Ardilistry Bay, Islay

Along the coast of Islay to the east of Port Ellen metamorphosed sills account for up to half of the succession. One of the sills, exposed along the shoreline of Ardilistry Bay, is quite rare in the British Isles.

Metapyroxenite rocks are relatively rare in the British Isles. They are found in the Lewisian Gneiss Complex of north-west Scotland, which are generally at a high metamorphic grade (amphibolite- to granulite-facies), and occur as very low-grade rocks, such as in the Unst and the Ballantrae Ophiolite-complexes. However, it is possible that the metapyroxenite at Ardilistry Bay may be the only greenschist-facies metapyroxenite preserved and exposed in the British Isles.
The metapyroxenite layer is up to 3 m thick and consists almost entirely of coarse-grained actinolite, a Mg-rich calcic amphibole that has replaced original clinopyroxene during metamorphism. It occurs in a sill that also has a 1 m-thick layer of meta-anorthosite, representing an original plagioclase-rich layer. The sill, therefore, is an excellent example of a layered basic igneous intrusion that has been metamorphosed to the greenschist facies. Whereas, deformation and fluids associated with the metamorphism have altered the margins of the sill, which now has a schistose fabric, the inner part has retained the original igneous textures, particularly in the metapyroxenite and meta-anorthosite layers. The good exposure, and the very distinctive appearance of this rock, enhances the geological attractiveness of an already geologically fascinating small corner of Islay.

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