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Summary Full report
Slieve Gullion Ring - Overview
Rec. Number:1118File Number:  
Locality Type:Crag, Crags, Inland exposure Status: PASSI
Grid Reference: J025203 Centroid
County: Armagh District:Newry & Mourne District Council
Period:Tertiary, Devonian
Stages:Devonian Undifferentiated, Palaeogene
Lithostrat:Newry Granodiorite, Slieve Gullion Complex
Site Description
Spectacular topographic expression of ring dykes. Type localities for some notable hypotheses such as fluidization, acid/basic magma mixing, transformation of effusive volcanics to coarse grained igneous rocks and ring-dyke emplacement.
The Ring of Gullion in South Armagh occurs SW of the town of Newry and immediately W of the Narrow Water, which connects Newry with Carlingford Lough. The more resistant acid igneous rocks of the ring itself can be traced as an almost complete circle, broken by NW-SE faults, from Sturgan Mountain and Sugarloaf Hill in the north, just W of Camlough Village, to Courtney Mtn., Slievenacappel, the smaller hills above Ballinasack Bridge, Mullaghbane Mtn., Slievebrack, Croslieve, and east of Forkhill village over Tievecrom, Daaikilmore, Slievebolea to Feede Mtn. Here the circle is broken by a NW-SE belt of later intrusions but continues north of Anglesey Mtn. above the Narrow Water to Fathom Mtn. and so to Ballymacdermot and Camlough Mountains. A more prominent NW trending wrench fault along Cam Lough itself separates this end of the ring from Sugarloaf Hill. These craggy and heath covered hills rise some 1-200 m above the lowlying land within the ring. An earlier Porphyritic Felsite builds the hills in the SW sector and is closely associated with Vent agglomerates. A later Porphyritic Granophyre defines the rest of the ring. Both rocks are intruded up a ring-fault and are associated with some brecciation due to continued movement on this fault. A later phase of eruptive activity produced a NW-SE belt of gabbro and granophyre - the central mass of Slieve Gullion which rises to a height of 573 m O.D. and stretches from Lislea in the north to Foughill, Carrickarman, Anglesey Mtn., and across the national border to Clermont Carn.
The ring structure was mapped by Egan, Nolan, and Traill in the 1870s but it was Richey and Thomas (1932) who first appreciated the nature of the ring and prompted the more detailed mapping of the Porphyritic Felsite ring-dyke by Emeleus (1962). Griffith (1840) noted the igneous origin of the older and younger granitic rocks of Slieve Gullion and their relationship to the basic 'greenstones'. He also appreciated the essential layered structure of this central massif. Reynolds (1937-54) explored the rocks of the NW belt and dated the small gabbro masses of the ring. Bailey and McCallien (1956) reinterpreted the rock relationships in this NW belt and drew attention to the co-existence of both acid and basic magmas: Elwell (1958) and with colleagues (1974) ably illustrated this latter association; Gamble et al (1976) explored the sub-surface rock sections exposed by a proposed pump storage hydroelectric scheme and further supported the work of Bailey and McCallien. More recent work has been highly specialised and geochemical.
The Tertiary central complex of Slieve Gullion is emplaced within an almost detached lobe of the older Caledonian 'Newry' Granodiorite at its SW end. Little remains of the basaltic shield volcano which once covered the site, but in the volcanic hearth now exposed by erosion one can see evidence for subsidence of the volcanic pile along a ring-fault some 20 km in diameter. This probably resulted in a summit caldera, though movement on the ring fault probably resulted from resurgence as well as subsidence. Movement of an acid magma up the SW quadrant of the ring-fault accompanied by explosive degassing produced spectacular vents filled with fragments of country rock. These vent agglomerates can be traced from Mullaghbawn Mtn. round to Slievebolea; in places they are composed almost entirely of Newry Granodiorite or Lower Palaeozoic greywackes, elsewhere they carry large slumped masses and smaller fragments of basaltic and trachytic lavas. The acid magma congealed in the ring-fracture and in the vents as a Porhyritic Felsite; flow structures within the rock indicate a steep outward dip to the ring-dyke - the outer felsite, but the vent intrusions emplaced within the bounding ring-fault are irregular in outcrop and apparently sheet-like with a dip of flow bands to the SW. Confocal banding in the rock points to feeding channels within the ring-dyke. In places the Porphyritic Felsite contains broken phenocrysts and shows eutaxitic flow banding; some disruptive vesiculation of the rising magma may have produced ash flow material now welded and compacted to a tuffisite.
