Earth Science Conservation Review
| Mournes - Cone-sheet and Dyke Swarm - 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: | Andesite, Basalt, Dolerite, Feldspar porphyry, Felsite, Granophyre, Leidleite, Porphyry, Quartz porphyry, Tholeiite |
| Interest | |
| Other interest: | cone-sheet, dyke, magma mixing, multiple intrusion, phenocrysts, spherulitic, xenocrysts, xenoliths, Intrusion |
Description:
THE CONE-SHEET INTRUSIONS ASSOCIATED WITH THE MOURNE GRANITES
An elliptical cone-sheet system surrounds all the granite intrusions of the Mourne centre. Its longer axis bears ENE-WSW and follows the shift of intrusive activity from the Eastern to the Western Mournes. Individual exposures of the quartz porphyry and composite basalt-quartz porphyry sheets are shown on the 1:50,000 geological map of the Mourne Mountains (I.G.S. 1978) and on fig. 29.17 (Emeleus 1982) in Sutherland's Igneous Rocks of the British Isles.
The Glasdrumman inclined sheet is splendidly displayed between Jenkin's Point to the Sherby Rocks at Glasdrumman Port. It is probably the same sheet which outcrops in the Annalong River. Similar sheets are exposed on Augrim Hill, Knockchree, Cloghachorcha, Formal, and Knockshee further west. Around the northern sector of the granites, inclined sheets occur on the foreshore north of the Bloody Bridge River, in the Tullybranigan River, on Slievenaman, and at several localities between this hill and Hilltown; these occurrences show a more pronounced spherulitic groundmass. West of the granites, quartz porphyry outcrops on Gruggandoo and Slieve Roe. In most instances these sheets dip at 35 deg. towards the granite and indicate a focus some 3 miles below the present surface.
Many of these occurrences are linked to a single cone-sheet fracture; the outcrop is extrapolated in both maps referred to above. This fracture is hypothetical and, where the line runs inland between Bloody Bridge and Glasdrumman Port, suspect, since the beach at Green Harbour exposes a typical composite cone-sheet, unrecorded by Tomkeieff and Marshall (1935) though described by Cole (1894 (B)).
Compared with other Tertiary Igneous central complexes in Ireland (Carlingford) and Scotland (Ardnamurchan and Mull) where cone-sheet intrusions occur in swarms and are associated with doming of the crust above the volcanic hearth, this sole example surrounding the Mourne Granites is unusual. Unusual too is its postulated path around all the Mourne Granites; it may be that the various cone-sheets are related to two more circular outcrops around each centre. Another anomaly, just as difficult to resolve, is the irregularity of the conical fractures. In general these show an inward dip towards a focal point beneath the Mournes; locally there is considerable divergence. At Glasdrumman Port the cone-sheet can be traced up and down the beach, at Green Harbour the contacts with the Silurian Slates are vertical and at the Bloody Bridge its disposition again appears to be corrugated. It could be that the well fractured Lower Palaeozoic country rock afforded a more irregular pathway to rising magma.
The coastal sections of the cone-sheet are by far the most informative of this episode of igneous activity. Exposures inland tend to be small and incomplete. Consequently the sites most worthy of conservation are those at Glasdrumman Port and Bloody Bridge. Only one occurrence shows the age of these cone-sheets relative to the Granites; this is the site mapped by Emeleus (1955) at Gruggandoo where the quartz porphyry sheet is truncated by G4 of the Western Mournes. Age relationships with the dyke swarm are shown at the coastal sites but one should remember that dykes were probably emplaced over an extended period of time.
THE MOURNE MOUNTAIN DYKE SWARM
Tomkeieff and Marshall (1935) offer a very detailed appraisal of the Mourne swarm of Tertiary dykes which for the most part are exposed in the coastal section from the lifeboat station at Newcastle to Ballykeel Point south of Annalong. Some 130 dykes occur along this 10 mile stretch of coast; their aggregate thickness indicating a 2.5% extension of the earth's crust.
In contrast to other linear dyke swarms of Tertiary age in Ireland they show a much greater variety of rock-types; rarely are neighbouring dykes of the same nature. Olivine-bearing dolerites/basalts and tholeiites and olivine-free equivalents are interspersed with andesitic tholeiites, andesites of different textural types, leidleites, quartz porphyries, feldspar porphyries, spherulitic granophyres, and felsites; the petrographic terms are taken from Tomkeieff and Marshall (1935). This wide spectrum of rock composition from basic to acid points to fractional crystallization processes already established during this early phase of magmatic activity.
This coastal section is unlikely to suffer any sudden, drastic change which could destroy the geological evidence. Small landslips could bury some exposures for a time but the tide would quickly redistribute the debris; reworking of the boulder beach by storm waves may cover one dyke section but other dykes could become better exposed.
