A contribution to the petrochemistry and petrogenesis of the composite sill of South Bute

Abstract

In 1913, W. R. Smellie described the Composite Sill of South Bute as composed of five sheets: a thick middle dolerite flanked by sheets of quartz-porphyry which in turn are ensheathed by an upper and lower dolerite. He believed the basic rocks to be parts of a single injection which was followed by injections of acid magma along belts of weakness between the massive marginal dolerites and the jointed middle dolerite. Reasons are given in the present paper for inferring that the marginal sheets of dolerite were formed by a first surge of basic magma and the middle dolerite by a later surge, <i>i.e.,</i> the sill is regarded as a composite multiple sill. The dolerites have been contaminated by the quartz-porphyry prior to emplacement and the latter carries xenoliths of dolerite picked up during its intrusion. Smellie’s estimate of the proportion of quartz-porphyry (27 per cent.) contained in the marginal dolerite is confirmed by chemical analyses. It is shown that the uncontaminated basic rock would have had a composition comparable with that of crinanite. A hypothetical mechanism is suggested to account for the emplacement of the acid members of the sill between the upper and middle dolerites and between the lower and middle dolerites. A modification of this mechanism suggests how acid magma obtains access to the interior of a basic intrusion, this being the relationship found in most Tertiary composite sills and dykes.