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

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.

The Geological Index of the Scottish Caledonides northwest of the Highland Boundary Fault

<p>The quantification and mapping of geodiversity have gained more interest in recent years due to practical application in natural resource management and conservation. The Geological Index (I<sub>Geo</sub>) represents the quantitative expression of geological features and is part of a broader Geodiversity Index (I<sub>Geodiv</sub>), which also includes geomorphological, pedological, paleontological and hydrological elements.</p><p>In Scotland, the area delimited by the Moine Thrust Zone to the northwest and the Highland Boundary Fault to the southeast represents a fragment of the Caledonian orogenic belt that extends across parts of North America, Greenland and Scandinavia. It includes the Highlands, most of the Inner Hebrides and the islands of Orkney and Shetland. The area is underlain by two tectonic blocks – the Northern Highlands Terrane and the Grampian Terrane – separated by a major strike-slip fault, the Great Glen. Both blocks consist of an Archaean-Paleoproterozoic basement covered by the Neoproterozoic metamorphic suites of the Moine and Dalradian Supergroups, together with a series of magmatic intrusions and other rocks of late Precambrian and Phanerozoic age.</p><p>The I<sub>Geo</sub> was obtained from lithostratigraphic and lithodemic units, mapped at group and suite/complex level respectively, major geologic contacts and faults and minor igneous intrusions from the British Geological Survey 1:625k digital datasets. These were reclassified and analyzed using QGIS and ArcGIS software.</p><p>The results show overall medium and high values of I<sub>Geo</sub>, with regional variations and well-individualized areas of very high and very low values. Conspicuous transitions between extremes are observed at the north and south edges of the study area.</p><p>High I<sub>Geo</sub> values occur in five major areas across the mainland: 1). on the north coast, which exhibits small outcrops of varied lithologies; 2). in the northeast Grampian Mountains, where the deformed Dalradian rocks are intruded by the Cairngorms suite of the Newer Granites; 3). along the Great Glen, the meeting place of adjacent tectonic blocks; 4). in the Firth of Lorne area and further inland, where Neoproterozoic and Paleozoic rocks come into contact with more recent Cenozoic rocks of the Hebridean Province; 5). at the southern tip of the Kintyre Peninsula that contains isolated exposures of rocks characteristic of the nearby Midland Valley.</p><p>Low I<sub>Geo</sub> values are encountered in three major areas of the mainland: 1). southeast of the Moine Thrust Zone, an area occupied by the oldest Moine group; 2). in the Pentland Firth area that consists of the Old Red Sandstone Supergroup; 3). in the Firth of Clyde area and further inland, around the main outcrop of the youngest Dalradian group.</p><p>Offshore, the islands of Orkney and Shetland have I<sub>Geo</sub> values at opposite ends of the spectrum. The first are made up of a monotonous sedimentary cover. The latter comprise a mosaic of rocks of Precambrian and early Phanerozoic age.</p>