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The small peralkaline microgranite intrusion that forms the island of Ailsa Craig in the Firth of Clyde represents the southern-most intrusion of the British Palaeogene Volcanic Province (BPVP) within Scotland. The granitic rocks of Ailsa Craig, famous for their use in making curling stones, are thought to be the result of fractionation from a mantle-derived basic magma with no crustal contamination. The rocks of Ailsa Craig also contain the comparatively rare alkaline ferromagnesian minerals riebeckitic arfvedsonite, hedenbergite-acmite and aenigmatite. These factors make the Ailsa Craig intrusion unique in the BPVP. The distinctive ‘spotty’ appearance of the Ailsa Craig granites, which is invaluable as a tracer in establishing Pleistocene glacial transport, results from the close association of ferromagnesian minerals with drusy cavities. This suggests that, unlike the more common dark minerals, they formed late in the cooling and crystallisation history of the magma. Ailsa Craig was selected as part of the Geological Conservation Review network covering the rocks of the British Palaeogene Volcanic Province, which for historic reasons, is known as the ‘Tertiary Igneous’ GCR block. Site descriptions for GCR sites of the Tertiary Igneous block are contained in the 4th published GCR volume (Emeleus and Gyopari, 1992). Ailsa Craig GCR site was, however, omitted for reasons unknown. A full GCR site description for Ailsa Craig is, therefore, provided in this paper in order to complete the published description of the Tertiary Igneous GCR block. |
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