Geological and geochemical aspects of the Loch Loyal Alkaline Complex, Sutherland

The object of this investigation was to categorize all geological and geochemical parameters applicable to the Loch Loyal syenites and to relate these both to processes operating there and to the broader problems of alkaline rock genesis. Whole rock and trace element contents have been determined for the Loyal syenites and compared with other alkaline intrusions in Scotland and both pegmatite chemistry and mineralogy have been detailed. Loch Loyal syenites consist of three separate bodies, Ben Loyal, Beinn Stumanadh, and Cnoc nan Cuilean each intrusive into grey flaggy psammitic Moinian granulites. They are best described as quartz-nordmarkites. Emplacement of the magma resulted in the formation of a feldspar flow lamination in the outer parts of the Loyal mass and to deformation of the surrounding country rocks. Core rocks crystallized as a one-feldspar type and outer laminated syenites as a two-feldspar assemblage. A pegmatitic phase was succeeded by low-temperature hydrothermal mineralisation and considerable deuteric activity in which strontium and barium carbonates, hematite and rare-earth elements were deposited either in veins or druses and, at a temperature close to 25000, a type of lead metasomatism formed green amazonitic feldspar. The intermediate nature of the Loyal nordmarkite has been established, it straddles the sodaclase-granite/nordmarkite division of Johannsen, is close to sphene-orthite type granitoids but contains a mineral with monazite structure, and the tantalo-niobate mineral polycrase occurs within the pegmatites contrary to a suggestion byPavlenko et. ale that such minoralo arc absent from sphene-orthite typo granitoids. Tho other alkaline intrusions in Sutherland, Loch Ailsh and Loch Borralan, were found to differ in trace-element chemistry from each other and from the Loch Loyal syenites. Even so a general alkaline petrographic province is postulated for the N.W. Highlands which persisted from early Caledonian times and included the metamorphosed syenite of Glen Dessarry, part of the Glenelg-Ratagain complex and scattered areas of soda-metasomatism. Compared with other rocks of the Scottish "Newer Igneous" suite Loyal syenites are markedly enriched in Pb,Ba,Nb, and the rare-earth elements and very greatly dbficient in calcium. Anomalous radioactivity associated with the Loyal intrusions is due chiefly to the thorium content of allanite. Within the pegmatites of Sgor Chaonasaid at the northern extremity of the Loyal range a zonation between galena and silicate minerals exists in which silver and copper form complex sulphides such as stromoyerite. The origin of the Loyal syenites is not thought to be connected with tho hypothesis of limestone assimilation for alkaline rock genesis as envisaged by Daly and Shand nor to tho specialised stable tectonic conditions considered important by Harker, but to anatexis at depth and subsequent extreme differentiation, connected with faulting, which resulted in an increased sodium content and a 'reworking' of surrounding sediments and/or granitic rocks, with consequent enhancement of U,Th,Nb,RE's,Ba and Sr over that usual for granitic rocks.