucite crystallizes. The composition progresses toward more Na-rich Borralan intrusion (Wooley et al., 1972; Coward, composition as the liquid evolves toward the eutectic point (E) where nepheline + feldspar crystallize at fixed compositions. 1985; Searle et al., 2010; Fox, 2015). To the north and south of Borralan, unmetamorphosed Cambrian sediments and Ordovician dolomites (Durness Group) are strongly imbricated but show no con- (Fig. 2), but the grade of country rock at Loch Urig- exposed in the Ledbeg quarry (Fig. 17). Medium- tact metamorphism. If the Borralan syenites were ill is far lower than that of the high-temperature grained pseudoleucite-bearing borolanite dikes intruded after the thrusting (Parsons, 1965a, 1965b, aureole at Ledbeg quarry. From the chemical data intrude into Durness Group dolomites. Relic bed- 1972; Goodenough et al., 2011), the high-tempera- alone, it would seem that the Loch Urigill carbon- ding in the Durness limestone-marble strikes at ture aureole would be expected to occur around atites are mantle-derived igneous rocks, related to 050° and dips 38°SE beneath the main Borralan the whole massif. However, it has been shown the alkaline intrusion of the Borralan complex, and syenite (Fig. 17B). The marble hosts minerals that imbricated Cambrian–Ordovician sedimen- not derived from the Durness limestone-dolomite. indicative of high-temperature (>600–700 °C) con- tary rocks abut the northern and southern margins tact metamorphism, notably forsterite, diopside, (Coward, 1985; Searle et al., 2010) and that a thrust antigorite serpentine, brucite, phlogopite, and must underlie the Borralan massif (the Ledbeg

■■ METAMORPHIC AUREOLE talc. Forsterite olivine (Mg2SiO5) in the marble of thrust of Elliott and Johnson [1980] and Coward Ledbeg quarry shows the highest temperatures [1985]; the Borralan thrust of Searle et al. [2010]) The high-temperature contact metamorphic of contact metamorphism adjacent to the borola- with major lateral ramps along the north and south

aureole of the Borralan intrusion is spectacularly nite. Fibrous wollastonite (CaSiO3), the product of margins of the syenite.

