Ledmore North Quarry: Alkaline intrusions and contact metamorphism

Just north of Ledmore, on the northwestern edge of the Loch Borralan Complex, a quarry has been excavated in carbonate rocks of the Durness Limestone within the Moine Thrust zone. Sheets of igneous rock, mostly varieties of borolanite, are exposed in the quarry, and the main mass of syenite is nearby.

The heat from the crystallizing magmas and the effects of fluids released from them has caused contact metamorphism of the limestones. They are now marbles with spectacular banding, veining and blotching caused by the growth of new minerals. At the time the photographs were taken, smooth sawn rock faces and slabs of marble could be examined in the quarry. Some of the banding is due to differences in chemical composition (silica, magnesium) between original beds of limestone that would not have been obvious in the original sediment. Other patterns are caused by magma and hot fluids passing along fractures in the rock, adding silica, potassium and other elements, which allow new minerals to grow in the limestone next to the fractures.

Sawn quarry face showing very complex patterns of nodular and banded marble.

Banded marble at a contact with nepheline syenite. The syenite was at the left. Chemical elements from the magma passed into the marble, causing the growth of new minerals in bands parallel to the contact.

Rock types in Ledmore Quarry

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