Lewisian mylonite, Moine Thrust Zone

From the Moine Thrust zone at Loch Eriboll, to the north of the Assynt area. Similar rocks are found along the Glencoul thrust plane.

Hand specimen

Mylonite formed from Lewisian Gneiss
The Moine Thrust Zone contains a lot of Lewisian Gneiss, especially in the Assynt area, and in places this too has been converted into mylonite. Because it is much richer in feldspar than the Moine Schists, the mylonite has a different texture. Feldspar is more resistant to deformation than quartz, and makes flattened pink lens shapes in this rock. The fine banding wraps around them.

Thin section

Mylonite formed from Lewisian Gneiss
Dusty feldspar makes up knobbly bands of strung-out larger crystals, alternating with quartz-rich bands.

Plane polarized light. Field of view 7 mm across.

Mylonite formed from Lewisian Gneiss
The view between crossed polars shows the large fractured feldspar crystals in lens-shaped layers. The bands in between, made mostly of quartz, are in contrast very fine grained. The bright streaks are strung-out bundles of mica flakes.

Crossed polars. Field of view 7 mm across.

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