Earth Science Conservation Review
| Mournes - The Granites - Overview | Down |
| Site Type: | Mountains |
| Site Status: | |
| Council area: | Down District Council, Newry & Mourne District Council |
| Grid Reference: | J1815 |
| Google maps: | 54.07113,-6.19743 |
| Rocks | |
|---|---|
| Rock Age: | Tertiary (Eocene) |
| Rock Name: | Granite |
| Rock Type: | Basalt, Felsite, Granite, Greisen, Greisenized Granite, Porphyry, Quartz porphyry |
| Interest | |
| Minerals: | Beryl, Cassiterite, Chrysoberyl, Fayalite, Fluorite, Hematite, Peridot, Stilbite, Topaz, Vermiculite, Zinnwaldite |
| Other interest: | cone-sheet, drusy, dyke, laccolith, magma mixing, multiple intrusion, ring-dyke, ring-fault, stoping, xenocrysts, xenoliths, Contact, Intrusion |
Description:
INTRODUCTION TO THE GEOLOGY OF THE MOURNE MOUNTAINS
The Mourne Mountains rise abruptly from the sea on an 8 km front just south of Newcastle, Co. Down, and extend westwards for some 21 km. They mark the site of a Central Igneous Complex of Tertiary age comprising an east and west centre of granitic intrusion with associated dyke-swarms and cone-sheets. Most of the granites are dated at 56 Ma BP using Rb-Sr and Ar-Ar methods.
The sequence of events which built this complex probably began with an access of basaltic magma deep in the crust below the region; crustal extension focussed on this structural weakness and a swarm of NW-SE fractures were filled with basic, intermediate, and acid intrusions. Many fissures were intruded repeatedly to form multiple and composite dykes. Dyke emplacement then continued to punctuate the later phases of igneous activity in the area. Individual dykes carry xenoliths of plutonic basic igneous rocks, gabbro and anorthosite, the only direct evidence of an underlying magma source inferred from the gravity 'high' recorded over the whole area of the Mournes, Slieve Gullion and Carlingford.
The next intrusive episode was more localised. Magmatic pressure beneath the Mourne centre resulted in distension and fracturing of the supra-crustal rocks. Cone-sheet intrusions invaded some of these fractures; inwardly dipping sheets of felsite and quartz porphyry encircle the later granites; their outcrop is elongated in an ENE-WSW direction. Around the Eastern Mournes the conical fractures were pre-heated by basic magma and the cone-sheets have basic margins with some mixing and hybridization between the contrasting magmas.
Granitic emplacement followed; this was the main phase of igneous activity and responsible for the spectacular mountain scenery of the present-day. The various granites were labelled G1 - G4 by Richey (1928) and later workers have used and expanded his abbreviations. Three intrusions, G1, G2 and G3, invaded the Lower Palaeozoic rocks of the Eastern Mournes; all show flat-lying or gently inclined roof contacts and, all but G1, have steep wall contacts against the country rock, now baked to diopside and biotite bearing hornfelses. Aplite, quartz porphyry, and composite basalt-quartz porphyry dykes cut the granites. Cauldron subsidence on ring-fractures is a possible emplacment mechanism and, as is often the case, the centre of crustal collapse shifted. A further two granites- G4, G5 - of similar nature were then emplaced in the Western Mournes.
Basalt dykes cross-cutting the granites mark the final igneous episode.
There is no evidence of surface vulcanicity associated with this centre of igneous activity as there is at Slieve Gullion and Carlingford. An oxygen and hydrogen isotopic study (McCormick et al. 1993) shows only a limited penetration of meteoric water into the intrusive complex when compared with other British Tertiary subvolcanic complexes. It is the margins of the plutons and the country rocks and minor intrusions which show the effects of water-rock interaction.
THE GRANITES
The mountains' conspicuous peaks and ridges, up to 800 m high, are formed mainly of granite. Rare minerals occur in the drusy cavities of the granite, a feature of high-level/near-surface granites, and this feature alone has attracted many geologists to the area.
Berger (1816) noted two types of granite, one porphyritic and bearing hornblende, the other granular in texture and characterised by biotite. Griffiths (1835) accurately traced the boundary of the granite on his geological map of Ireland (4 miles to the inch). Both he and Bryce (1853) claimed a Tertiary age for the intrusions, an age supported by the chemical analyses of Haughton (1858). Traill (1870-73) mapped the Mourne area in detail for the Geological Survey and with Hull (1871) published his results. Hull (1881) also wrote the explanatory memoir to the 1-inch sheets which covered the area. Though Traill failed to recognise the successive intrusions which make up the granite complex he did appreciate the basic structure of the granite mass.
