Abstract

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.

First Page Preview

First page PDF preview
You do not currently have access to this article.