Granophyre and Hybrid Pipes in a Dolerite Layer of Slieve Gullion

Abstract

This communication deals with an unusual form of relationship between granophyre and dolerite, found in the Tertiary igneous complex of Slieve Gullion, Northern Ireland. The rocks of Slieve Gullion (described by Richey, 1932; Reynolds, 1951; and Bailey and McCallien, 1956) form a pile of almost flat-lying sheets, dolerites alternating with granophyres. The layers are cut by veins of granophyre. In one of the dolerite sheets, which is a sill, granophyre also takes the form of narrow, vertical pipes. From the macroscopic evidence it is argued that the pipes, which connect downward with the vein system, must have formed by the rise of granophyre magma into dolerite magma. Such a "two-liquid" interpretation of relationships between granophyre and basic rock has already been proposed by Wager and Bailey (1953) for certain associations, in St. Kilda and Slieve Gullion, in which the basic rock shows pillow forms within the granophyre. Chilled edges of the basic rock against the granophyre constitute an important part of the evidence adduced by the authors cited. Many of the Slieve Gullion pipes also have fine-grained edges of dolerite at their contacts, at any rate near their lower ends. From various considerations this is interpreted as a chilling phenomenon; and such a conclusion is consistent with the two-liquid theory of pipe formation.