There are however a number of dyke members which illustrate more interesting rock relationships, or carry evidence of deeper seated magmatic activity and these examples could be selected for designation as ASSIs.
An elliptical cone-sheet system surrounds all the granite intrusions of the Mourne centre. Its longer axis bears ENE-WSW and follows the shift of intrusive activity from the Eastern to the Western Mournes. Individual exposures of the quartz porphyry and composite basalt-quartz porphyry sheets are shown on the 1:50,000 geological map of the Mourne Mountains (I.G.S. 1978) and on fig. 29.17 (Emeleus 1982) in Sutherland's Igneous Rocks of the British Isles.
The Glasdrumman inclined sheet is splendidly displayed between Jenkin's Point to the Sherby Rocks at Glasdrumman Port. It is probably the same sheet which outcrops in the Annalong River. Similar sheets are exposed on Augrim Hill, Knockchree, Cloghachorcha, Formal, and Knockshee further west. Around the northern sector of the granites, inclined sheets occur on the foreshore north of the Bloody Bridge River, in the Tullybranigan River, on Slievenaman, and at several localities between this hill and Hilltown; these occurrences show a more pronounced spherulitic groundmass. West of the granites, quartz porphyry outcrops on Gruggandoo and Slieve Roe. In most instances these sheets dip at 35 deg. towards the granite and indicate a focus some 3 miles below the present surface.
Many of these occurrences are linked to a single cone-sheet fracture; the outcrop is extrapolated in both maps referred to above. This fracture is hypothetical and, where the line runs inland between Bloody Bridge and Glasdrumman Port, suspect, since the beach at Green Harbour exposes a typical composite cone-sheet, unrecorded by Tomkeieff and Marshall (1935) though described by Cole (1894 (B)).
Compared with other Tertiary Igneous central complexes in Ireland (Carlingford) and Scotland (Ardnamurchan and Mull) where cone-sheet intrusions occur in swarms and are associated with doming of the crust above the volcanic hearth, this sole example surrounding the Mourne Granites is unusual. Unusual too is its postulated path around all the Mourne Granites; it may be that the various cone-sheets are related to two more circular outcrops around each centre. Another anomaly, just as difficult to resolve, is the irregularity of the conical fractures. In general these show an inward dip towards a focal point beneath the Mournes; locally there is considerable divergence. At Glasdrumman Port the cone-sheet can be traced up and down the beach, at Green Harbour the contacts with the Silurian Slates are vertical and at the Bloody Bridge its disposition again appears to be corrugated. It could be that the well fractured Lower Palaeozoic country rock afforded a more irregular pathway to rising magma.
The coastal sections of the cone-sheet are by far the most informative of this episode of igneous activity. Exposures inland tend to be small and incomplete. Consequently the sites most worthy of conservation are those at Glasdrumman Port and Bloody Bridge. Only one occurrence shows the age of these cone-sheets relative to the Granites; this is the site mapped by Emeleus (1955) at Gruggandoo where the quartz porphyry sheet is truncated by G4 of the Western Mournes. Age relationships with the dyke swarm are shown at the coastal sites but one should remember that dykes were probably emplaced over an extended period of time.
THE MOURNE MOUNTAIN DYKE SWARM
Tomkeieff and Marshall (1935) offer a very detailed appraisal of the Mourne swarm of Tertiary dykes which for the most part are exposed in the coastal section from the lifeboat station at Newcastle to Ballykeel Point south of Annalong. Some 130 dykes occur along this 10 mile stretch of coast; their aggregate thickness indicating a 2.5% extension of the earth's crust.
In contrast to other linear dyke swarms of Tertiary age in Ireland they show a much greater variety of rock-types; rarely are neighbouring dykes of the same nature. Olivine-bearing dolerites/basalts and tholeiites and olivine-free equivalents are interspersed with andesitic tholeiites, andesites of different textural types, leidleites, quartz porphyries, feldspar porphyries, spherulitic granophyres, and felsites; the petrographic terms are taken from Tomkeieff and Marshall (1935). This wide spectrum of rock composition from basic to acid points to fractional crystallization processes already established during this early phase of magmatic activity.
This coastal section is unlikely to suffer any sudden, drastic change which could destroy the geological evidence. Small landslips could bury some exposures for a time but the tide would quickly redistribute the debris; reworking of the boulder beach by storm waves may cover one dyke section but other dykes could become better exposed.
There are however a number of dyke members which illustrate more interesting rock relationships, or carry evidence of deeper seated magmatic activity and these examples could be selected for designation as ASSIs.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Mournes - Cone-sheet and Dyke Swarm - Overview, County Down, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1131. Accessed on 2024-12-26 |
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