Achmelvich: the Lewisian Gneiss Complex (2)

Folded Lewisian gneisses on the coast near Achmelvich. These are highly metamorphosed igneous rocks of both acid and basic compositions. They have been strongly deformed several times. The earlier deformations flattened and stretched the original igneous rocks so that they have a compositional banding (i.e. they are gneisses). Later deformation has folded and crumpled this banding (centre of photo).

Metamorphic rocks with Scourian structures

Achmelvich lies on the coast about 4 km NW of Lochinver. Here, the Lewisian gneisses show banding and folds that formed during the Scourian episode. However, the minerals have been affected by the Laxfordian metamorphism, during which the temperature was lower than that of the Scourian metamorphism. So, the rocks now contain amphibole (hornblende) as their typical mafic mineral, rather than pyroxene. At Achmelvich, therefore, the Scourie dykes are straight and cross-cutting (except for slight shearing at their margins), but their igneous minerals have been totally converted to a hornblende-rich assemblage.

The sheet of dark rock, dipping steeply to the right in the centre of the photo, is a small Scourie dyke about 1.5 metres wide. It has sharp contacts against the lighter-coloured gneiss, and it cuts right across the banding in the gneiss, which on both sides of the dyke can be seen dipping about 50 degrees to the left.

Laxfordian shear zone

Passing just to the north of Achmelvich is an east-west-trending zone about a kilometre wide in which the gneisses have changed their texture and appearance because of intense deformation during the Laxfordian episode at about 1750 million years ago. These rocks are finer grained and contain more strongly aligned minerals. Scourie dykes are converted into schistose amphibolites.

Looking west along the zone of steeply-dipping gneisses produced during the Laxfordian episode. Notice the strongly developed layering visible in the gneisses near the shoreline in the centre of the photo.

Rock types at this locality

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