IV.—The Geology of Slieve Gullion, Foughill and Carrickcarnan: An Actualistic Interpretation of a Tertiary Gabbro-granophyre Complex
Authors: Reynolds Doris L. 
Published: January 2013

Synopsis:

An interpretation of the Tertiary rocks of the area, which are shown to be denudation relics of a gigantic volcano, is found by following the actualistic principle, that is by interpreting the unknown with reference to the known or actual. “Actualistic”, i.e. with reference to actual causes—commonly used by continental geologists—is chosen in preference to “uniformitarian” since the latter stands in contradistinction to “catastrophic”, and is not, therefore, altogether appropriate to some of the more violent aspects of volcanic activity. Gabbros and granophyres, hitherto thought to be plutonic intrusions emplaced within Caledonian granodiorite, are shown to be highly metamorphosed basaltic and rhyolitic lava-flows, agglomerates and tuffs, together with some gabbro sills. These rocks form a layered series which is almost horizontal at Slieve Gullion, and dips at various angles at Foughill and Carrickcarnan. A Tertiary ring-dyke of granophyre and felsite encircles the layered rocks and outlines a caldera within which the basaltic lavas (including pillow-lavas) of Slieve Gullion accumulated. During the formation of a later caldera these lavas became inset within the underlying Caledonian granodiorite. The acid layers of Slieve Gullion represent rhyolites, incandescent tuff-flows, and agglomerates, the materials of which were derived from the Caledonian granodiorite. The time of eruption of these acid rocks is correlatable with periods of caldera formation.

The layered series is dissected by arcuate faults, and contacts previously thought to be intrusive are now demonstrated to be faulted. Transgressive granophyres follow the faults and form intrusion-breccias and net-vein systems with the layered series. Evidence is presented for concluding that these granophyres originated as glassy acid tuffisite—an intrusive pyroclastic rock (Cloos, 1941)—derived from the basement Caledonian granodiorite, and emplaced by rising gas-streams. That the gas sometimes had a temperature high enough to fuse basalt is witnessed by the fact that the basaltic rocks commonly become exceedingly fine grained where they adjoin transgressive granophyre. Such fine-grained edges are cross-cutting to the layers and cannot therefore be interpreted as normal chilled edges. Hybrid rocks, e.g. marscoite, are associated with the transgressive granophyres. They are shown to represent mechanical mixtures of acid tuffisite, carried upwards by rising gas-streams, with fragmented (tuffisitised) and partially fused basic material derived from the walls of the channels. The transgressive granophyres and associated hybrids are related in origin to incandescent tuff-flows, and are regarded as marking the routes by which such flows reached the surface.

The metamorphism of the basaltic flows, and the transformation of the acid layers and transgressive acid tuffisites to granophyre, were probably accomplished during the pneumatolytic and hydrothermal phase of volcanic activity. Transformation of the acid rocks to granophyre involved introduction of K and Si, and removal of Na, Al, Fe, Ca, Ti, P and Mn. Similar chemical interchanges transformed basaltic rocks at Foughill and Carrickcarnan to quartz-dolerites. The subtracted materials have been located in small-scale basic fronts margining granophyres; within masses of gabbro that sank within gas-tuff streams from higher levels; and in dispersed form within the basic rocks of Slieve Gullion.