ESCR - Introduction to the Caledonian Igneous Complexes of Northern Ireland

Earth Science Conservation Review

Summary Full report
Introduction to the Caledonian Igneous Complexes of Northern Ireland	Antrim, Armagh, Down, Londonderry, Tyrone

Site number:	595
Locality Type:	Various
Grid Reference:
Google maps:	,
County:	Antrim, Armagh, Down, Londonderry, Tyrone
Period:	Devonian, Silurian, Ordovician, Precambrian
Lithostrat:	Tyrone Volcanic Group, Newry Igneous Complex, Old Red Sandstone, Pomeroy Granite, Slieve Gallion Granodiorite, Tyrone Plutonic Group

Site Description
Introduction:
Since the 1960's the evolution of the world's metamorphic belts, including the Caledonides have been viewed in the context of global plate tectonic theory. Plate tectonics and in particular the key mechanism of continental drift, have revolutionized the way that geologists have modeled the evolution of the continents. Plate tectonic theory offers a mechanism whereby, through geological time, large areas of lithospheric crust (plates) can migrate large distances across the surface of the Earth. Oceans open and close, and in the process create sedimentary depositional basins and mountain chains. Plate tectonic models also provide the key to our understanding of the origins of magmatic activity and the important relationship between igneous rocks type and tectono-stratigraphic environment.
Description:
PART I LOWER PALAEOZOIC TERRANES, PLATE TECTONICS & PALAEOGEOGRAPHY 1. THE TERRANES OF NORTHERN BRITAIN The basement rocks of Northern Britain have been divided into a number of distinct tectono-stratigraphic "terranes" separated by major northeast-southwest trending faults (Bluck et al., 1992). Rocks with common geological provenance and a shared metamorphic and deformational history characterize each terrane. Terranes range in age from the Archaean granulite facies rocks of northwest Scotland (Hebridean Terrane about 2.9Ga) to the Silurian, lower greenschist facies turbidites of southern Scotland and Ireland (Southern Uplands Terrane about 430Ma). The rocks and sites described here fall exclusively within the Midland Valley (Tyrone-Girvan sub-Terrane) and the Southern Uplands Terrane which includes the Northern Belt Sub-Terrane. A. The Midland Valley Terrane The Midland Valley Terrane extends through central Scotland southwest into Northern Ireland. In Scotland, the northern boundary of this terrane is clearly defined by the Highland Boundary Fault and the southern boundary by the Southern Uplands Fault. In Ireland, the northern boundary of the Midland Valley Terrane is less clearly defined than it is in Scotland. It is partially concealed by south-easterly directed thrusting along the Omagh Fault during a later, post-Carboniferous (Hercynian?) orogenic event. The northern boundary of this terrane is marked by a series of Caledonian fault structures extending from Torr Head to Clew Bay. The southern limit of the Midland Valley Terrane is delineated by the continuation of the Southern Uplands Fault from Scotland into Northern Ireland. In Ireland, this southern boundary is overlapped and concealed beneath Palaeozoic and younger cover rocks. No pre-Lower Palaeozoic basement rocks are exposed within the Midland Valley Terrane at the present erosion level in Scotland and Northern Ireland. B. The Tyrone-Girvan Sub-Terrane The Tyrone-Girvan Sub-Terrane is a lithologically distinct component contained within the Midland Valley Terrane. It comprises fragments of oceanic (ophiolite) crust and related basic to intermediate magmatic arc-volcanic rocks. These rocks were accreted and erupted along the northern margin of the Iapetus Ocean starting from mid-Ordovician times. C. The Southern Uplands Terrane The Southern Uplands Terrane extends southeastwards from the Southern Uplands of Scotland to the Down-Longford Massif of northern and central Ireland. The northern boundary of this terrane is the Southern Uplands Fault and, its unexposed extension into Ireland. The southern terrane boundary is marked by the Iapetus Suture, a zone of transpressional convergence that marks the line of final closure of the Iapetus Ocean and the join between northern and southern Britain. The Iapetus Suture extends from the Berwickshire coast of Scotland through Ireland to County Limerick south of the River Shannon. The Southern Uplands Terrane consists of a broad belt of turbiditic and pelagic sediments formed in an open geosynclinal basin during the Ordovician and Silurian. As the Iapetus Ocean closed these rocks were accreted along the northern continental margin of Iapetus where they were deformed and stacked to form an accretionary prism, a series of fault-bound wedges of tightly folded rock (McKerrow et al. 1977). 