Studies on the granophyres and related rocks of the Slieve Gullion tertiary igneous complex, Ireland

The Slieve Gullion district is the site of one of the four Tertiary Centres in the North-East of Ireland. The area was mapped during the Geological Survey of Ireland about 1870. Since then a number of more detailed studies of certain aspects of the geology have been carried out. Of these, the more important were the investigations of Richey and Thomas (1932), and Reynolds (1943, 1901).
The Tertiary Centre is neatly inset at the south-western end of the Newry granodiorite complex, of Caledonian age. There are at least two distinct units within the Slieve Gullion centre, a ring dyke and, situated within this, a later layered complex. The ring dyke is made up of two major intrusive bodies, the Porphyritic Felsite intruded into vents opened along the ring fracture in the south-west, and a later Porphyritic Granophyre intruded into the other three-quarters of the ring fracture. (Richey, 1932). The ring intrusions were preceeded by violent explosive activity in the south-west of the complex, and within the vent agglomerates of Forkill, Richey recognised the presence of foundered masses of lavas. These Thomas showed to be petrographically similar to lavas seen at the southern edge of the Antrim Lava Sheet, some 10 miles to the north of the complex, and known to be of Tertiary age. Apart from the very close structural and other similarities with the centres of proven Tertiary age in the west of Scotland, this is the most convincing evidence available in support of a Tertiary age for the Slieve Gullion centre.
From the results of a gravity survey of the north of Ireland, it is considered that the Tertiary Centres, including Slieve Gullion, are underlain by a considerable body of basic rock at a depth of four or five miles. (Cook & Murphy, 1952.)
During the present investigation, the western half of the ring dyke was remapped on the scale of six inches to the mile, particular attention being paid to the Porphyritic Felsite and its associated agglomerate. Specimens collected from the ring dyke, the Central Complex of Slieve Gullion, and other of the Irish and Scottish Tertiary Centres, were examined petrographically, chemically and spectrographically at Oxford, under the supervision of Professor L.R. Wager.
Newry granodiorite forms the country rock to the centre, and at several localities this has been found to be deeply rotted. The rotting is evidently pre-glaciation, since sound granodiorite may readily be found in the overlying drift. Thin sections reveal the preferred alteration of the plagioclase within the granodiorite, and it is considered that the decomposition of the granodiorite is due to hydrothermal alteration of the granodiorite during the evolution of the ring complex. The alteration does not appear to be connected with the Central Layered Complex (Reynolds, 1951).
Thin sections of the gabbro from the small plugs along the south-west of the complex show this to be a relatively iron-rich gabbro.
Richey demonstrated that there had been two phases of intense dynamic crushing of rocks of the ring dyke complex (Richey & Thomas, 1932), and the crushing was shown to be closely connected in place with the granophyre ring dyke. This has been verified, and it is considered to be possible to confine each phase of crushing within fairly narrow limits during the development of the ring dyke. Possible causes for the extensive crushing are examined and the suggestion is made that the crushing developed as a result of the central block of country rook within the ring dyke being pushed upwards, due to the pressure of underlying acid magma. Comparisons are made with other areas. Reynolds (1951) has advanced the suggestion that gas action played an important, and probably predominant part in the formation of the crush rocks, or Cam Lough breccias as Richey termed them. This suggestion is examined and the conclusion reached that dynamic crushing has played the more important role in the formation of the breccias. The Porphyritic Felsite, which is earlier than the extensively crushed Porphyritic Granophyre, has completely escaped the effects of dynamic crushing. This is shown to be probably accidental.
