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. meta morphosed 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, © 2021 The Authors GEOSPHERE | Volume 17 | Number 4 Fox and Searle | Alkali intrusions in Moine thrust zone, Scotland Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/4/1126/5367553/1126.pdf 1126 by guest on 01 October 2021 Research Paper 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 pro- file 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