Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern Scotland

Abstract

During the Caledonian orogeny, the Moine thrust zone in northwestern Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods, westward over the Laurentian passive margin in the northern highlands of Scotland. The Laurentian margin comprises Archean–Paleoproterozoic granulite and amphibolite facies basement (Scourian and Laxfordian complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well exposed in the Assynt window. Two highly alkaline syenite intrusions crop out within the Moine thrust zone in the southern Assynt window. The Loch Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite) to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust zone, syenites intrude up to the Ordovician Durness Group limestones and dolomites, forming a high-temperature contact metamorphic aureole with diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry. Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites were intruded prior to thrusting or intruded syn- or post-thrusting. Borolanites contain large white leucite crystals pseudomorphed by alkali feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened and elongated during ductile shearing. The minerals pseudomorphing leucites show signs of ductile deformation indicating that high-temperature (~500 °C) deformation acted upon pseudomorphed leucite crystals that had previously undergone subsolidus breakdown. New detailed field mapping and structural and petrological observations are used to constrain the geological evolution of both the Loch Ailsh and the Loch Borralan intrusions and the chronology of the Moine thrust zone. The data supports the interpretation that both syenite bodies were intruded immediately prior to thrusting along the Moine, Ben More, and Borralan thrusts.

Structure of the Assynt window, Moine Thrust Zone and relationship of thrusts to alkaline igneous complexes, Caledonian orogeny, NW Scotland

“…Wood (2015) remapped the central part of the Assynt window, particularly the area where many of the Assynt minor intrusions crop out. Fox (2015) and Fox and Searle (2021) remapped the area around the Loch Ailsh and Loch Borralan syenites in detail and constructed balanced and restored sections from the Sole thrust east to the Moine thrust. Three further balanced cross-sections across the Assynt window are presented here with a restored section across the central part of the Assynt window, and a SSW-NNE lateral section.…”

“…The southern part of the Assynt window was mapped by Searle et al . (2010) and Fox and Searle (2021) who also presented detailed balanced and restored cross-sections. A simplified geological map of the Assynt window based on all these studies is shown in Figure 3.…”

“…Within the MTZ, two major intrusions sandwiched between the Moine, Ben More and Borralan thrusts are the Loch Ailsh syenite and the Loch Borralan syenites (Parsons, 1965a, b, 1972; Wooley, 1970, 1973; Parsons & McKirdy, 1983; Coward, 1985; Searle et al . 2010; Fox, 2015; Fox & Searle, 2021). Numerous sills of alkaline igneous rocks intrude the Cambrian-Ordovician sedimentary rocks.…”

“…1). Three major alkaline igneous complexes occur in and around the Assynt window, the Loch Ailsh and Loch Borralan syenite complexes within the MTZ, and the Canisp porphyry intruding the foreland sequence (Parsons, 1999; Goodenough et al 2006, 2011; Searle et al 2010; Fox & Searle, 2021). Numerous minor alkaline dykes and sills intrude all thrust sheets within the MTZ (Goodenough et al .…”

“…Previous studies presented balanced and restored cross-sections across the Loch Ailsh and Loch Borralan plutons in the southern Assynt window (Elliott & Johnson, 1980; Coward, 1985; Searle et al . 2010; Krabbendam & Leslie, 2010; Fox & Searle, 2021).…”