Temperature known as the “Metamorphic Burn,” shows excel- Melanite: 1137 °C Leucite: 1100 °C Pseudomorphing: Deformation: lent outcrops of the intrusive relationships. Parsons 650-550 °C 450-550 °C (1965a, 1965b, 1968) identified three intrusive syenite phases (S1, S2, S3), which can be separated Time on the basis of cross-cutting field relationships, but are all broadly contemporaneous. Mafic pods (pyroxene glomerocrysts) and basic syenites (S1) occur as xenoliths in S2 and S3 syenites (Fig. 19). Dikes and veins of S3 alkali-feldspar syenites (perthosites) cut the earlier ultramafics and S1 and S2 syenites (Fig. 20). Also in the upper River Oykel (1) Liquid >1100 °C (2) Leucites crystallize (3) Leucite undergoes (4) Nepheline breaks down to section, peralkaline rhyolite dikes (grorudites) of Melanite is the rst <1100 °C (tetragonal pseudomorphing analcite, cancrinite, and mica phase to crystallize [pseudocubic] habit) reactions 650-550 °C to (5) Tectonic deformation the Assynt minor intrusives cut the Loch Ailsh K-feldspar & nepheline produces subgrains syenites (Fig. 21), showing that these dikes must in ductile regime be younger than 430.6 ± 0.3 Ma, the U-Pb age of (6) Late brittle deformation crystallization of the Loch Ailsh pluton (Goode- overprint (minor) nough et al., 2004, 2011). The peralkaline rhyolite is compositionally most similar to the late suite of Figure 15. Crystallization and deformation history of leucite in borolanite deduced from scanning electron microscope and electron backscatter diffraction analyses. Initial crystallization temperature of leucite is the Borralan intrusion. 1100 °C at 1 kbar. The temperature axis on the cooling path is not to scale. Two-feldspar geothermometry indicates crystallization temperatures of between 1060 °C at 3 kbar and 750 °C at 1 kbar (Parsons, 1965a). Alkali-feldspar and pyroxene compositions and perthitic ■■ LOCH AILSH INTRUSION is rarely exposed, but some river sections show textures suggest that the fractionation trend of spectacular structural relationships. Geophysical the leucosyenites becomes more peralkaline with The Loch Ailsh pluton covers 10 km2 of leuco- data show the magnetic pyroxenites along the mar- time. Most of the Loch Ailsh intrusion is composed syenites that were initially described by Peach et gin of the Loch Ailsh intrusion to be steeply dipping of sodium-rich leucocratic syenites with a minor al. (1907) and Phemister (1926) and later mapped to the southeast (Parsons, 1965a). amount of quartz but no nepheline, unlike the struc- in detail by Parsons (1965b). Figure 18 shows a Near the northwestern corner of the Loch Ailsh turally lower Borralan complex. S1 and S2 syenites cross-section across the Moine thrust zone and the intrusion, a small tributary of the River Oykel, are early pyroxene or riebeckite (alkali amphibole) Loch Ailsh syenite body. Phemister (1926) described the intrusion as a stratified laccolith, but Parsons (1965b) refuted this using geophysical evidence, which suggested that ultramafic rocks form sub- Durness Group vertical margins of the Loch Ailsh syenite intrusion. dolostone Mapping also shows that the syenites cut folded and thrusted foreland rocks at Sgonnan Mor (Elliott Intrusive and Johnson, 1980; Coward, 1983; British Geolog- carbonatite ical Survey, 2007; Searle et al., 2010), including Lewisian basement, Torridonian sedimentary cover, and the unconformably overlying Cambrian–Ordo- vician sedimentary sequence (Fig. 18). The Loch Ailsh complex and the surrounding country rocks are all carried above the Ben More thrust below and truncated by the Moine thrust above (Searle et al., Figure 16. Carbonatite–country rock contact along the north shore of Loch Urigill (location: 24700 10388, coordinate 2010). The contact between the syenite and the sur- reference system: EPSG: 32630 WGS84 / UTM 30N) showing a sharp intrusive contact and chilled margin, implying an rounding Cambrian–Ordovician sedimentary rocks igneous intrusion into cold, unmetamorphosed dolomite outside of the contact metamorphic aureole.

4 m WIDE BOROLANITE DYKE

TOWARDS THE LOYNE MASS

VIEWPOINT FOR PANORAMA 38 HAUL ROAD ROCK FACE 2 (NEW)

Inset Map 1 km EXIT ROCK FACE 1 HAUL ROAD (OLD) FENCE LINE QUARTZITE

DURNESS 32 LIMESTONE 38

STOCKPILE LOCH BORRALAN INTRUSION

ENTRANCE

SETTLING POND

FENCE LINE

CAR PARK Scale 100 m (POORLY EXPOSED) 0 25 50 75 100

Figure 17. Field photos of the contact metamorphic aureole along the western margin of the Borralan syenite at Ledbeg quarry (location: 25280 13623; coordinate reference system: EPSG: 32630 WGS84 / UTM 30N). A newly exposed surface shows borolanite mafic nepheline syenite dikes and sills (outlined in orange) intruding into Durness Group dolostones. Relic bedding can be seen in marbles (blue lines), which strike 230° and dip 39°S. Marbles contain green diopside and forsterite, with yellowish brucite and pale talc indicative of high-temperature contact metamorphism. The borolanite sills can be traced to link with the main Borralan pluton on a strike of 150°. (A) Basemap of the quarry complex. (Continued on following page.)

