The Petrochemistry of the Ardara Aureole

Abstract

Summary The chemical variation in a single horizon of pelitic schists, both within and without the contact aureole of the granodioritic diapir of Ardara in County Donegal, is given in detail. In both environments the schists have been carefully sampled and some 50 analyses made by using rapid spectrophotometric methods. By establishing an original compositional variation across this horizon it is shown that the contact metamorphism is largely isochemical. Close to the pluton, however, certain slightly higher values for potassium and sodium are recorded and, throughout the aureole, the water content is significantly reduced. The variations in the chemical composition of the rocks are clearly reflected by changes in the mode ; most important are the appearance of a new muscovite and a progressive change in the composition of the ferromagnesian micas. The results of this study make it quite clear that a knowledge of the original chemical variation of the sediments is a necessary prerequisite to the petrochemical study of their metamorphic derivatives. Certain of the contact aureoles of the Donegal Granite Complex in North-West Ireland provide excellent opportunities for the geochemical investigation of metamorphic processes. Almost ideal sites for such researches are given by the beautifully exposed aureole surrounding the diapiric pluton of Ardara. The Dalradian country rocks include a thick horizon of pelitic schists which can be followed into this contact zone, thus providing a single medium in which to determine and compare any changes in chemical composition. This study is concerned with a comparison of the composition of