Earth Science Conservation Review
| Loughmacrory | Tyrone |
| Site Type: | Inland exposure |
| Site Status: | ESCR |
| Grid Reference: | H599765 |
| Google maps: | 54.63357,-7.0724 |
| Rocks | |
|---|---|
| Rock Age: | Ordovician |
| Rock Type: | Quartz porphyry, Rhyolite |
| Interest | |
| Other interest: | Caledonian, Intrusion |
Introduction:
The Loughmacrory site is located ~2.5km northeast of Milltown in County Tyrone.
On the first edition of the one inch to one mile geological map (Ballygawley (Sheet 34) (Geological Survey of Ireland, 1913) the rocks at this site were mapped and identified as quartziferous porphyry or elvanite within hornblendic and pyroxenic rocks. Initially, the porphyries in Tyrone were believed to be metamorphosed lava (Egan, 1881; Nolan, 1884).
On the second (1:50,000 scale edition) of geological Sheet 34 (renamed Pomeroy) (Geological Survey of Northern Ireland, 1979) several large, sheet-like and elongate quartz porphyry bodies were mapped aligned parallel to the dominant Caledonian strike. The relationship between the quartz porphyry intrusions, granitoid bodies and Ordovician country rock is by no means fully understood. On the Pomeroy sheet, the Carrickmore granite is shown as cut by quartz porphyry dykes. 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. These sills have mineralogy similar to both the Slieve Gallion Granodiorite and the dacite lavas: they are probably the intrusive equivalent of the latter. An attempt is currently being made to obtain a radiometric date for the Loughmacrory porphyry (NIGL work in progress).
On the first edition of the one inch to one mile geological map (Ballygawley (Sheet 34) (Geological Survey of Ireland, 1913) the rocks at this site were mapped and identified as quartziferous porphyry or elvanite within hornblendic and pyroxenic rocks. Initially, the porphyries in Tyrone were believed to be metamorphosed lava (Egan, 1881; Nolan, 1884).
On the second (1:50,000 scale edition) of geological Sheet 34 (renamed Pomeroy) (Geological Survey of Northern Ireland, 1979) several large, sheet-like and elongate quartz porphyry bodies were mapped aligned parallel to the dominant Caledonian strike. The relationship between the quartz porphyry intrusions, granitoid bodies and Ordovician country rock is by no means fully understood. On the Pomeroy sheet, the Carrickmore granite is shown as cut by quartz porphyry dykes. 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. These sills have mineralogy similar to both the Slieve Gallion Granodiorite and the dacite lavas: they are probably the intrusive equivalent of the latter. An attempt is currently being made to obtain a radiometric date for the Loughmacrory porphyry (NIGL work in progress).
| Enlander, I., Dempster, M. & Doughty, P., 2024. Loughmacrory, County Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=593. Accessed on 2024-12-26 |
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