Amphibolite, Lewisian Gneiss Complex

This sample is from about 1 km north of Achmelvich, in a shear zone formed during the Laxfordian episode

Outcrop

Amphibolite, Lewisian Gneiss Complex, Achmelvich
This outcrop lies inland, away from the clean outcrops on the shore and from fresh roadside cuts. It illustrates one of the geologist's common problems: the outcrop surface is partly covered by lichens and moss, and the true rock surface is rather difficult to distinguish. A geologist has to learn to look past the encrusting material and not be distracted by it! If you are sharp-eyed, and look at the areas below the geologist's boot and above the hammer handle, you can make out that the rock consists of dark green and white minerals with a speckled texture. You may also detect that the banding and mineral alignment are stronger at the right of the picture than they are at the centre. This is an amphibolite, consisting of hornblende and plagioclase feldspar.

Hand specimen

Amphibolite, Lewisian Gneiss Complex, Achmelvich
This rock is from one of the Scourie dykes. Because it now has a metamorphic texture, with minerals aligned in a banding, we know that these minerals formed later than the time of dyke intrusion (2000 million years). Therefore, the metamorphism we see here happened in the Laxfordian episode (1800 to 1750 million years) rather than the earlier Scourian episode. The rock consists almost entirely of two minerals, very dark green hornblende and white feldspar. Such rocks are called amphibolites, and they are medium-grade metamorphic rocks formed usually from basic igneous rocks like basalt and gabbro. Notice how the grain size is finer and the banding is more intense in the lower half of the specimen, whereas in the upper half the white feldspar specks are less flattened. Rocks are seldom deformed evenly, and this one has been sheared more intensely along certain zones.

Thin section

Amphibolite, Lewisian Gneiss Complex, Achmelvich
This amphibolite formed by metamorphism of a gabbro (in one of the Scourie dykes). The part of the rock that is less strongly sheared has rather broad, spongy-looking blue-green hornblende crystals and irregular patches of feldspar. It is obviously a metamorphic rock, but it has only a weak banding, and the distribution of dark and light minerals is not unlike that of the pyroxene and feldspar in the original gabbro.

Plane polarized light, field of view 3 mm across

  Amphibolite, Lewisian Gneiss Complex, Achmelvich
Other parts of the amphibolite sample, only centimetres away, have been dramatically changed by shearing and recrystallization. Here the grain size is much finer. The amphibole crystals are strongly aligned and arranged in a banding of dark and light minerals.

Plane polarized light, field of view 3 mm across

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