RELICT CNOC NA STROINE BEDDING LOCH BORRALAN INTRUSION (400 m) 10m 3m

INTRUSIVE BOROLANITE MARBLED DURNESS LIMESTONE

Figure 17 (continued). (B) Photo panorama of the new quarry face showing a newly exposed surface at Ledbeg quarry. Grid reference: [25280 13623]. Looking SE (red line of map indicates field of view). Borolanite dykes and sills intrude into Durness Group dolostone producing brucite-bearing marble. Orange lines highlight borolanite (mafic nepheline-syenite) contacts with thermally metamorphosed dolomite. Local bedding (dark blue) of the Durness Group dolostone has a strike and dip of 230°/38°S (beneath the intrusion) while the dikes can be traced towards the Borralan on a bearing of 150°. (C) Relict bedding in the Durness dolomite strikes and dips at 230°/38°S beneath the Borralan intrusion. Field of view is 11 m wide. (D) Brucite (dark green) and talc (light green) are associated with cracks highlighting the role fluids play during calc-silicate contact metamorphism at Ledbeg Quarry. (E) Above the newly exposed quarry face, a 4-m-wide borolanite dike (top right corner) dips SE towards the Borralan.

syenites, termed “pulaskites” and “shonkinites” the syenite are mainly medium- to coarse-grained the Borralan complex showed that the ultramafics in older literature (Phemister, 1926). Chemical meta-dolomites containing diopside, brown amphi- and the syenites have clear intrusive contacts into compositions of the amphiboles with prominent bole, biotite, magnetite, sphene, and apatite, with the marble or the stratigraphically lower quartzites blue-violet pleochroism reveal a solid solution some hornblendite containing minor albite and (Parsons and McKirdy, 1983), and the same field between ferro-richterite and richterite and may be orthoclase (Parsons, 1965b). These rocks have relationships are likely at Loch Ailsh. altered pyroxenes rather than primary minerals been interpreted as metasomatic skarns along (Fox, 2015). S3 rocks are alkali-feldspar syenites the syenite-dolomite contact (Shand, 1910; Par- (perthosites), and all phases have clear igneous sons, 1968; Johnson and Parsons, 1979), where ■■ MOINE THRUST MYLONITES textures. the country rock, mainly diopside marbles, has Vertical sheets of ultramafic rocks mark the been partially assimilated to form the S2 syenites. Mylonites along the Moine thrust crop out east southeastern margin of the intrusion, where The later S3 syenites form an extensive zone of of the easternmost exposures of syenite. Syenites syenites have intruded up to the Durness Group mixing between basic syenite and feldspathic along the eastern margin show mylonitization limestone-dolomites. The country rocks adjacent to syenite. However, drilling around the margin of fabrics superimposed on igneous fabrics in a few

RIVER LOANAN BEINN FHUARAIN KLIPPEN SGONNAN MOR RIVER OYKELL

MOINE THRUST WNW ESE [247 164] Metamorphic Burn [337 126] (along-strike projection) T 1000 GLEN OYKELL THRUST 1000 900 900 800 800 700 BEN MORE THRUST 700 600 BEN MORE THRUST L MOINE THRUST 600 500 T GLEN OYKELL THRUST 500 400 400 300 SOLE THRUST T 300 200 200 100 Prp ? L 100 0 BaQ 0 L LOCH AILSH (m) Elevation -100 SOLE THRUST -100 -200 T BEN MORE THRUST SYENITES -200 -300 T -300 -400 L L -400 -500 -500 -600 -600

SOLE THRUST

Equal horizontal & vertical scale

1 km BaQ = Basal quartzite L = Lewisian basement Approximate thermal aureole Prp = Pipe Rock T = Torridonian

Figure 18. Cross-section of the Moine thrust zone and Loch Ailsh syenite oriented WNW-ESE at location end points 247 164 and 337 126 (coordinate reference system: EPSG: 32630 WGS84 / UTM 30N), constructed from the British Geological Survey (2007) Assynt sheet. The bold black line is the present-day topography. Key is same as in Figures 2 and 3.

