The Contaminated Tonalites of Loch Awe, Argyll

Abstract

I. Introduction At the south-eastern corner of the great Cruachan “granite” mass, near the head of Loch Awe, there is a small area of the “granite” (tonalite) which is isolated from the main mass (see Fig. 1). It is bounded on the north and west by diorite and porphyrite, on the east by quartzite (the Glencoe quartzite) and on the south by a series of variable sediments which are now thermally metamorphosed to a considerable degree. The present contribution is concerned mainly with a small portion of the south-eastern side of this tonalite area. At its immediate contact with the surrounding sediments the tonalite becomes extremely rich in xenoliths in certain parts, and holds large blocks and smaller fragments of the sedimentary rocks in all stages of disintegration and dissolution. As exposed in the old quarry, the xenolithic band is about 30 feet wide, but in other parts it is much narrower. The phenomena to be described are, therefore, of strictly local character, though this does not detract from their interest. Outside this xenolithic band there is little scattering of the xenoliths, and the zone passes sharply into tonalite which is almost free from them. As the sedimentary series into which the tonalite is intruded on the southern side is banded, the xenoliths vary from one area of the tonalite margin to another (Fig. 2). Locally, they have modified the original tonalite in different ways, and in the following pages the main types of contaminated tonalite with their accompanying xenoliths