Loch Borralan: Alkaline igneous intrusions

The largest body of igneous rock in this area is found around Loch Borralan, and it has been named the Loch Borralan Complex. It is an intrusion composed of a variety of coarse-grained alkaline rock types, from dark to light in colour. The most abundant light-coloured rocks are not granites but syenites, made up mostly of potassium feldspar. Dark varieties are rich in mafic minerals such as pyroxene and garnet. One intermediate variety is so distinctive that it has been given its own name, borolanite (from an old spelling of the name of the loch).

The borolanite can be studied in the small disused quarry at Aultivullin.

The importance of the Loch Borralan igneous rocks for the history of the area is that some of the varieties were intruded before the movements on the Moine Thrust zone, and so are deformed, whereas others were intruded after them, and so cut across the thrusts. Radiometric dating of minerals in the igneous rocks tells us their time of intrusion (about 430 million years) so we can also deduce the age of the thrust movements.

This is the typical white-spotted nepheline syenite, borolanite. However, it also contains a dark green fragment about 10 cm across of a pyroxene-rich mafic rock. Such rocks occur elsewhere in the Complex, and were intruded at an earlier stage in its development. This piece was probably broken off and carried in with the nepheline syenite magma as a xenolith (literally, a foreign rock). Xenoliths are common in many igneous rocks.

Crucial relationships between crystallisation of the 430 million year old igneous rocks and movements in the Moine thrust zone are shown by the syenites in Alltivullin quarry. In this view, the white spots in the borolanite have been flattened (since they are solid crystals, this would only be possible at high temperature, before the rock had fully cooled). Also, just to the left of the hammer, a small, later zone of sheared rock cuts the borolanite.

Other igneous rocks in the Assynt area

Other small intrusions in neighbouring areas include sills and dykes, commonly emplaced into the Cambrian sediments. Sills are sheets of igneous rock intruded parallel to the bedding of the enclosing sediments, and dykes are steep, cross-cutting sheets. A sill would normally be connected somewhere to a dyke that acted as its feeder, bringing magma up from depth. Dykes and sills, being relatively thin, crystallize more rapidly that larger intrusive bodies, but more slowly than lavas. As a result they tend to have an intermediate grain size.

Rock types mentioned here

D.J. Waters, Department of Earth Sciences, May 2003