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Earth Science Conservation Review
| Loughmacrory | Tyrone |
| Site number: | 593 | ||
| ESCC: | EO | ||
| Locality Type: | Inland exposure | Status: | ESCR |
| Grid Reference: | H599765 Centroid | ||
| Google maps: | 54.63357,-7.0724 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Ordovician | ||
| Site Description | |
|---|---|
| Highlights: | |
Exposure of a quartz porphyry (rhyolite) body believed to be slightly younger than early Ordovician Caledonian granites | |
| 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). | |
| Description: | |
An extensive area of natural exposure can be approach via a farm track leading off the Loughmacrory Road. Quartz porphyry occurs in the northern two thirds of the site and is in contact with rocks of the Copney Pillow-lava Formation (Ordovician Tyrone Volcanic Group) though the relationship is unclear. The porphyry appears mottled green with numerous orange-pink phenocrysts. Exposures are extensively sheared and contain many near vertical discontinuities with slickensides. In thin section, the quartz porphyry can be seen to consist of up to 20% phenocrysts in an altered, devitrified rhyolite. Phenocryst phases include quartz, alkali feldspar and a heavily altered mafic phase, probably amphibole. Quartz phenocrysts (<4.5mm) are rounded and embayed, suggesting that they were out of equilibrium with the host magma and have been partially resorbed. The quartz has strain extinction either due to high magma viscosity or, more likely, to tectonism. The groundmass was probably originally glassy but now comprises a finely crystalline snowflake-textured intergrowth of quartz and feldspar (Merriman and Hards, 2000). | |
| Importance: | |
The Loughmacrory site provides accessible exposure of relatively fresh quartz porphyry. Samples from this site should therefore provide the best chance of establishing dates of intrusion via geochronology. If dating is successful, the relationship with granitoids of Slieve Gallion and other similar granites and surrounding country rocks may be revealed. | |
| Interpretation: | |
The age of granitoids and porphyries remain uncertain; it is hoped that NIGL dating will provide an age. Field relations suggest that the porphyry post-dates the granodiorite. Cameron et al. (1997) have suggested that the porphyry was intruded through slightly older granitoids forming sills at the base of the Ordovician volcanics. | |
| Conclusions: | |
The age of the porphyries remains uncertain. Field relations suggest that the porphyries are younger than the granitoids. Magma must have been sourced from below the obducted ophiolite (Hutton et al., 1995), but current thinking places little certainty on the role of mantle, pre-existing and newly generated crust in the Caledonian orogenic setting. | |
| 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 592 - Tintagh, Slieve Gallion 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 | |
| Keywords | |
|---|---|
| Rocks: | Quartz porphyry, Rhyolite |
| Structures: | Caledonian |
| Relations: | Intrusion |
| Access | |
|---|---|
| Approach: | Access is via a farm track leading off the Loughmacrory Road. |
| Management: | |
Given extensive natural exposure at this site, there is no need to restrict hammering or sampling. The rocks of the Tyrone Volcanic Group and the closely associated porphyry intrusives are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout much central Tyrone. The area which includes this site is likely to attract further exploration activity in the future. | |
| Map(s): | GEOLOGICAL SURVEY OF IRELAND, 1913. 1:63,360 Geological Sheet 34 (Ballygawley). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1979. Pomeroy, Northern Ireland Sheet 34. Solid. 1:50,000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 04 MAR 2001 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Loughmacrory, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=593. Accessed on 2026-11-06. |
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