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Earth Science Conservation Review
| Tintagh, Slieve Gallion | Londonderry |
| Site number: | 592 | ||
| ESCC: | EO | ||
| Locality Type: | Inland exposure | Status: | ESCR |
| Grid Reference: | H28103872 Centroid | ||
| Google maps: | 54.29678,-7.5684 | ||
| County: | Londonderry | Council area: | Fermanagh & Omagh District Council |
| Period: | Devonian, Ordovician | ||
| Lithostrat: | Old Red Sandstone, Slieve Gallion Granodiorite | ||
| Site Description | |
|---|---|
| Highlights: | |
The Slieve Gallion granodiorite intrusion is one of a chain of four plutons that are aligned parallel to the dominant Caledonian strike in Counties Tyrone and Londonderry. These plutons resemble the "Newer Granite" intrusions of Scotland and but field relationships indicate that they were intruded during the early Ordovician. The quartz porphyry intrusions on Slieve Gallion are related to the granites and are probably slightly younger than the granodiorite. | |
| Introduction: | |
Tintagh is located on the southeastern side of the Slieve Gallion Complex and comprises an extensive area of natural exposure. Access is via a farm track located ~6km northwest of Moneymore in County Londonderry. Portlock's report on the "Geology of the County of Londonderry and parts of Tyrone and Fermanagh" (1843) contained one of the earliest accounts of the granites on Slieve Gallion. Subsequently, these rocks were mapped on the first editions of the Draperstown (Sheet 26) and Cookstown (Sheet 27) one-inch to the mile geological maps (Geological Survey of Ireland, 1882 and 1880) and were discussed in accompanying memoirs. On these early maps, the rocks were all considered to be of Pre-Cambrian age. The early survey described granite that in places was transitional into quartz porphyry and syenite (hornblende rich igneous rocks). The porphyry was initially believed to be metamorphosed lava (Egan, 1881). Cole (1897) was first to recognise the true nature of the granitic rocks and described intrusive contacts between granite and lavas at Letteran. Hartley (1933, 1936) also believed that the granodiorite (biotite-granite) post-dated the lavas, he concluded that the original granitic magma was contaminated by lavas at its contact resulting in hornblende and biotite bearing varieties and that the aplites represent uncontaminated granites. Slieve Gallion was resurveyed and published as 1:50,000 scale second editions of Sheet 27 (Cookstown) and Sheet 26 (Draperstown) (Geological Survey of Northern Ireland, 1983, 1995). On the Draperstown Sheet, granodiorite was identified and mapped as the main granitoid body. Within the granodiorite, areas of diorite were mapped and related to the early Ordovician Tyrone Basic Plutonic Complex. The main part of the granitoid body occurs on the Cookstown Sheet and was described as "biotite granodiorite". The summary of geological history on the Cookstown map indicates that the granites are older than the Ordovician Tyrone Volcanic Group. There is also thought to be a relationship between the Slieve Gallion granodiorite and the intrusions at Pomeroy, Carrickmore and Beragh with the possibility that these plutons join up at depth. On the southeastern side of Slieve Gallion there are exposures of porphyry, which has a mineralogy very similar to both the Slieve Gallion Granodiorite and the dacite lavas: they are probably the intrusive equivalent of the latter. The porphyry appears to strike beneath the Ordovician rocks. However, at several exposures at the contact between the two, the porphyry is clearly chilled against the volcanics. Cobbing (pers. comm.) has suggested that the porphyry intruded the granodiorite and then formed sills at the base of the Ordovician volcanics. | |
| Description: | |
Outcrops of light pinkish red coarse-grained granite with no obvious fabric occur adjacent to the track which runs through the site. From thin section this rock has been classified as a biotite-bearing granite consisting mainly of quartz (25%), alkali feldspar (35%), plagioclase (34%) and biotite (5%). The quartz occurs as ragged anhedral grains (<6mm) commonly showing embayed and internally corroded shapes caused by magmatic reactions (Merriman and Hards, 2000). Quartz porphyry is exposed within the site north of an animal shelter that appears to straddle the porphyry/granodiorite igneous contact. In fresh specimens, the porphyry is pink and fine-grained with numerous phenocrysts of quartz and plagioclase. Exposures are characterised by a strong platy jointing. A boundary between granite and quartz porphyry is exposed in the stream section in the northwest corner of the site, but the relationship is unclear. Cameron and Old (1997) described a sharp contact 800m south-southeast of Tintagh Mountain and within the site boundary. As a result of examination of a thin section, this rock, classified as a quartz porphyry, was reclassified as a feldspar-quartz-phyric rhyolite consisting of phenocrysts of feldspar (20%) and quartz (15%) in a fine-grained groundmass (60%) of quartz and feldspar. The phenocrysts of quartz and plagioclase are most striking with the quartz appearing to be deeply corroded and embayed with few crystal faces remaining (Merriman and Hards, 2000). | |
