No abstract

Concern about security of supply of critical elements used in new technologies, such as the Rare Earth Elements (REE), means that it is increasingly important to understand the processes by which they are enriched in crustal settings. High REE contents are found in syenitedominated alkaline complexes intruded along the Moine Thrust Zone, a major collisional zone in north-west Scotland. The most northerly of these is the Loch Loyal Syenite Complex, which comprises three separate intrusions. One of these, the Cnoc nan Cuilean intrusion, contains two mappable zones: a Mixed Syenite Zone in which mafic melasyenite is mixed and mingled with leucosyenite and a Massive Leucosyenite Zone. Within the Mixed Syenite Zone, hydrothermal activity is evident in the form of narrow altered veins dominated by biotite and magnetite; these are poorly exposed and their lateral extent is uncertain. The REE mineral allanite is relatively abundant in the melasyenite and is extremely enriched in the biotite-magnetite veins, which have up to 2 % total rare earth oxides in bulk rock analyses. An overall model for development of this intrusion can be divided into three episodes: (1) generation of a Light Rare Earth Element (LREE)-enriched parental magma due to enrichment of the mantle source by subduction of pelagic carbonates; (2) early crystallisation of allanite in melasyenite, due to the saturation of the magma in the LREE; and (3) hydrothermal alteration, in three different episodes identified by petrography and mineral chemistry, generating the intense enrichment of REE in the biotite-magnetite veins. Dating of allanite and titanite in the biotite-magnetite veins gives ages of c. 426 Ma, overlapping with previously published crystallisation ages for zircon in the syenite.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 overlying sediments were deformed and metamorphosed during oblique collision of the 45 Laurentia, Baltica, and Eastern Avalonia continental blocks (Soper et al., 1992; Torsvik et al., 46 1996;McKerrow et al., 2000;Dewey & Strachan, 2003). The resulting Caledonian orogenic 47 belt extends from Scandinavia and East Greenland, through the British Isles and beyond to 48 the Appalachians of North America. Scotland and Ireland (within Laurentia) underwent an 49 early orogenic phase, the Grampian arc-continent collision, with later Silurian Laurentia-Avalonia collision known as the Scandian event (c.435-425 Ma) (Coward, 1990). 51 Caledonian deformation within the NW Highlands was due to this Scandian event. In the block (Johnstone and Mykura, 1989). 59 During the Scandian event, numerous magmatic intrusions were emplaced along the Moine 60 Thrust Zone. These are predominantly alkaline in composition, and range from mafic and 61 ultramafic early phases, to diorites and high Ba-Sr granites and syenites (Thompson & 62 Fowler, 1986;Tarney & Jones, 1994;Fowler & Henney, 1996;Fowler et al., 2008). The most 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 (Fowler, 1988;Fowler, 1992;Fowler et al., 2008). 92The Loch Loyal Syenite Complex is part of the NW Highlands alkaline plutonic suite, 93 comprising volumetrically small yet highly variable alkaline intrusions of Caledonian age that 94 occur along, and on both sides of, the Moine Thrust Zone. This suite also includes intrusions 95 at Loch Borralan and Loch Ailsh in Assynt (Johnstone & Mykura, 1989;Parsons, 1999; 96 Atherton & Ghani, 2002;Fowler et al., 2008;Goodenough et al., 2011). These magmatic 97 events were related to the subduction of Iapetus oceanic crust below Laurentia, resulting in 98 the production of ultramafic, mafic, granitic, and syenitic magmas (Thompson & Fowler, 99 1986;Fowler et al., 2001;Atherton & Ghani, 2002;Fowler et al., 2008). Thus there is an 100 association between strongly alkaline magmas and a zone of active crustal shortening, 101 rather than extension, in the NW Highlands (Goodenough et al., 2004). This magmatism 102 occurred during and immediately after the Scandian collisional event at 435-425 Ma 103 (Goodenough et al., 2011). 104The ages of the alkaline intrusions have been used to constrain the timing of regional 105 deformation and development of the Moine Thrust Zone (Halliday et a...