A composite tholeiite dyke at imachar, Isle of Arran: its petrogenesis and associated pyrometamorphism

Abstract

This NW.-SE. dyke comprises two texturally distinct tholeiitic members. The outer (earlier) bears porphyritic plagioclases with essential olivine, plagioclase, and augite in a chloritized mesostasis; the inner carries porphyritic plagioclases in an olivine-free matrix showing ophitic and intersertal textures and disseminated varioles. The outer component had little metamorphic effect on its country-rock, but where the inner transgresses the outer to come into contact with the schistose-grits the latter are transformed to buchite. The outer component, after initial chilling, crystallized as a virtually closed system; the inner, on the contrary, shows no signs of chilling and bears evidence of crystallization as an open system. The variolitic structures in the inner component result from liquid immiscibility between droplets of acidic melt and basic host magma, the former originating as buchitic liquid caught up by the dyke magma at its contacts with mobilized country rock. Both components afford textural evidence of the immiscibility of a sulphide melt in the tholeiitic magma. The petrography and thermal behaviour of the inner dyke-member support its recognition as a probable former feeder to fissure eruption.