ESCR - Craigballyharky and Craigbardahessiagh

Earth Science Conservation Review

Summary Full report
Craigballyharky and Craigbardahessiagh - Caledonide Igneous Complex	Tyrone

Site number:	590
ESCC:	EO
Locality Type:	Inland exposure	Status:	ESCR
Grid Reference:	H73337573,H72187433 Centroid, Not Entered
Google maps:	54.62462,-6.86514
County:	Tyrone	Council area:	Mid Ulster District Council
Period:	Silurian, Ordovician
Stages:	Arenig, Llandovery, Llanvirn
Lithostrat:	Little River Group, Tyrone Volcanic Group, Desertcreat Group, Tyrone Plutonic Group

Site Description
Highlights:
Together these two sites demonstrate relationships between the Tyrone Basic Plutonic Complex, the Tyrone Volcanic Group and marginally younger granitoids at Craigballyharky and Craigbardahessiagh. Age data and rock relationships indicate formation of all of the above in a relatively short time span (Ordovician, Arenig-Llanvirn).
Introduction:
The Craigballyharky and Craigbardahessiagh sites occur ~3-5km northeast of Pomeroy, County Tyrone. Craigballyharky Hill is located within an extensive but patchy area of natural exposure. It is approached via a single track used for farm and reservoir access. Craigbardahessiagh is an area of extensive but patchy exposure that is best approached via a farm from the junction of Bardahessiagh Road and Bardahessiagh Lane. The first edition of the Ballygawley (Sheet 34) one-inch to the mile geological map (Geological Survey of Ireland, 1913) and accompanying explanatory memoir (Nolan, 1878) shows and describes an intrusive body of syenite and syenitic granite. The map shows a single body intruded into hornblendic and pyroxenic country rocks, while the southern margin of the body is shown in contact with sedimentary rocks of Silurian age. On the second edition of Sheet 34 (renamed Pomeroy) two parts to the complex were recognized (Geological Survey of Northern Ireland, 1979). One was centered on Craigballyharky and the other around Craigbardahessiagh. Both were emplaced in the Tyrone Basic Plutonic Complex and include large roof pendants of the Tyrone Basic Plutonic Complex and Ordovician Tyrone Volcanic Group. Graptolitic shales and turbidites of the Ordovician Desertcreat Group (Caradoc-Ashgill) and Silurian (Llandovery) Little River Group overly the intrusive complexes at the southern contact.
Description:
CRAIGBALLYHARKY The area immediately under and east of the reservoir is underlain by fine-grained, dark green basaltic rock belonging to the Ordovician Tyrone Volcanic Group. The rocks often appear brecciated and show evidence of deposition as pillow lavas. Immediately west of the reservoir is an area composed of coarse-grained, dark green, gabbro; Angus (1977) shows this as an area of quartz-diorite. In thin section the gabbro consists of green amphibole, probably hornblende (40%), feldspar predominantly plagioclase (40%), quartz (17%) and Fe-oxide. The rock appears to have undergone metamorphic alteration, including some degree of deformation relating to tectonism (Merriman and Hards, 2000). Coarse-grained, pinkish white tonalite occurs west of the reservoir and has diffuse contacts with the gabbro/quartz- diorite. Prominent, euhedral quartz phenocrysts are common within the tonalite, but are also seen within the gabbro. Irregular, diffuse boundaries are apparent as well as sharp well-defined boundaries. In thin section the tonalite consists of dusty, brown, heavily altered feldspar (60%), quartz (30%), hornblende (7%) and Fe-oxide (Merriman and Hards, 2000). The rock has been subjected to syntectonic recrystallisation. A sample from the magma-mingling zone (gabbro-tonalite) is probably most accurately classified as a tonalite. Like previous samples, the rock appears to have undergone metamorphic alteration, including some degree of tectonisation. The rock is composed of hornblende (25%), quartz (45%), sodic plagioclase feldspar (25%) and Fe-oxide. At Craigballyharky, there are also sheets of microgranodiorite (<1m thick) resembling marginal finer grained granodiorite associated with the Craigbardahessiagh site. CRAIGBARDAHESSIAGH Exposures in the southwestern part of the site [27220 37430] are composed of orange coloured, coarse to very coarse-grained granodiorite. The rock appears massive with faint fabric developed in places. Finer grained granodiorite can be seen in contact with tonalite and around the mapped margin of the granodiorite intrusion. After examination in thin section, the rock was confirmed as granodiorite composed of dusty brown, turbid feldspar (50%), quartz (40%), and primary and secondary mafic phases (10%). Primary mafic phases include biotite and probably hornblende. The entire sample shows clear evidence of strain and some degree of recrystallisation (Merriman and Hards, 2000). Towards the northwest of the site at [27215 37450] there are outcrops of coarse-grained, pinkish white tonalite. The tonalite has a conspicuous fabric. In thin section this rock is seen to be metatonalite which has undergone a higher degree of tectonisation and grain size reduction than the granodiorite. It is composed of dusty brown, heavily altered feldspar (52%), quartz (35%), green amphibole (10%) and Fe-oxide (Merriman and Hards, 2000). Transitional boundaries between tonalite to granodiorite were observed during field inspection.