Richey (1928) gave the first detailed petrography of the various granite intrusions and interpreted their structural relationships as a consequence of cauldron subsidence above a deep seated volcanic hearth. He defined two centres of activity, an early centre in the east which later shifted some 9 km to the WSW. Three granite ring-dykes, G1 (Feldspathic), G2 (Quartzose), and G3 (Aplitic) in order of age, were mapped within the Eastern Mourne centre, and though he only described a single granite ring-dyke (G4-Pink variety) from the Western Mournes he prompted Emeleus (1955) to remap and subdivide the granite into G4 (medium and even grained, pink/purple in colour) and G5 (fine and even grained, blue-grey in colour). Richey (1928) effectively invoked the mechanism of sub-surface cauldron subsidence, and intrusion up the ring-fractures to emplace within the upper crust ring-dykes with laccolithic tops. Emeleus (1955) agreed with this mode of emplacement and further noted a WSW migration between the foci for the two Western Mourne granites. The absence of any foliation within the Mourne Granites and the general lack of country-rock enclaves lends support to this mechanism for passive emplacement of granite magma.
Later work has been largely geochemical in its nature but, under the guidance of Meighan (see Meighan et al. 1984 and the works of Hood 1981, Gibson 1984, and McCormick 1989) has resulted in a large scale rearrangement of the geological map of the Eastern Mournes, see the special 1-inch sheet for the Mourne Mountains (1978). Here Richey's G1 is restricted in occurrence to small roof cappings on Slieve Donard, Slieve Commedagh, and Slieve Corragh and the steep wall/ring-dyke parts of G1 are incorporated into the geochemically similar G2 as pulses of the same magma; a concept first illustrated for the original G2 ring-dyke by Harry and Richey (1963). Hood (1981) found several magmatic pulses in both the G2 and G3 Granites; Gibson (1984) noted two pulses for the G4 and another two for the G5 intrusions.
Robbie (1955) described extensive pneumatolytic alteration in the Slieve Binnian Tunnel; surface expression of this kind of kaolinization is small-scale and local. Nockolds and Richey (1939) found greisen veins in the granite especially well developed at Lindsay's leap. McCormick's (1989) isotopic studies throw light on the role of meteoric water during and postdating the intrusive activity.
The radiometric age of the granites G1 to G4 is established as 56 Ma BP (Gibson et al. (1995)); that of G5 is less certain at the younger age of 51 Ma (Rb-Sr), 54 Ma (Ar-Ar).
All the granites, except G1, contain drusy cavities where exsolved gases collected against the cooler marginal/roof zones of an intrusion. It is these cavities which yield a variety of unusual minerals. Haughton (1856) records beryl, chrysoberyl, topaz, fluorspar and peridot. Smith (1864) found stilbite, Sollas (1890) identified zinnwaldite, Seymour (1903) recorded cassiterite, and Greg and Lettsom (1858) and Richey (1928) discovered fayalite. Richey (1928) completes the list with a bright green mica belonging to the vermiculite group, hematite, and kaolin.
CONSERVATION
Sites of geological importance must represent not only the rock types of this igneous complex but their inter-relationships and relative ages. For convenience these sites can be selected from the principal episodes of the igneous history of the Mourne Mountains outlined above; the sites chosen are listed below. For site specific information see the associated reports.
Dyke Swarm sites:
Mournes - Green Harbour Dyke no. 55 (Site 1112)
Mournes - Dunmore Head, dyke nos. 60, 61, 63 (Site 1109)
Mournes - Samuel's Port, Annalong, dyke no. 124 (Site 1108)
Cone-Sheet sites:
Mournes - Bloody Bridge Cone-sheet (Site 1102)
Mournes - Glasdrumman Port Cone-sheet (Site 1111)
Mournes - Gruggandoo quartz porphyry sheet (Site 1106)
Granite sites:
Mournes - Eagle Rock (Site 1105)
Mournes - Bloody Bridge River section (Site 1104)
Mournes - The Diamond Rocks (Site 1110)
Mournes - Lindsay's Leap (Site 1103)
Mournes - Ben Crom (Site 1100)
Mournes - Spelga Dam (Site 1114)
Mournes - Rocky Mountain (Site 1113)
Mournes - Eagle Mountain, Slievemoughanmore, Pigeon Rock Mountain (Site 1101)
Mournes - Kilbroney River section (Site 1107)
For general information on the cone-sheet intrusions associated with the Mourne Granites, and the Mourne Mountains dyke swarm, see;
Mournes - Cone-sheet and Dyke Swarm - Overview (Site 1131)
In the site descriptions the older petrographic terms of Tomkeieff and Marshall (1935) are used for the dyke and cone-sheet intrusions. Some of these are now out-dated but it would entail a detailed petrographic study to up-date them.
