The Structure of the Granophyric Quartz-Dolerite Intrusion of Centre 2, Ardnamurchan and the Problem of Net-Veining

Abstract

The Quartz-dolerite intrusion of Sgùrr nam Meann, Ardnamurchan, is shown to be a ring-dyke which has sill apophyses extending from its outer wall. This is a form of intrusion which has not been recognized before, and its field relations are described in detail. The quartz-dolerite is extensively veined by acid rocks which are ubiquitous in all parts of the intrusion, and most abundant in the sills. From the evidence of the distribution and nature of the veining it is concluded that the acid and basic components of the intrusion must have been ernplaced simultaneously. It is suggested that a gas-charged, acid residual magma may have separated from a crystallizing basic parent under the influence of sudden reductions of external pressure acting upon the system.