2. THE CALEDONIDES Sometime during late-Cambrian or early-Ordovician times, the Laurentian continental plate divided and North America became separated from what subsequently became a Northern European Plate (Baltica) and a central European Plate (Avalonia) which at that time incorporated Nova Scotia and Newfoundland. The separation of these plates resulted in the formation of the Iapetus Ocean (named after Iapetus, father of Prometheus and Atlas in Greek mythology), sometimes referred to as the Pro-Atlantic Suture. During the time that Iapetus was open, thick piles of turbiditic sediments were deposited in this geosynclinal ocean basin. From early-Ordovician times onwards, the Laurentian and BalticaAvalonia plates were converging and the Iapetus Ocean began to close. In northern Britain the closure of Iapetus was in part accommodated by north-westerly directed subduction of the ocean floor as it was progressively pushed beneath the Laurentian Continental Plate Margin. This situation was mirrored along the north-western margin of Avalonia where the Iapetus Ocean floor was subsumed along a south-easterly dipping subduction zone. In northern Britain, the compressive forces arising from convergence and plate collision, gave rise to Caledonian folding and faulting of the sediments within the Iapetus Ocean (the Paratectonic Caledonides), as well as the primary deformation of the Dalradian rocks which had been deposited in earlier formed basins along the Laurentian margin (the Orthotectonic Caledonides). Prior to the formation of the present day North Atlantic Ocean, the Caledonides formed a continuous mountain range which stretched from the north-eastern seaboard of the United States to Scandinavia. Today, remnants of this now disjointed Caledonides range can be found in the Appalachians, northeastern Canada, Greenland, northwest Ireland, Scotland and Scandinavia. 3. CALEDONIAN MAGMATISM During the continental collision leading to the Caledonian Orogeny, basaltic ocean crust and the silicic sediment contained within the Iapetus Ocean basin were subducted beneath the northern and southern continental margins of Iapetus. This process resulted in partial melting and fractional crystallization of the subducted material and crust on the continental foreland. The resultant magmatism ranged from basic to acid in composition. The majority of these magmas were erupted at surface in shallow water and sub-aerial environments along the continental margin and eventually formed island arcs. Early formed granitoid plutons (tonalite) also formed at about this time. Remnants of these arcs can be seen today at Clew Bay (South County Mayo), in County Tyrone and at Ballantrae (Scotland). This situation is mirrored along the southern margin of Iapetus where volcanic arc related rocks occur in Wexford, Snowdonia Wales and the English Lake District. Continued subduction during the later stages of the Caledonian times caused further widespread partial-melting of the silicic crustal rocks and resulted in much more potassium rich acid magmatism. In northern Britain, granitoid intrusions are the most common and widespread manifestation of this Caledonian magmatic activity. Referred to as the "Newer Granites", these intrusions have long been regarded as a prime example of calc-alkaline orogenic magmatism (Soper, 1986). These rocks are widely distributed in northern Britain and in the north of Ireland are found throughout both the Orthotectonic and Paratectonic Caledonides. PART II DISTRIBUTION OF CALEDONIAN IGNEOUS ROCKS IN NORTHERN IRELAND In Northern Ireland, Lower Palaeozoic igneous rocks occur in counties Antrim, Down, Armagh, Tyrone and south Londonderry where they form parts of the Southern Uplands Terrane, the Tyrone-Girvan SubTerrane of the Midland Valley Terrane and the Southern Highlands (Grampian) Terrane. 