A slightly later Porphyritic Granophyre ring-dyke occupies the ring-fault in the remaining quadrants. It may be a multiple intrusion with a xenolithic core to the NW near Lislea. Its steep outward dip is evident in the disused quarry W of Cam Lough (lake) and at its margins later movement, possibly upward movement on the ring fracture, has produced extensive crushing - mylonite veins and the Camlough Breccias.
The ring-dykes are cut by a NW belt of basic and acid igneous rocks which build the sheeted centre of Slieve Gullion and extend to the SE through Foughill and Carrickcarnan. Most workers since Griffith have recognised the layered structure of this central complex. Reynolds (1951) identified 13 layers and interpreted them as original effusive volcanics - rhyolites, basalts, and tuffs, now recrystallised to dolerites, gabbros, granophyres, and microgranites; two olivine dolerite sills, layers 6 and 10, intruded the volcanics. This actualistic transformation was attributed to the pneumatolytic and hydrothermal phases of volcanicity during the extended lifetime of this shield volcano, and associated with the repeated formation of subsidence caldera. The acid net-veining of these rocks was linked to the escape of incandescent tuff flows through the fractured rock pile. Bailey and McCallien (1956) regarded all the rocks as intrusive with screens of Newry Granodiorite between; the lowermost layer, the Lislea Granophyre, they figured as a peripheral, steeply transgressive, intrusion probably integral with the summit microgranites; the whole an inner series of ring-dyke intrusions. Drill core and tunnel sections, essential to the hydroelectric pump storage scheme (Gamble et al. 1976), bear out Bailey and McCallien's claim. The realization that basic magma could chill against acid magma (Wager and Bailey 1953) led to a reinterpretation of many of the granophyre/dolerite contacts; the pillow lavas of Reynolds became sacs and lobes of basic magma chilled against the acid melt which enveloped them. Elwell (1958) described pipes of granophyric and hybrid magma rising from a remelt of acid igneous rock at or within the base of layer 10 into the still liquid dolerite magma of this intrusive sill. Curvilinear and rectilinear net-veins were defined by Elwell et al (1974) and related to the mixing of two contrasting magmas, giving lobate and crenulate margins, and when the basic component had solidified its penetration by a very fluid gas-rich acid melt along planar fractures.
To the SE where this NW Belt of acid and basic rocks intersects the bounding ring-fracture a stock of granophyric microgranite cuts both the Porphyritic Granophyre ring-dyke and the rocks of the NW Belt and is itself cut by basaltic cone-sheets belonging to the Carlingford central complex.
In the past the Slieve Gullion central complex of Tertiary volcanic rocks has attracted geologists and vulcanologists from all corners of the world. Within Ireland it is the finest example of a Tertiary igneous centre and within the British Isles it is the most spectacular expression of a ring-dyke intrusion. Internationally it is known as a site where many new hypotheses were proposed to explain the unusual rock relationships; hypotheses that were accepted and used by geologists worldwide. These are:
1. Cauldron subsidence Bailey in 1909 first proposed this mechanism to explain the occurrence of ring-intrusions emplaced around a bounding ring-fault in Glen Coe. In 1924 he applied his hypothesis to the Tertiary igneous rocks of Mull. In later years Richey found many similar structures in both Scottish and Irish Tertiary volcanic centres.
2. Transformation Reynolds who strongly believed in the granitisation of crustal rocks to account for the occurrence of huge bathyliths of granite, applied a similar argument to coarse-grained intrusive igneous rocks in the very stronghold of the magmatists. Her actualistic interpretation attracted considerable attention and dispute.
3. Fluidization Reynolds accounted for the extensive net-veining of dolerite by granophyre in the NW Belt by explosive release of hot gases and vapours carrying finely comminuted rock fragments through the highly fractured rock pile. This tuffisite solidified to granophyre. This concept proved far more acceptable to geologists than her transformation of volcanics to seemingly intrusive rocks and has been used repeatedly since.