Within the Forkill agglomerates the distribution of the different rock types has been mapped, and from this distribution and other considerations, it appears likely that part of the agglomerate now exposed originated at a deeper level In the vents, being brought into its present position by bodily upwards displacement by the intruding felsite, a suggestion which has also been made by Richey. Small-scale structures within the agglomerates are described and figured. These are attributed to the flow of particle-charged gases through the partially brecciated rocks. The manner of formation of the breccias is examined, and drawing on examples from others of the Tertiary Centres and using the descriptions given by Reynolds (1954) of fluidized systems of solids suspended in gases, the development of the agglomerates is traced and a sequence of decreasing explosive activity described. It is considered that Richey's suggestion that the explosive activity during which the agglomerates were formed is connected with escape of gas from the felsite magma, is correct. The writer would not go so far as to say that the escape of gas took place when the felsite was being intruded into the upper parts of the ring fracture.
Within the Porphyritic Felsite a pattern of fluxion structures has been mapped, this indicates several centres of upwelling of the felsite, and strengthens Richey's conclusion that the large masses of felsite were vent intrusions. Macroscopic and microscopic structures within the felsite are described; these lead to the conclusion that the felsite was emplaced as an extremely viscous and relatively cool mass.
Chemical, optical and X-ray work on the alkali feldspar phenocrysts from the felsite shows that these were originally homogenous 'high temperature' forms, which now show various stages in unmixing from sanidine-anorthoclases. Some high temperature optics are preserved in marginal specimens. From the chemical composition of phenocrysts and rocks, it appears that prior to emplacement the felsite may have been differentiated into relatively sodic and potassic parts, now reflected in slight differences between the inner and marginal parts of the felsite. Despite this, and the evidence of centres of flow banding, there is no evidence of multiple intrusion in that no internal chilled contact was found. The problem of embayments and apparent corrosion features in phenocrysts, especially quartz, is examined and the suggestion made that for the ring dyke rocks these features may equally well be due to rapid growth of the phenocrysts, though the evidence available is inconclusive either way. Fayalite is described from the felsite (and the granophyre), and the conditions suitable for the precipitation of iron-rich olivine from an acid melt are discussed.
It had been suggested that the felsites were possibly welded tuffs. This is not thought to be so, but the presence of numerous fragments of phenocrysts is taken as an indication that at some stage the felsite magma may have violently disintegrated, possibly when the gases were given off to form the agglomerates. It is thought that at this stage the small sedimentary xenoliths found throughout the otherwise inclusion-free felsite, may have been incorporated at this stage.
Within the Porphyritic Granophyre ring dyke a zone of inclusions has been mapped. The inclusions are practically identical chemically to their enclosing granophyre, but have a very different, and distinctive, texture consisting of quartz and idiomorphic feldspar intergrown, sometimes in spherulitic aggregates. Through the inclusions there are small pieces of granodiorite, and on Slieve Gullion some of the altered granodiorites develope somewhat similar textural features. The suggestion is made that the inclusions may represent granodiorite altered at depth by basic magma. Texturally identical inclusions are found in two of the Tertiary granites from Skye, these are described.
From evidence provided by the contacts of granophyre and felsite, the granophyre would appear to have been emplaced very soon after the felsite, possibly when the inner parts were only partially solid. Reynolds has made the suggestion that the granophyres were emplaced as tuffisites, and altered to granophyre subsequently. This is discussed, and reasons advanced for rejecting her suggestion.
Trace element analyses of various rocks from Slieve Gullion, Mourne, Antrim and Skye are described. Variations are found within the rock types of Slieve Gullion and Mourne, and from one to another. It is suggested that the differences may be accounted for if the rocks be considered as representing different stages in differentiation from a more basic parent, or parents. The relatively high Rb/K ratio for the Mourne rocks analysed is thought to indicate that these are extreme differentiates. The high zirconium in certain of the acid rocks from the Tertiary Centres in discussed. The concentration of Zr in the acid rocks may originate through a combination of selective remelting and assimilation of country rock by basic magma, and strong fractionation within the magma.
Duplicate analyses were made of rocks from Skye that had been previously analysed for their major and trace elements. The trace element determinations were carried out at the Macaulay Institute, Aberdeen, in the first place, and the duplicates made in the Department of Geology and Mineralogy at Oxford provide a standard by which results from the two laboratories may be compared.