The composite granitic gneiss of western Ardgour, Argyll

Abstract

Summary The Moine Upper Psammitic Group of western Adrgour, Argyll, is steeply folded into an anticline running across country for a number of miles. Along the axis of the anticline is exposed a belt of composite gneisses with a foundation of pelitic Moine schist. The gneisses adjacent to the Moine Upper Psammitic Group are usually oligoclase-biotite-quartz-gneiss still presenting many of the features of pelitic schist. At deeper structural levels the composite gneisses are of granitic composition, containing quartz, microcline, oligoclase and biotite. Replacement textures are pronounced. This composite granitic gneiss develops a subordinate amount of augen-gneiss due to the growth of microcline porphyroblasts. Both the granitic and oligoclase-biotite-quartz-gneiss show migmatitic structures. The latter rock includes types megascopically identical with the lit-par-lit regional injection gneisses extensively developed in adjacent districts of the West Highlands of Scotland. The granitic gneiss was formed by syntectonic advance along pelitic schist of a silica-potash “front” of metasomatism preceded by a Na, Ca, Mg “front” of felspathization, forming oligoclase-biotite-quartz-gneiss. “ Lit-par-lit injection” types of the latter are ascribed to metamorphic segregation of quartz-felspar folia. Both gneisses are replacements essentially in situ. An attempt is made to explain the textural varieties within the granitic gneiss (augen-gneiss, aplite etc.) by the variable interplay of crystallization and earth-movement during granitization, combined in some cases with metamorphic differentiation.