Earth Science Conservation Review
| Tintagh, Slieve Gallion | Londonderry |
| Site Type: | Inland exposure |
| Site Status: | ESCR |
| Grid Reference: | H28103872 |
| Google maps: | 54.29678,-7.5684 |
| Rocks | |
|---|---|
| Rock Age: | Devonian, Ordovician |
| Rock Name: | Old Red Sandstone, Slieve Gallion Granodiorite |
| Rock Type: | Granite, Granodiorite, Porphyry |
| Interest | |
| Other interest: | Caledonian, pluton, Intrusion |
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.
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.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Tintagh, Slieve Gallion, County Londonderry, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=592. Accessed on 2024-12-26 |
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