| Previous Site | Next Site |
Earth Science Conservation Review
| Black Rock | Tyrone |
| Site number: | 580 | ||
| ESCC: | EO | ||
| Locality Type: | Inland exposure | Status: | ESCR |
| Grid Reference: | H685836 Centroid | ||
| Google maps: | 54.69624,-6.93759 | ||
| County: | Tyrone | Council area: | Mid Ulster District Council |
| Period: | Ordovician | ||
| Lithostrat: | Tyrone Volcanic Group | ||
| Site Description | |
|---|---|
| Highlights: | |
The Black Rock site contains numerous, readily accessible outcrops and affords an opportunity to study lithology and contact relationships between some of the main rock units elements which form part of the Tyrone Plutonic Group. | |
| Introduction: | |
Black Rock is a prominent topographic feature in otherwise flat-lying country about 2.5km north of Dunnamore and 400m south of the stone circles at Beaghmore. Access to the eastern side of the site is from the Blackrock Road while access to the southern and western sides is from the Beaghmore Road. On Portlock's early geological map of the area, (1843) all rocks in the vicinity of Black Rock were included within the category "metamorphic rocks of hornblendic type". Portlock's map did not delineate individual rock types but did include specific lithological qualifiers for "trap", "diallage", "slate" and "porphyry." On the first edition of the one-inch to the mile scale geological map (Sheet 26, Draperstown, Geological Survey of Ireland, 1882) the rocks at Black Rock were grouped together and referred to as "Pyroxenic Rocks". These formed part of the "Metamorphic and Igneous Rocks" on the map and were referred to as being "Probably of Pre-Cambrian or Upper Laurentian Age". In the memoir which accompanied the map, Nolan (1884) referred to the "wonderful variety in the rocks" occurring at Black Rock. Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane. He incorporated the rocks at Black Rock within a division which he referred to as the "Crush Complex", part of what he termed the "Tyrone Igneous Series". The crush complex occurs immediately north and south of the Central Tyrone Metamorphic Inlier. Hartley characterized the crush complex by the close intermingling of many igneous lithologies. On the second edition (1:50,000 scale) of Sheet 26(Draperstown) (Geological Survey of Northern Ireland, 1995), the rocks at Black Rock are included within the Tyrone Plutonic Group which is broadly equivalent to Hartley's Crushed Complex. The Tyrone Plutonic Complex surrounds and is in faulted contact with the high grade metamorphic inlier (Corvanaghan Formation) The Tyrone Plutonic Group forms part of the Tyrone Ophiolite Complex, interpreted as being a fragment of Ordovician island arc or back-arc oceanic crust (Hutton et al., 1985). These rocks form part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane as defined by Bluck et al. (1992 and can broadly be correlated with comparable ophiolitic rocks at Ballintrae in Scotland and Clew Bay in western Ireland. | |
| Description: | |
Several different igneous lithologies occur within this site, and the complex cross-cutting contact relationships between several lithologies can be seen in individual outcrops. The main lithology at Black Rock is coarse to very coarse-grained hornblende gabbro which has an extremely variable texture ranging from equigranular gabbro to very coarse-grained auto-intrusions of pegmatitic gabbro in which individual hornblende and plagioclase crystals are in excess of 2cm diameter. The rock is locally sheared but has no regional schistosity. This is evidence of local but not regional deformation. The gabbro is extensively altered and most original pyroxene has been replaced by amphibole. The main amphibole is bright green, fresh hornblende and is sometimes accompanied by fibrous actinolite. Some original pyroxene remains and is visible mainly in thin sections. The plagioclase has been completely altered and replaced by quartz sericite and epidote. The gabbro is in contact and contains xenoliths of an earlier suite of dolerites and their finer grained equivalents. These earlier rocks are deformed and locally schistose and show evidence of earlier alteration with extensive and elaborate net-vein systems with fine stringers of yellowish green epidote. At Black Rock and throughout the outcrop of the Tyrone Plutonic Complex, the gabbro is cut by a series of younger dolerite and basaltic intrusions mainly in the form of 1 to 2m wide dykes. Several such intrusions can be seen at the Black Rock. These dolerites represent the youngest phase of basic activity in the Complex. In the field they are distinctive and look moderately fresh and undeformed. The dolerite occurs both as porphyritic and non-porphyritic varieties, the former (EQ-173) consisting of 1-2mm diameter rounded and angular lathes of plagioclase in a fine-grained ophitic or intergranular matrix These dolerite, is extensively uralitised and bright green amphibole (hornblende) replaces pyroxene (augite?) as the principal mafic mineral. Secondary constituents of the rock include chlorite, biotite, pyrite, quartz and epidote. On the crag directly south of Beaghmore Road [268525 383400], an outcrop of