The Mourne Mountains rise abruptly from the sea on an 8 km front just south of Newcastle, Co. Down, and extend westwards for some 21 km. They mark the site of a Central Igneous Complex of Tertiary age comprising an east and west centre of granitic intrusion with associated dyke-swarms and cone-sheets. Most of the granites are dated at 56 Ma BP using Rb-Sr and Ar-Ar methods.
The sequence of events which built this complex probably began with an access of basaltic magma deep in the crust below the region; crustal extension focussed on this structural weakness and a swarm of NW-SE fractures were filled with basic, intermediate, and acid intrusions. Many fissures were intruded repeatedly to form multiple and composite dykes. Dyke emplacement then continued to punctuate the later phases of igneous activity in the area. Individual dykes carry xenoliths of plutonic basic igneous rocks, gabbro and anorthosite, the only direct evidence of an underlying magma source inferred from the gravity 'high' recorded over the whole area of the Mournes, Slieve Gullion and Carlingford.
The next intrusive episode was more localised. Magmatic pressure beneath the Mourne centre resulted in distension and fracturing of the supra-crustal rocks. Cone-sheet intrusions invaded some of these fractures; inwardly dipping sheets of felsite and quartz porphyry encircle the later granites; their outcrop is elongated in an ENE-WSW direction. Around the Eastern Mournes the conical fractures were pre-heated by basic magma and the cone-sheets have basic margins with some mixing and hybridization between the contrasting magmas.
Granitic emplacement followed; this was the main phase of igneous activity and responsible for the spectacular mountain scenery of the present-day. The various granites were labelled G1 - G4 by Richey (1928) and later workers have used and expanded his abbreviations. Three intrusions, G1, G2 and G3, invaded the Lower Palaeozoic rocks of the Eastern Mournes; all show flat-lying or gently inclined roof contacts and, all but G1, have steep wall contacts against the country rock, now baked to diopside and biotite bearing hornfelses. Aplite, quartz porphyry, and composite basalt-quartz porphyry dykes cut the granites. Cauldron subsidence on ring-fractures is a possible emplacment mechanism and, as is often the case, the centre of crustal collapse shifted. A further two granites- G4, G5 - of similar nature were then emplaced in the Western Mournes.
Basalt dykes cross-cutting the granites mark the final igneous episode.
There is no evidence of surface vulcanicity associated with this centre of igneous activity as there is at Slieve Gullion and Carlingford. An oxygen and hydrogen isotopic study (McCormick et al. 1993) shows only a limited penetration of meteoric water into the intrusive complex when compared with other British Tertiary subvolcanic complexes. It is the margins of the plutons and the country rocks and minor intrusions which show the effects of water-rock interaction.
THE GRANITES
The mountains' conspicuous peaks and ridges, up to 800 m high, are formed mainly of granite. Rare minerals occur in the drusy cavities of the granite, a feature of high-level/near-surface granites, and this feature alone has attracted many geologists to the area.
Berger (1816) noted two types of granite, one porphyritic and bearing hornblende, the other granular in texture and characterised by biotite. Griffiths (1835) accurately traced the boundary of the granite on his geological map of Ireland (4 miles to the inch). Both he and Bryce (1853) claimed a Tertiary age for the intrusions, an age supported by the chemical analyses of Haughton (1858). Traill (1870-73) mapped the Mourne area in detail for the Geological Survey and with Hull (1871) published his results. Hull (1881) also wrote the explanatory memoir to the 1-inch sheets which covered the area. Though Traill failed to recognise the successive intrusions which make up the granite complex he did appreciate the basic structure of the granite mass.
