The Ard Bheinn Area of the Central Igneous Complex of Arran

Abstract

Summary The complex is largely surrounded by Old Red Sandstone formations, which were evidently domed up at an early stage in its development. A succession of sediments ranging in age from Permian to Cretaceous, surmounted by Tertiary lavas, is represented within the complex as “remanié masses” which are interpreted as relics of a block that foundered during the formation of a volcanic caldera, the main subsidence being estimated at 3000 feet. Subsequently there arose on the caldera floor a series of volcanic cones, characterized in general by the sequence andesite-dacite-rhyolite, but also showing a great diversity of pyroclastic formations. The position of the various centres is recognized by the disposition of concentric and radial structures. Quasi-horizontal formations in the most elevated parts of the Ard Bheinn area are considered to correspond to remnants of the basal portions of the volcanic superstructures. The andesites and andesitic breccias are evidently extrusive in character, but the later dacites and rhyolites are represented only by their intrusive equivalents. Finally, granites were emplaced, mainly in conformity with the general structure of the caldera, and in all but the Ard Bheinn area are the prevalent formations at the present level of erosion. I. Introduction The complex occupies an oval area about three miles from north to south and nearly four miles from east to west almost in tne centre of the Isle of Arran and is emplaced within strata ranging in age from Old Red Sandstone to Permian. Topographically the area is a dissected plateau, the highest