IV.—The Geology of Slieve Gullion, Foughill and Carrickcarnan: An Actualistic Interpretation of a Tertiary Gabbro-granophyre Complex

Abstract

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.

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

“…Recently D. L. Reynolds (1950) has attempted to explain net-veining in terms of intrusive acid tuff lubricated and transported by abundant streams of gas. The broken, and slightly displaced, blocks of dolerite are compared with phenomena displayed in the Swabian tuff-pipes and described by Cloos.…”

Partially fused plagioclases in the rocks of Slieve Gullion

“…From the presence within the transgressive granophyres of oligoclase crystals that show oscillatory zoning like those of the Caledonian granodiorite, and from chemical considerations, the parental tuff can be identified as fragmented Caledonian grano diorite derived from the basement rocks (Reynolds, 1951). Similar oligoclase crystals derived from the Caledonian grano diorite are present within the layers of granophyre on Slieve Gullion.…”

“…Evidence has already been presented from which it has been deduced that these transgressive granophyres are tranformation products of intrusive acid tuff emplaced by gas-streams that rose along the arcuate faults (Reynolds, 1951). This interpretation is based on (a) Cloos's (1941) structural work on the Swabian tuff-pipes, (b) Richey's (1930) observation that glassy tuff, the shards of which are aligned by flowage, is emplaced as net-veins in Ardnamurchan, and (c) the assemblage of concordant evidence from the Slieve Gullion area.…”

The Viscosity of Rock-Glass of Granitic Composition under Various Physical Conditions: a correction and an Addendum

“…In the latter event, if the temperature were such that the droplets were liquid (saturated with water) a variety of ignimbrite would be expected to remain after escape of the gaseous transporting agent. Ignimbrite does, in fact, occur as net-veins in Ardnamurchan (Reynolds, 1951(Reynolds, , 1954. Furthermore, ignimbrite, like the powdered synthetic glass used in hydrothermal synthesis by Bowen and Tuttle, may be equally expected to devitrify or crystallize under appropriate physical conditions.…”