The Differentiated Dyke of Newmains, Dumfriesshire, and its Contact and Contamination Phenomena

Abstract

I. Introduction Within the Southern Uplands of Scotland, the highly folded Ordovician and Silurian sediments are intruded by a suite of calc-alkaline igneous rocks of Lower Old Red Sandstone age which includes a well-developed dyke swarm. The dykes trend in a north-east and south-west direction and, although individually thin, collectively they represent a considerable body of magma. The only literature dealing with the subject, however, is Teall's generalized account in the Southern Upland Memoir (1899, pp. 625–31), a paper by Read (1926) on the micalamprophyres of Wigtownshire, and a short section in the description by Gardiner and Reynolds of the Loch Doon granite (1932, pp. 19–26). The Newmains dyke, which forms the subject of the present paper, is a member of the Southern Upland swarm and crops out on the hillside immediately north and north-east of Newmains farm, some 4½ miles west of Dumfries and within Sheet 9 of the 1-inch Geological Survey Map (Fig. 1). The dyke is about 15 feet wide and runs for a little more than a mile in a general north-east and south-west direction, in conformity with the strike of the Silurian (Llandovery-Tarannon) greywackes into which it is intruded. It is a typically heterogeneous, differentiated intrusion and consists of three main rock types associated in an intimate manner. The main rock is a granophyric spessartite or markfieldite, which passes on the one hand into a basic, hornblende-rich type and on the other into a leucocratic variety poor in ferromagnesian constituents. These three rock types are