Syn-orogenic alkaline magmatism and its relationship to the Moine Thrust Zone and the thermal state of the Lithosphere in NW Scotland

The alkaline complexes of NW Scotland lie in a zone sub-parallel to the Moine thrust and represent the NW edge of the otherwise overwhelmingly calc-alkaline late-orogenic magmatism of the Caledonian fold-belt. The emplacement of intrusives in Assynt overlapped the thrust movements and new U-Pb zircon ages for the Loch Ailsh intrusion (439 ± 4 Ma) provide are liable bracket, when combined with existing age data for the Loch Borralan intrusion (430 ± 4 Ma), for the main movements on the Ben More thrust plane. A detailed history of the main tectonic and igneous events in the Moine Thrust Zone is tabulated. Zircons from the undeformed Cnoc-nan-Cuilean syenite, emplaced in Moine rocks near Loch Loyal, give an age of 426 ± 9 Ma, in contrast to the deformed Glen Dessary syenite for which earlier work gave 456 ± 5 Ma. The linear distribution of the alkaline complexes to the west of the calc-alkaline plutons, and the ultra-potassic character of the Loch Borralan complex (the most westerly), are consistent with, but do not prove, generation related to a W-dipping subduction zone. The persistence of the magmatism, over 30 Ma, in a belt being shortened by thrusting, suggests a source in the mantle lithosphere. The absence of magmatism further to the NW and the production of calc-alkaline magmas to the SE of this zone, is compatible with a model in which the thermal state of the lithosphere exercises the most important control on the nature of Late Caledonian magmatism.