Rockall central complex, Hebridean Igneous Province
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
This body of peralkaline granite forms a precipitous islet, about 20 m high and 30 m across, in the Atlantic Ocean about 250 km west of St Kilda (Harrison, 1975; P914120). The nearby micro-gabbro of Helen's Reef is thought to be part of the same intrusive centre, and a variety of igneous rocks have been recorded from the adjacent sea bed (Morton and Taylor, 1991). Gneisses, which have also been recovered from the sea bed, form the country rock and are Early Proterozoic in age. The granite is unusually rich in aegirine and riebeckite (13 to 27 per cent), and there is a particulary high concentration of ferromagnesian minerals (up to 68 per cent) in the variety 'rockallite', which forms vein-like segregations in the granite. Accessory minerals include elpidite, rare monazite and the barium-zirconium silicate bazarite (first described from Rockall). Leucophosphite occurs as a late-stage replacement mineral, formed by reaction between phosphate-rich fluids derived from sea-bird droppings and the potassium feldspars and ferromagnesian minerals of the granite.