localities. Large perthitic alkali feldspars have and microstructures (490–410 °C; Passchier and nordmarkite sill at the Allt nan Sleach stream sec- recrystallized to fine-grained albite aggregates. Trouw, 1996) are generally ~100 °C higher than tion was intruded after the earliest mylonite fabrics A new quarry close to Ben More Lodge (grid ref- deformation temperatures in northern Assynt were formed during the initiation of the Moine erence NC31846 09986, British Geological Survey, (Thigpen et al., 2010a, 2010b). This temperature thrust but was probably a part of the Loch Ailsh 2007, Assynt sheet) has uncovered spectacular out- increase could be the result of residual heat from intrusion, which is truncated by the Moine thrust crops of Lewisian-derived mylonites that show tight the Loch Ailsh syenite intrusion and some frictional above and was carried on the Ben More thrust folding along a northeast-southwest axial-plane heating along the Moine thrust. beneath after intrusion. strike (Fig. 22). The fold has an axial plane strik- One quartz-bearing microsyenite (nordmarkite) ing 120°, an upright to steep southwest dip, and sill has been reported as cross-cutting the Moine an axis plunge of 6°–10°NW. This unique outcrop mylonites and quartz mylonites at one locality, the ■■ THRUST GEOMETRY AND demonstrates that some northeast-southwest com- Allt nan Sleach stream section southeast of Loch RESTORATIONS pression, perpendicular to the direction of thrust Ailsh. Sabine (1953), Christie (1960, 1965), and Law transport, occurred in this region after mylonite et al. (1986) all mapped this area and described the The eastern contact of the Loch Ailsh pluton is formation. Compositional banding is evident, with sill as cutting mylonitic fabrics that show WNW not well exposed, but all maps show the entire plu- feldspar porphyroclast–rich layers interbanded with shear sense. Apart from the large Loch Loyal ton being structurally below (west of) the mapped quartz-rich mylonites. Temperatures of deformation syenite intrusion in the far north of Sutherland, no and inferred position of the Moine thrust (Peach et during ductile mylonite formation deduced from other alkaline sills are reported as intruding across al., 1907; Parsons, 1965; Elliott and Johnson, 1980; quartz c-axis opening-angle thermometry (470 °C) the main Moine Supergroup. We conclude that the British Geological Survey, 2007; Searle et al., 2010).

N Geophysical evidence implies that the syenites and ultramafic margin may extend eastward beneath the shallow-dipping Moine thrust (Parsons, 1965). The western and southern margin of the Loch Ailsh syenite lies structurally above a thrust originally termed the Sgonnan Mor thrust (Peach et al., 1907; River Oykell Phemister, 1926), nowadays called the Ben More thrust (Fig. 2). The syenites are assumed to have S1 - Loch Ailsh a subvertical western contact and cut across the pyroxene-bearing syenite tight folds (horizontal axial planes) of the Sgonnan Mor fold structures, which affect a slice of Lewisian basement, the unconformably overlying Torrido- nian clastic sediments, and the unconformably overlying Cambrian–Ordovician sediments (Milne, 1978; Elliott and Johnson, 1980; Searle et al., 2010, their figure 4). Thus, the Loch Ailsh pluton was intruded after initial folding and thrusting but is cut by thrusts both above (Moine thrust) and below S1 S2 - Loch ma c (Ben More thrust). Ailsh syenite pod The sequence of thrusting in the Moine thrust zone is thought to be dominantly a foreland-prop- Figure 19. Field photo of the Loch Ailsh syenite lithologies, showing cross-cutting relationships, from the agating sequence of thrusting from the Moine to confluence of the River Oykel south of the Sail au Ruathair ridge (location: NC 32590 12738, coordinate the Ben More, Glencoul, and Sole thrusts with time. reference system: EPSG: 32630 WGS84 / UTM 30N). Mafic syenites of the first intrusive phase (S1) enclose The Moine thrust zone forms a critical taper wedge, ultramafic pyroxenite xenoliths and pods, intruded by S2 syenites. increasing in thickness to the east. The Moine thrust shows some of the earliest motion recorded in the deep ductile mylonites, but also shows late-stage out-of-sequence motion where the thrust truncates all underlying imbricate slices and lithologies in the footwall to lie almost directly above the foreland, S2 at Knockan Crag for example. Elliott and John- son (1980), Coward (1985), and Searle et al. (2010)