| Importance: | |
The Tintagh site provides access to typical granodiorite and the rhyolitic porphyry that together comprise much of the volume of Slieve Gallion. The exposures are relatively fresh and therefore provide the best chance of establishing dates of intrusion via geochronology. If dating is successful, the relationship of Slieve Gallion with similar granitoids and surrounding country rocks may be revealed. | |
| Interpretation: | |
The Slieve Gallion and associated intrusions resemble younger Caledonian granites of late Ordovician to Silurian age. However, Cameron and Old (1997) described relationships elsewhere in the region as follows, "On the south-east of Slieve Gallion the porphyry outcrops appear to strike beneath the Ordovician rocks but several exposures at the contact between the two show that the porphyry is chilled against the volcanics". Cobbing (pers. comm.) contends that the porphyry was intruded through the granodiorite and then formed sills at the base of the Ordovician volcanics. The granite must have been sourced from somewhere below the obducted ophiolite of Hutton et al. (1995). | |
| Conclusions: | |
The age of the Slieve Gallion granodiorite and associated porphyry remains uncertain. Field relations indicate that the porphyry post-dates granodiorite and is not considered to be a marginal phase of the granodiorite. Cameron and Old (1997) described relationships elsewhere in the region as follows, "On the south-east of Slieve Gallion the porphyry outcrops appear to strike beneath the Ordovician rocks but several exposures at the contact between the two show that the porphyry is chilled against the volcanics". Cobbing (pers. comm.) contends that the porphyry was intruded through the granodiorite and then formed sills at the base of the Ordovician volcanics. The granitoids must have been sourced from below the obducted ophiolite of Hutton et al. (1995) although current thinking places little certainty on the role of mantle, pre-existing and newly generated crust in the Caledonian orogenic setting. It is hoped that radiometric age dating currently being carried out on the granodiorite and porphyry (NIGL work in progress) will shed light on the age of these rocks. | |
| Notes: | |
For information and references on sites within the Tyrone Basic Plutonic Complex see the following sites: Key Site 580 - Black Rock Key Site 581 - Scalp Hill For information and references on sites within the Tyrone Volcanic Group see the following sites: Key Site 582 - Mveela More Key Site 583 - Creggan Key Site 584 - Copney Hill Key Site 585 - Beaghbeg Key Site 586 - Bonnety Bush, Altihaskey Key Site 587 - Tullybrick Key Site 588 - Stuhanleanantawey Burn For information and references on other Granitoid sites within the Caledonian Complexes of N. Ireland see the following sites: Key Site 589 - Cashel Rock Key Site 590 - Craigballyharky and Craigbardahessiagh Key Site 591 - Craiganawork, Pomeroy Key Site 593 - Loughmacrory Key Site 594 - Cushendun For general information and references on the Caledonian Igneous Complexes of N. Ireland, and for directions to other regional sites, see the following site: Key Site 595 - Introduction to the Caledonian Igneous Complexes of N. Ireland For more information on the geology of the area around Slieve Gallion, see the following site: Key Site 105 - Slieve Gallion | |
| Keywords | |
|---|---|
| Rocks: | Granite, Granodiorite, Porphyry |
| Structures: | Caledonian, pluton |
| Relations: | Intrusion |
| Access | |
|---|---|
| Approach: | Access is via a farm track located ~6km northwest of Moneymore in County Londonderry. |
| Management: | |
Given the extensive natural exposure at Tintagh, there is no need to impose hammering or sampling restrictions. Intrusive granitoids are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout central Tyrone. The area which includes this site is likely to attract further exploration activity in the future. | |
| Map(s): | GEOLOGICAL SURVEY OF IRELAND, 1880. 1:63,360 Geological Sheet 27 (Cookstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF IRELAND, 1882. 1:63,360 Geological Sheet 26 (Draperstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1983. Cookstown, Northern Ireland Sheet 27. Solid. 1:50,000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1995. Draperstown. Northern Ireland Sheet 26. Solid and Drift Geology. 1:50,000. (Keyworth, Nottingham: British Geological Survey). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 04 MAR 2001 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Tintagh, Slieve Gallion, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=592. Accessed on 2026-11-07. |
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