green and grey variegated and finely laminated rock is surrounded by coarse-grained gabbro. This rock is volcaniclastic in origin and was derived from a mixed volcanic / siliciclastic source. Although the contact between tuff and gabbro is not exposed, the tuff most probably represents a raft or selvage of Tyrone Volcanic Group rock incorporated within the gabbro. Viewed in thin section (NI4691), the tuff rock appears well-sorted and consists of alternating mafic and silicic rich domains. The mafic bands consist mainly of green hornblende, plagioclase and epidote while the silicic units consist of well sorted quartz and minor plagioclase. The rock has been deformed and the primary granular depositional textures have been modified and overprinted by pressure solution producing an interlocking mosaic texture in the silicic bands. Porphyritic granitoid and fine-grained non-aphyric acid intrusions post-date the Tyrone Plutonic Group and the nearby Tyrone Volcanic Group. These intrusions form a series of small bosses and sill-like sheets which cut all of the principal lithological components at Black Rock. Two examples of porphyry are worth noting. On the eastern edge of Black Rock, close to the junction between the Blackrock Road and a farm lane [26860 38400] there are several slabby outcrops of porphyry. This rock consists of conspicuous rounded and embayed quartz (up to 5mm dia.), feldspar and hornblende (uralitised pyroxene?) phenocrysts set in a grey fine-grained groundmass. A porphyritic dacite intrusion is exposed on the western edge of the site, beside the farm track at [268250 383500]. Contacts are clearly exposed and early-formed coarse-grained gabbro is cut by dolerite with both basic lithologies in turn by the granitoid. This rock (JTP-159) is greenish-grey in hand specimen and has conspicuous quartz, zoned epidotised feldspar and biotite phenocrysts (extensively replaced by chlorite) in a fine-grained matrix comprising quartz feldspar intergrowths. | |
| Importance: | |
This is a site of national importance. A wide range of lithologies characteristic of the Tyrone Plutonic Group is exposed in a comparatively compact area. The site affords an opportunity to study key lithologies and contact relationships between the principal rock types. | |
| Interpretation: | |
The gabbroic and doleritic rocks at Black Rock are components of the Tyrone Plutonic Complex which has been interpreted as being part of the "Tyrone Ophiolite" (Hutton et al., 1985). These rocks form part of the more extensive Tyrone-Girvan sub-terrane of the Midland Valley Terrane (Bluck et al., 1992) and are broadly equivalent to the ophiolite fragments at Ballintrae (Scotland) and Clew Bay on the west coast of Ireland. The Tyrone Ophiolite is part of the Caledonian-Appalachian Orogen which extends from Scandinavia through northern Britain and Ireland into the northeastern seaboard of America. These rocks are fragments of the ancient oceanic floor of the Iapetus Ocean. This material was obducted onto the continental margin of Laurentia at the northern margin of the Iapetus Ocean some time during Ordovician times. | |
| Conclusions: | |
Black Rock is a site of national importance affording an opportunity to study the principal lithologies and contact relationships within part of the Tyrone Ophiolite (Tyrone Plutonic Group). These rocks form part of the distinctive Tyrone Girvan sub-terrane and are important elements in the development of the Caledonian Orogen in Ireland and Scotland. | |
| Notes: | |
For information and references on the other site within the Tyrone Basic Plutonic Complex see the following site: 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 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 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 | |
| Keywords | |
|---|---|
| Minerals: | Epidote |
| Rocks: | Gabbro, Tuff, Dacite |
| Structures: | Caledonian, phenocrysts |
| Relations: | Intrusion |
| Access | |
|---|---|
| Approach: | Access to the eastern side of the site is from the Blackrock Road while access to the southern and western sides is from the Beaghmore Road. |
| Management: | |
No threats to the integrity of the site were evident at the time of the site visit. Much of the area is too craggy for building and much is unsuitable even for rough-grazing. In recent years several new dwellings have been built on the site, mainly on flat-lying, rock free land adjacent to the Beaghmore and Blackrock Roads. These developments have not seriously compromised the integrity of the site. Future development on the site need not be precluded. The rocks of the Tyrone Plutonic Group 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. There is abundant and varied accessible outcrop at this site. It is unnecessary to manage the site in a proscriptive manner. There is sufficient outcrop of a robust nature present at Black Rock to make hammering and sampling restrictions unnecessary. | |
| Map(s): | 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, 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. Black Rock, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=580. Accessed on 2026-11-05. |
| Previous Site | Next Site |