The petrology of the Dippin sill, Isle of Arran

Abstract

The Dippin sill, which is emplaced into the Triassic sediments of SE Arran, is a basic alkaline intrusion of Tertiary age within which four main rock types have been identified. Examination of vertical sections through the sill, including two drill holes, indicates that it consists essentially of a thick central crinanite unit bordered by thin marginal teschenite facies with minor units of augite teschenite and basic pegmatite occurring at various levels. The compositions of crinanites, marginal teschenites and augite teschenites plus pegmatites define three distinct chemical trends which formed during a complex continuous differentiation and emplacement process. This process began with the intrusion of a mantle-derived, alkaline, basic magma into a deep crustal reservoir. Crystallization and gravitational settling of olivine and plagioclase resulted in the magma becoming compositionally stratified. Relatively fractionated magma was subsequently expelled from the top of this reservoir and intruded at a shallow depth to form the marginal facies of the sill. This was followed by the main intrusion of less fractionated olivine-bearing magma to form the thick central portion of the sill. Further differentiation occurred during and after the uprise and emplacement of these magmas resulting in the evolution of the three distinct trends.