Gabbro, Dyke in Lewisian Complex

Sampled from the "Graveyard Dyke", Scourie.

Outcrop

Gabbro, Scourie dyke in Lewisian Gneiss Complex, Scouriemore
The dyke is a vertical sheet of igneous rock that cuts through the Lewisian gneisses. This particular one, known to geologists as the "graveyard dyke", is about 40 metres wide. Because it weathers more rapidly than the surrounding felsic gneisses, it has been hollowed out by the sea, forming this small bay just west of Scourie village. Here, we are looking westwards along the southern edge of the dyke. The students are looking at the outcrop of the dyke itself, and the southern margin of the dyke is marked by the steep vertical face at the left-hand side of the beach.

Hand specimen

Gabbro, Scourie dyke in Lewisian Gneiss Complex, Scouriemore
This rock shows an igneous texture of blocky dark pyroxene crystals with light-coloured feldspar occupying the irregular spaces between the pyroxenes. The texture indicates that pyroxene started crystallizing first from the magma, before the feldspar. (Many gabbros show the opposite relationship, and have oblong feldspars with pyroxene filling the space between them). There is also a little garnet in clusters and streaks of red crystals. This did not crystallize from the magma, but grew later in the solid rock.
This sample comes from near the centre of the dyke. Nearer to its margins, where the magma cooled more rapidly, the rock has a much finer grain size.

Thin section

Gabbro, Scourie dyke in Lewisian Gneiss Complex, Scouriemore
Most of the rock is made of greenish and reddish pyroxenes and clear feldspar. However, in detail the pyroxene is not very fresh, and has partly broken down to hornblende and dark iron-rich material. This shows that the rock suffered some metamorphism after it was intruded. Radiometric dating on this dyke reveals that it was intruded 2000 million years ago, and the later mineral breakdown happened at 1750 million years, during the Laxfordian episode.

Plane polarized light, field of view 4 mm across

Gabbro, Scourie dyke in Lewisian Gneiss Complex, Scouriemore
In this view the spongy texture of the pyroxene (centre left, and right) shows where it began to break down. The crystal outlines of the feldspar and pyroxene are not as regular and distinct as they would have been in the fresh igneous rock.

Crossed polars, field of view 4 mm across

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