S1 mapped a thrust, the Borralan thrust, underlying the Borralan syenite complex together with its high-temperature contact metamorphic aureole. The base of the Borralan complex is not exposed, but a borehole drilled along the southwestern margin showed syenite intruding quartzites (Parsons and McKirdy, 1983). Searle et al. (2010) interpreted S3 this as showing intrusive relationships above a basal thrust, the Borralan thrust, which ramps up Pyroxene glomerocryst S1 to the surface west of Ledbeg (Fig. 2). Because the “clots” high-temperature metamorphic aureole is exposed only in the Ledbeg quarry area and not around the Figure 20. Field photo of the Loch Ailsh syenites from the River Oykel (location: 32603 12756, coordinate reference system: whole massif, these field constraints imply that the EPSG: 32630 WGS84 / UTM 30N). Coarse-grained mafic syenites (first intrusive phase, S1) with clots or glomerocrysts of ultramafic pyroxenites (dark green), cut by veins of pyroxene syenite (S2), and cut by later S3 alkali-feldspar syenites northern and southern margins are structural fea- (perthosites). tures—lateral ramps—rather than intrusive contacts.

River Oykell Figure 21. Field photo of peralkaline rhyolite dikes intruding Rhyolite Flow banding syenite of the second intrusive phase (S2), upper reaches of dike the River Oykel (location: 32687 13643, coordinate reference system: EPSG: 32630 WGS84 / UTM 30N). Chilled margin

S2 Tension fractures in lled with rhyolite

Y X Loch Ailsh FOLD AXIS NORTH

Direction of Transport (NE) Z Ben More Thrust Moine Mylonite Quarry PPL N

30 mm Figure 22. Photos from the new “mylonite quarry” near C’ fabric Ben More Lodge (location: 31846 09986, coordinate ref- Moine Thrust erence system: EPSG: 32630 WGS84 / UTM 30N), view to the southeast, showing folded Lewisian-derived my- MYLONITE FABRIC Inset Map 1 km lonite fabric (yellow lines) along an axial plane striking 120°, suggesting a local post-mylonite phase of north- Fabrics east-southwest compression, perpendicular to the Moine thrust. Mylonites show compositional banding from feldspar porphyroclast–rich mylonites to quartz mylonites. Three-dimensional thin-section images are oriented to show the S-C and C′ fabrics from the new quarry (PPL— C fabric plane-polarized light; XPL—cross-polarized light).

XPL

Fold axial planes and imbricate thrusts in the Brea- have been restored using standard line-balancing U-Pb zircon dating of igneous intrusions to pro- bag dome north of Borralan strike north-south and techniques (Fig. 23). They show that the Loch Ailsh pose a detailed structural evolution for the Moine are abruptly truncated by the east-west lateral ramp complex has been transported ~3 km westward thrust zone (Fig. 24). Folding, thrusting, crustal along the northern margin of the syenites. To the along the Ben More thrust, and that the Borralan shortening and thickening, and ductile shearing west of Borralan, the Cam Loch klippe consists of complex has been transported ~7 km westward in the Moine schists in the hanging wall of the Lewisian basement and Cambrian quartzite, lying above the Borralan thrust, in rough agreement with Moine thrust occurred from ca. 475 to 430 Ma as structurally above the Sole thrust sheet (Fig. 2). Res- Coward’s (1985) assessment. a result of the collision of Laurentia and Avalonia toration of balanced sections across this section (Thigpen et al., 2013; Bird et al., 2013; Ashley et al., shows that the Cam Loch thrust must be a separate 2015; Mako et al., 2019). Thrusting propagated from thrust to the Ben More thrust, and not directly con- ■■ GEOLOGICAL EVOLUTION OF THE east to west across the Scottish Highlands, both nected (Searle et al., 2010). MOINE THRUST ZONE in the Moines and across the Moine thrust zone. The three cross-sections presented in this work The Moine thrust initiated by movement along the across the Loch Ailsh profile (Fig. 18), Loch Bor- We can use the structural relationships from ductile mylonites that have protoliths from the Lew- ralan profile (Fig. 3), and Loch Urigill profile (Fig. 5) detailed mapping and the age constraints from isian basement, the Cambrian quartzites, and the