Richey (1928) gave the first detailed petrography of the various granite intrusions and interpreted their structural relationships as a consequence of cauldron subsidence above a deep seated volcanic hearth. He defined two centres of activity, an early centre in the east which later shifted some 9 km to the WSW. Three granite ring-dykes, G1 (Feldspathic), G2 (Quartzose), and G3 (Aplitic) in order of age, were mapped within the Eastern Mourne centre, and though he only described a single granite ring-dyke (G4-Pink variety) from the Western Mournes he prompted Emeleus (1955) to remap and subdivide the granite into G4 (medium and even grained, pink/purple in colour) and G5 (fine and even grained, blue-grey in colour). Richey (1928) effectively invoked the mechanism of sub-surface cauldron subsidence, and intrusion up the ring-fractures to emplace within the upper crust ring-dykes with laccolithic tops. Emeleus (1955) agreed with this mode of emplacement and further noted a WSW migration between the foci for the two Western Mourne granites. The absence of any foliation within the Mourne Granites and the general lack of country-rock enclaves lends support to this mechanism for passive emplacement of granite magma.
Later work has been largely geochemical in its nature but, under the guidance of Meighan (see Meighan et al. 1984 and the works of Hood 1981, Gibson 1984, and McCormick 1989) has resulted in a large scale rearrangement of the geological map of the Eastern Mournes, see the special 1-inch sheet for the Mourne Mountains (1978). Here Richey's G1 is restricted in occurrence to small roof cappings on Slieve Donard, Slieve Commedagh, and Slieve Corragh and the steep wall/ring-dyke parts of G1 are incorporated into the geochemically similar G2 as pulses of the same magma; a concept first illustrated for the original G2 ring-dyke by Harry and Richey (1963). Hood (1981) found several magmatic pulses in both the G2 and G3 Granites; Gibson (1984) noted two pulses for the G4 and another two for the G5 intrusions.
Robbie (1955) described extensive pneumatolytic alteration in the Slieve Binnian Tunnel; surface expression of this kind of kaolinization is small-scale and local. Nockolds and Richey (1939) found greisen veins in the granite especially well developed at Lindsay's leap. McCormick's (1989) isotopic studies throw light on the role of meteoric water during and postdating the intrusive activity.
The radiometric age of the granites G1 to G4 is established as 56 Ma BP (Gibson et al. (1995)); that of G5 is less certain at the younger age of 51 Ma (Rb-Sr), 54 Ma (Ar-Ar).
All the granites, except G1, contain drusy cavities where exsolved gases collected against the cooler marginal/roof zones of an intrusion. It is these cavities which yield a variety of unusual minerals. Haughton (1856) records beryl, chrysoberyl, topaz, fluorspar and peridot. Smith (1864) found stilbite, Sollas (1890) identified zinnwaldite, Seymour (1903) recorded cassiterite, and Greg and Lettsom (1858) and Richey (1928) discovered fayalite. Richey (1928) completes the list with a bright green mica belonging to the vermiculite group, hematite, and kaolin.
CONSERVATION
Sites of geological importance must represent not only the rock types of this igneous complex but their inter-relationships and relative ages. For convenience these sites can be selected from the principal episodes of the igneous history of the Mourne Mountains outlined above; the sites chosen are listed below. For site specific information see the associated reports.
Dyke Swarm sites:
Mournes - Green Harbour Dyke no. 55 (Site 1112)
Mournes - Dunmore Head, dyke nos. 60, 61, 63 (Site 1109)
Mournes - Samuel's Port, Annalong, dyke no. 124 (Site 1108)
Cone-Sheet sites:
Mournes - Bloody Bridge Cone-sheet (Site 1102)
Mournes - Glasdrumman Port Cone-sheet (Site 1111)
Mournes - Gruggandoo quartz porphyry sheet (Site 1106)
Granite sites:
Mournes - Eagle Rock (Site 1105)
Mournes - Bloody Bridge River section (Site 1104)
Mournes - The Diamond Rocks (Site 1110)
Mournes - Lindsay's Leap (Site 1103)
Mournes - Ben Crom (Site 1100)
Mournes - Spelga Dam (Site 1114)
Mournes - Rocky Mountain (Site 1113)
Mournes - Eagle Mountain, Slievemoughanmore, Pigeon Rock Mountain (Site 1101)
Mournes - Kilbroney River section (Site 1107)
For general information on the cone-sheet intrusions associated with the Mourne Granites, and the Mourne Mountains dyke swarm, see;
Mournes - Cone-sheet and Dyke Swarm - Overview (Site 1131)
In the site descriptions the older petrographic terms of Tomkeieff and Marshall (1935) are used for the dyke and cone-sheet intrusions. Some of these are now out-dated but it would entail a detailed petrographic study to up-date them.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Mournes - The Granites - Overview, County Down, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1129. Accessed on 2024-12-26 |
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