Importance:
These two sites demonstrate the relationships between the Tyrone Basic Plutonic Complex, the Tyrone Volcanic Group and the marginally younger granitoids at Craigballyharky and Craigbardahessiagh. Age data and rock relationships indicate formation of all of the above in a relatively short time span early in the Caledonian mountain building event as it affected Northern Ireland (Ordovician, Arenig-Llanvirn).
Interpretation:
Angus (1977) divided the Craigballyharky complex into zones composed of quartz-diorite, tonalite and biotite granodiorite. He suggested that an initial pulse of tonalitic magma caused metasomatic alteration of surrounding gabbro and lead to the formation of the quartz-diorite. Angus included the Craigbardahessiagh biotite granodiorite as part of a Craigballyharky Complex. Rock relationships show the granodiorite to be a later intrusive pulse that occurred after cooling of the tonalite and formation of the quartz-diorite envelope. An interesting relationship is seen between tonalites and quartz diorites, where irregular and diffuse boundaries are apparent. These boundaries contain large quartz crystals with hornblende rims. These were noted as ocelli by Angus (1962 & 1977), who interpreted their formation as resulting from hybridization. More recently it is thought that ocelli formation is the result of coeval acid-basic magma mingling (Vernon, 1986). Transitional boundaries between tonalite to granodiorite were observed during field inspection suggesting contemporaneous intrusion and cooling. Although the gabbro (in its altered form quartz diorite) of the Tyrone Basic Plutonic Complex is clearly older than the Craigballyharky and Craigbardahessiagh granitoids, Hutton et al. (1985) suggested that the gabbro is only slightly older as indicated by the contact relationships described above. A U/Pb zircon age for the Craigballyharky tonalite of 471±4Ma (Hutton et al., 1985) indicates emplacement during the early Ordovician (Arenig). Significant uplift and erosion prior to deposition is indicated by the presence of blue tinged quartz grains and clasts derived from Craigballyharky and Craigbardahessiagh in the nearby Ordovician and Silurian sediments. Isotope and graptolite ages along with the rock relationships described above indicate formation of the Tyrone Volcanic Group, Tyrone Basic Plutonic Complex and the granitoids of Craigballyharky and Craigbardahessiagh in a relatively short time span (Ordovician, Arenig-Llanvirn). Very low concentrations of uranium in the Craigballyharky is thought to indicate melt derived from oceanic and continental crust (Hutton et al., 1985) and points to a model in which the Craigballyharky tonalite ascended through ophiolite very soon after it had been obducted (Hutton et al., op. cit.).
Conclusions:
Craigballyharky and Craigbardahessiagh together demonstrate contact relationships between the Tyrone Basic Plutonic Complex, Tyrone Volcanic Group and the marginally younger granitoids Age data and rock relationships indicate formation of all of the rocks in a relatively short time span (Ordovician, Arenig- Llanvirn). Uranium isotope data indicate that the Craigballyharky tonalite pluton rose through ophiolite very soon after it had been obducted (Hutton et al., op. cit.).
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 591 - Craiganawork, Pomeroy Key Site 592 - Tintagh, Slieve Gallion 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 Craigbardahessiagh, see the following site: Key Site 29 - Craigbardahessiagh and Bardahessiagh River

Keywords
Rocks:	Granodiorite, Quartz-diorite, Tonalite
Structures:	Caledonian
Relations:	Intrusion

Access
Approach:	Craigballyharky Hill is approached via a single track used for farm and reservoir access. Craigbardahessiagh is best approached via a farm from the junction of Bardahessiagh Road and Bardahessiagh Lane.
Management:
Access to these sites is via farm lanes. Parking is constricted and the permission of landowners should be sought. Both sites are currently used for rough grazing. Further expansion of the Reservoir on Craigballyharky could conceivably threaten parts of the site. Otherwise, at both sites, outcrop is relatively abundant and there is no need to restrict hammering or sampling. The rocks of the Tyrone Volcanic Group and closely associated intrusive granitoids such as the Craigballyharky tonalite 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.
Uses
Uses:	Both sites are currently used for rough grazing.


Map(s):	GEOLOGICAL SURVEY OF IRELAND, 1877. 1:63,360 Geological Sheet 34 (Pomeroy). Dublin, Ordnance Survey for Geological Survey of Ireland. 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. Craigballyharky and Craigbardahessiagh - Caledonide Igneous Complex, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=590. Accessed on 2026-11-06.