LOCH AILSH SECTION WEST EAST BREABAG THRUST SOLE THRUST BEN MORE THRUST GLEN OYKELL THRUST MOINE THRUST Moine mylonites 1 km Durness Group An t-Sron formation Equal horizontal and vertical scale Eriboll formation LOCH AILSH Lewisian SYENITES Torridonian Minor intrusions Torridonian Sole thrust ramps up Moine Supergroup (east of Sole thrust) over An t-Sron formation

LOCH BORRALAN SECTION Pre-Borralan WEST BREABAG THRUST EAST intrusion thrust Ledbeg quarry Four Burns locality

SOLE THRUST BORRALAN THRUST BEN MORE THRUST MOINE THRUST

Durness Group An t-Sron formation Eriboll formation LOCH BORRALAN

Lewisian ? SYENITES ? 1 km Equal Horizontal and Vertical Scale Parsons and McKirdy (1983) excavation site Moine Supergroup

LOCH URIGILL SECTION WEST EAST

SOLE THRUST MOINE THRUST Not to scale 1 km Durness Group An t-Sron formation Equal vertical and horizontal scale Eriboll formation Lewisian

Durness Group imbricates, poorly exposed on the north side of Moine Supergroup Loch Urigill. Thrusts inferred from southern side of Loch Urigill.

Figure 23. Restored sections from the Loch Ailsh profile (top, Fig. 18), the Loch Borralan profile (middle, Fig. 3), and the Loch Urigill profile (bottom, Fig. 5). See text for discussion.

WEST EAST Ordovician limestone-dolomites (Law, 1998; Law Active pBT Sgonnan Mor folds et al., 2010; Law and Johnson, 2010). Rb-Sr dat- Future BMT ing of micas from the Moine thrust mylonites at Knockan Crag gave ages of 437–408 Ma (Freeman Middle Cambrian et al., 1998). Dallmeyer et al. (2001) obtained Rb-Sr Lower Cambrian Torridonian ages 428–413 Ma from the northern section of the Lewisian Active GOT Moine thrust zone. Within the Moine thrust zone, basement the earliest structures are the tight folds and thrusts Future BMT (e.g., Glen Oykel thrust) within the Sgonnan More (A) Late Ordovician (ca. 440 Mya) region that are intruded and cut by the Loch Ailsh syenites (Fig. 24A). Future BMT Sgonnan Mor Ductile MT Uranium-lead zircon ages from the Canisp por- folds Locked pBT MT phyry (430.4 ± 0.4 Ma) on the foreland, the Borralan syenite early suite (431.1 ± 1.2 Ma), the Borralan Ailsh MT intrusion quartz syenites (429.2 ± 0.5 Ma), and the Loch Ailsh Borralan intrusion syenite (430.6 ± 0.3 Ma), all from Goodenough et al. (2011), show that despite some compositional dif- Lewisian Future BT Future BMT ferences, all the alkaline intrusions in Assynt were basement intruded between ca. 431 and 429.2 Ma. Goode- (B) Silurian: Llandovery (430 Mya) nough et al. (2011) suggested that the total amount of displacement along the Glencoul and Ben More thrusts, estimated at ~50 km (Elliott and Johnson, Active BT Active BMT Sgnonnan Mor Future brittle MT folds 1980), occurred between the ages of emplacement Future ST MT of the Loch Ailsh suite (430.6 ± 0.3 Ma) and of the late suite of the Borralan complex (429.2 ± 0.5 Ma). Borralan Ailsh intrusion intrusion However, nowhere do the late Borralan quartz ST BT syenite or the late undeformed Borralan pegmatite Lewisian basement dikes cut the thrusts, so this narrow time bracket BMT is not warranted. All that can be determined is that (C) Silurian: Llandovery (429 Mya) the Moine, Ben More, and Borralan thrusts must have moved after 430–429 Ma. It remains unclear why this major regional alka-