4. Back-veining Granitic veins in a basic igneous rock imply a later age for the intrusive granite. Reynolds demonstrated for some of the Slieve Gullion rocks that basic magma which intruded an older granitic rock would be able to melt it and the rheomorphic acid melt would stay liquid after the basic magma had congealed and in some instances back-vein it.
5. Mixing of two contrasting magmas Wager and Bailey accepted the possibility of back-veining, but where the basic component showed clear chilled margins against the acid, argued that when two magmas of contrasting composition came in contact, the basic melt at 1100 deg. cent. would chill against the cooler acid melt at 7-800 deg. cent. Further the interface between the two melts would persist because of different viscosities and become lobate or crenulate by flow movement. Prolonged stirring of the two melts would inevitably lead to hybrid melts. The NW Belt offers numerous fine examples of such relationships between dolerite/gabbro and granophyre/microgranite. This concept too found worldwide application.
Since the geological structure of the Slieve Gullion region is so spectacularly displayed by the topography, this aspect should be fully protected within the 'Ring of Gullion' Area of Outstanding Natural Beauty. The most important features would be that of the ring itself and the glacial tail above Drumintee.
Further there should never be any lack of exposure of all the principal rock types. The geological sites of interest to the Earth Science Conservation Review can then be limited to special rock types and rock relationships, preferably those figured in the literature. In some instances the localities are so overgrown by afforestation that access is extremely uncomfortable and the known localities difficult to find; these sites will be mentioned but designation need not be considered until the ground is cleared and resurveyed.
The following sites within the Slieve Gullion Ring are Proposed Areas of Special Scientific Interest. For site specific information please see the associated reports.
Ring-dyke sites: Key Site 1119 - Cam Lough Quarry Key Site 1120 - Crooked Road Key Site 1121 - Mullaghbane North Key Site 1122 - Mullaghbane Central Key Site 1123 - Mullaghbane South Key Site 1124 - Glendesha Forest
NW Belt sites: Key Site 1125 - Slieve Gullion Key Site 1126 - Cloghinny 'pillow lavas' Key Site 1127 - Sarah Daly's Bridge Key Site 1128 - Forest Quarry
Please note;
I - An access tunnel driven into the NE shoulder of Slieve Gullion, as one stage of a proposed hydroelectric pump storage scheme, offers a continuous section through the lower dolerites of the NW Belt. Layer 1 is shown to be in steep contact with the dolerite thus supporting Bailey and McCallien's claim of a ring-dyke intrusion. The dolerites yield abundant examples of granophyric net-veining; some are due to mixing of two contrasting magmas (the curvilinear net-veining of Elwell et al), others are of rectilinear net-veining emplaced after the basic component solidified and then fractured by explosive release of volatiles from the acid melt. The tunnel is currently sealed off and the lower end at least is probably flooded. It does offer a remarkable insight into the geology of the NW Belt and this aspect should be borne in mind when the fate of the tunnel is in question.
II - The area from the E slopes of Tievecrom to the summits of Daaikilmore and Slievebolea has made a useful field excursion area in the past. A wealth of different rock types associated with the ring-fault can be examined, though inter-relationships are more difficult to establish. Good contacts between the Porphyritic Felsite and the vent agglomerate are now obscured by afforestation and the whole area is badly overgrown. Any improvement in the quality of the rock exposures should be noted and the area reassessed.

No Notes

Minerals:No data
Rocks:Dolerite, Felsite, Gabbro, Granodiorite, Granophyre
FossilGroups:No data
Fossil List:
Structures:fluidization, magma mixing, net veins, ring-dyke, ring-fault
NonGeol:Area of Outstanding Natural Beauty
Length:No dataWidth:No dataHeight:No data
Depth:No dataArea:No data  
Approach:Slieve Gullion itself lies approximately 7 km SW of Newry, and 2 km immediately W of the village of Meigh, which can be reached by leaving the main Dublin road at the Dublin Road Bridge just south of Newry.
Restrictions:Not entered
Uses:Not entered
Potential:Not entered
Educ. Level:Not entered


None entered

Map No:None entered
Rec Type ESCR report Recorder:  
Enterer: E M Porter
Updates: 3 May 2003 / 31 JAN 97 / 27 JAN 97 / 24 JAN 97 / 23 JAN 97
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