BMT = Ben More thrust MT = Moine thrust 3 km 3 BT = Borralan thrust pBT = Pre-Borralan Thrust line igneous suite, comprising the Loch Ailsh and 20 km Ma = Million years ago ST = Sole thrust Loch Borralan complexes and the Assynt dikes, intruded along the Laurentian plate margin at this time. One possibility is that the alkaline intrusions may have triggered initiation of the Moine thrust. The Loch Loyal syenite intrusion east of Loch Eriboll in the far north of the Moine thrust zone has a single concordant zircon 206Pb/238U age of ca. 425 Ma (Goodenough et al., 2011). If this age is accurate, it would suggest that folding and shearing along the Moine thrust zone was finished by 425 Ma, but Figure 24. Proposed geological evolution of the Moine thrust zone and Loch Ailsh and Borralan syenite more precise dating is needed to define the age intrusions. (1) Early folding of the Sgonnan Mor structures and ductile shearing of the Glen Oykel thrust more accurately. Thrusts propagated from the (GOT), followed by (2) cross-cutting intrusion of the Loch Ailsh and Borralan syenite intrusions, followed by (3) thrusting propagating from the Moine thrust to the Ben More thrust to the Borralan thrust, truncating and Moine to the Ben More thrust with time, cutting carrying the syenites, followed by late thrusting of the Cam Loch thrust and Sole thrust. See text for discussion. and carrying the Loch Ailsh syenite above the Ben

More thrust to the Borralan thrust, which carries the Borralan syenites are high-temperature tectonic Bird, A.F., Thirlwall, M.F., Strachan, R.A., and Manning, C.J., 2013, Lu-Hf and Sm-Nd dating of metamorphic garnet: Evidence the Borralan complex with its high-temperature fabrics superimposed on the earlier magmatic fab- from accretion events during the Caledonian orogeny in contact metamorphic aureole above (Searle et al., ric during ductile shearing. A high-temperature Scotland: Journal of the Geological Society, v. 170, p. 301– 2010). Major lateral ramps dropped the imbricated, contact metamorphic aureole shows olivine- and 317, https://doi.org/10.1144/jgs2012-083. British Geological Survey, 2007, Assynt, Scotland: Bedrock: Brit- structurally higher-level, Cambrian–Ordovician clinopyroxene-bearing marbles, brucite, and ser- ish Geological Survey 1:50,000 Geology Series, Scotland sedimentary rocks down to the north and south pentinite calc-silicates and was formed by heat from Special Sheet, scale 1:50,000. (Fig. 24B). The Borralan thrust carried the Borralan the high-temperature (~1100 °C) syenite intrusion. Butler, R.W.H., 1982, A structural analysis of the Moine thrust complex and its contact metamorphic aureole west- Dikes and sills of borolanite emanating off the zone between Loch Eriboll and Foinaven, NW Scotland: Journal of Structural Geology, v. 4, p. 19–29, https://doi.org ward over the Cam Loch thrust sheet. Finally, the main pluton intrude quartzites and marbles of the /10.1016/0191-8141(82)90003-7. youngest thrust is the Sole thrust sheet, which contact metamorphic aureole, exposed in the Led- Butler, R.W.H., 1987, Thrust sequences: Journal of the Geologi- carries imbricated Cambrian and Ordovician sed- beg quarry. The northern and southern contacts of cal Society, v. 144, p. 619–634, https://doi.org/10.1144/gsjgs .144.4.0619. imentary rocks in its hanging wall (Fig. 24C). The the Borralan syenites are lateral ramps that have Butler, R.W.H., 2010a, The geological structure of the North-West Moine thrust also must have had some late, out- dropped imbricated Cambrian quartzites, clastic Highlands of Scotland—Revisited: Peach et al. 100 years of-sequence motion, given that it truncates many sedimentary rocks (Fucoid Beds and Salterella Grit on, in Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabben- of the imbricate thrust sheets along the southern dam, M., and Strachan, R.A., eds., Continental Tectonics and Members of the An t-Sron Formation), and Ordo- Mountain Building: The Legacy of Peach and Horne: Geo- margin of the Assynt window and carries the Moine vician limestone-dolomites of the Sole thrust sheet logical Society of London Special Publication 335, p. 7–27, schist all the way westward to rest almost directly down relative to the Borralan complex. The U-Pb https://doi.org/10.1144/SP335.2. Butler, R.W.H., 2010b, The role of thrust tectonic models in above the foreland at Knockan Crag. ages of the syenites (430.6–429.2 Ma; Goodenough understanding structural evolution in NW Scotland, in Law, et al., 2011) constrain the ending of alkali magma- R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M., and tism and the beginning of thrusting along the Moine Strachan, R.A., eds., Continental Tectonics and Mountain ■■ CONCLUSIONS thrust zone in the Assynt region. It is possible that Building: The Legacy of Peach and Horne: Geological Soci- ety of London Special Publication 335, p. 293–320, https:// the mantle-derived alkaline magmatism might have doi.org/10.1144/SP335.14. The east-dipping, west-verging Moine thrust initiated thrusting along the Moine thrust at depth. Butler, R.W.H., and Coward, M.P., 1984, Geological constraints, zone in northwestern Scotland is the Caledonian structural evolution, and deep geology of the northwest orogenic front, placing the regional metamorphic Scottish Caledonides: Tectonics, v. 3, p. 347–365, https:// doi.org/10.1029/TC003i003p00347. rocks of the Moine Supergroup over the Laurentian ACKNOWLEDGMENTS Christie, J.M., 1960, Mylonitic rocks of the Moine Thrust-zone foreland comprising Lewisian basement (granu- We thank Eleni Wood and Nick Gardiner for field assistance, in the Assynt region, north-west Scotland: Transactions of lite and amphibolite facies gneisses), Torridonian Lars Hansen and Dave Wallis for help with the scanning elec- the Geological Society of Edinburgh, v. 18, p. 79–93, https:// tron microscope, and Rick Law for numerous discussions on doi.org/10.1144/transed.18.1.79. clastic sediments, and Cambrian–Ordovician pas- Scottish geology and for comments on an earlier version of the Christie, J.M., 1965, The Moine Thrust in the Assynt region, sive-margin sediments. Major thrust sheets within manuscript. We also thank reviewers Rob Strachan and Ryan northwest Scotland [discussion]: The Journal of Geology, the Moine thrust zone include the Ben More (and Thigpen and editor Robert Hildebrand for extremely detailed v. 73, p. 672–681. Coward, M.P., 1983, The thrust and shear zones of the Moine Glencoul) and the Sole thrust sheets, which in comments, which greatly improved the paper. We thank the UK Ordnance Survey and the British Geological Survey for use of thrust zone and NW Scottish Caledonides: Journal of the general propagated from east to west with time. In their base maps. Base maps contain OS data © Crown copyright Geological Society, v. 140, p. 795–811, https://doi .org /10 .1144 the southern part of the Assynt window, two large and database right 2020. /gsjgs.140.5.0795. syenite intrusions, the Loch Ailsh and Loch Borralan Coward, M.P., 1984, A geometrical study of the Arnaboll and Heilam thrust sheets, NW of Ben Arnaboll, Sutherland: syenites, are sandwiched between thrusts above Scottish Journal of Geology, v. 20, p. 87–106, https://doi and below. 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