1. COUNTIES DOWN & ARMAGH: THE NEWRY IGNEOUS COMPLEX & ASSOCIATED SITES The Newry Igneous Complex is one of a series of granitic complexes emplaced within the Southern Uplands and Midland Valley Terranes in Ireland and Scotland during Caledonian (late Silurian to early Devonian) times. The Newry Igneous Complex intrudes folded Silurian rocks of the Down-Longford Massif. The Complex extends over an area of about 45km2 from Slieve Croob in the northeast to Forkhill in south Armagh. The Newry Complex comprises a series of overlapping plutons consisting of a variety of intrusive acidic, intermediate and ultramafic rocks. The southern end of the Newry Complex is cut by the younger (Palaeogene) Slieve Gullion Ring Complex. The current and generally accepted age of the Newry Igneous Complex is 399±21Ma based on a whole-rock isotope analysis (Rb-Sr) of granodiorite from the south-western part of the intrusion (O'Connor, 1975). Meighan & Neeson (1979) produced a 403±3 Rb-Sr age and a 399±3 age for granodiorite from the northeast pluton. Field relations indicate that the ultramafic-intermediate complex represents the earliest major phase of intrusion. On the first edition (one inch to one mile scale) Geological Survey of Ireland, maps which cover the Newry Complex (Sheet 48 (Banbridge), Sheet 59 (Keady), Sheet 60 (Newry) and Sheet 70 (Crossmaglen)) the Newry Igneous Complex was represented as a single homogenous intrusion. The rocks were referred to as "granite" and porphyritic and non-porphyritic varieties were identified. Ultramafic and intermediate rocks, which we now know form the northeastern end of the Newry Igneous Complex, were not recognized, and were therefore not represented on these maps. However, in the memoirs accompanying these early maps, there were some indications of the existence of these other lithologies. For example, Egan (1872) described "syenitic granite", a term used for rocks that contained "hornblende in great abundance". Early research was undertaken by Reynolds (1931; 1934; 1936; 1943), who was the first worker to investigate and publish details of the occurrence and petrology of these rocks. Reynolds also produced the first detailed lithological map showing the distribution of rock types and internal contacts within the ultramafic - intermediate complex, and its relationship to the more extensive granodiorite plutons and surrounding country rock. The early opinions of Reynolds (1934) and Bailey (1936) favoured a magmatic origin for the rocks. However, Reynolds (1944) subsequently proposed a non-magmatic 'transformist' theory for the origin of the Newry Igneous Complex which involved massive, in situ metasomatism of the Silurian sedimentary country rocks. More recent investigations of the structure and petrology of the north-eastern end of the Newry Complex (Grant, 1961; Neeson & Meighan, 1977; Meighan & Neeson, 1979) has led to a better understanding of the overall evolution of the Newry Complex. The work of Grant (1961) returned scientific thinking to the magmatic model of origin for the complex. More recent work (Neeson & Meighan, 1977; Meighan & Neeson, 1979) led to the subdivision of the complex into three distinct plutons and has involved the integration of geochemical data with field evidence and petrography. The results of this research support the magmatic the origin of the major rock types that constitute the complex. In addition, Meighan & Neeson (1979) proposed that ultramafic rocks, and associated meladiorites, were produced by a process of crystal accumulation from intermediate magma compositions. The most recently published research is incorporated into the second edition of the 1:250,000 scale Solid Geology Map of Northern Ireland (Geological Survey of Northern Ireland, 1997). Key Sites 568-577 represent a comprehensive network encompassing the lithological variation within the Newry Igneous Complex and surrounding contact metamorphosed country rocks. County Down: Lamprophyre Dykes Dykes of Caledonian age are intruded into interbedded greywacke sandstones and shales of Silurian age in Co. Down. Although the best exposures of these dykes are found along the Irish Sea coast of the Ards peninsula, their geographical extent covers much of Co. Down east of a line between Ballynahinch and Newtownards. The intrusions have a predominantly northeast to southwest trend, which is broadly concurrent with the strike of country rocks. The intrusions are lamprophyres and in thin section they display characteristic lamprophyre textures being porphyritic and containing numerous idiomorphic mafic phenocrysts. This dyke swarm is believed to be associated with the nearby Newry Igneous Complex. Two main phases of lamprophyre dyke intrusion have been recognized by Reynolds (1931) and confirmed by the work of Anderson (1962). Reynolds used the word 'crushed' to describe an older series of dykes, and showed them to be geographically restricted to the south of a line joining Ringburr Point and Ringboy Point on the Ards peninsula. These older dykes were probably intruded syn-tectonically during the Caledonian Orogeny. Later and generally undeformed dykes were termed the "younger" or 'uncrushed' series (Reynolds, 1931),and are a supposedly post- tectonic suite. Dykes of this younger suite are most common north of the Ringburr Point-Ringboy Point line. 2. COUNTIES TYRONE, LONDONDERRY & ANTRIM: IGNEOUS COMPLEXES Sites are located for the most part in Counties Tyrone and Londonderry with one site occurring in Antrim. They form parts of the Tyrone Volcanic Group, Tyrone Plutonic Complex and various related granitoid intrusions. These are remnants of ophiolite and island arc volcanic sequences and arc-related acid intrusions. Together, these sites are part of the Tyrone-Girvan Sub Terrane of the Midland Valley, which includes the Ballintrae Ophiolite Complex in Scotland and the Clew Bay Complex in County Mayo. The Caledonian igneous complexes of County Tyrone play a key role in the wider understanding of the Caledonian Orogen in Northern Britain. Outcrops occur over an area of about 300km2 in Northern Ireland and represent by far the most extensive outcrop of obducted Ordovician island arc and arc related rocks remaining along the northern margin of Iapetus in Northern Britain. Unlike the Newry Igneous Complex, the Caledonian Igneous rocks in Tyrone and Londonderry have attracted only sporadic and limited research interest. Excluding one radiometric age date for the Craigballyharky intrusion, there remains a paucity of published radiometric dates for most of the major Caledonian elements. A dating programme funded as part of this project is underway and is expected to result in some valuable Ur-Pb age data (NIGL in prep). New radiometric ages for the main intrusive elements in Counties Tyrone and Londonderry should help answer questions about the timing of events during the plate collision and subsequent transpressive terrane accretion. They may also allow comparisons to be made with dates for similar events recorded at Clew Bay in County Mayo and in the Girvan-Ballintrae ophiolite complex in Scotland, thereby shedding light on the timing and nature of the plate collision and closure of the Iapetus Ocean. Such data should enhance the value of the ESCR sites and confirm the international as well as national geological importance of the Northern Ireland site network. Mid-County Tyrone contains a wide variety of intrusive and extrusive igneous lithologies. Despite poor exposure, it has been possible through GSNI mapping and related research, to build a general stratigraphy based on field relationships. In 1845, Portlock produced a pioneering report titled "Geological Report on the Geology of Londonderry and parts of Tyrone, Fermanagh". This report contained a detailed geological map on which almost all of the principal Caledonian igneous rocks in Tyrone and south Londonderry were represented. First editions of one inch to one mile geological maps for Sheet 26 (Draperstown), Sheet 27 (Cookstown) and Sheet 34 (Pomeroy) and the accompanying memoirs represented and described most of the major lithological subdivisions of the Lower Palaeozoic igneous rocks. More detailed research studies carried out from the 1930's onwards have gradually built up a fuller understanding of the rocks and their regional context and helped unravel the geological history of this area. Although there is scope for further detailed research in this area, the published work, together with revised 1:50,000 scale second editions of the principle component geological maps constitutes a substantial body of geological data upon which this Earth Science Conservation Review has been based. ----------------------------------------------------------------- ESCR Site Ranking of Caledonian Igneous Sites in Northern Ireland: * - Site of International interest - Site of National interest
Notes:
For information and references on the sites within the Newry Igneous Complex see the following sites: Key Site 568 - Ardglass, Co Down Key Site 569 - Slieve Croob Key Site 570 - Shannaghan Hill Key Site 571 - Ballymagreehan Quarry Key Site 572 - Rathfriland Key Site 573 - Gransha Key Site 574 - Aughnagon Quarry Key Site 575 - Goraghwood Quarry Key Site 576 - Newry By-pass Key Site 577 - Cam Lough Quarry Key Site 578 - Kearney Point Key Site 579 - Ballyhalbert Pier For information and references on the 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 the 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 the 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

Keywords
Rocks:	Basalt, Breccia, Breccia, Volcanic, Chert, Feldspar porphyry, Gabbro, Granite, Granodiorite, Granophyre, Greywacke, Hornfels, Microgranite, Mylonite, Pegmatite, Pillow lava, Porphyry, Quartz porphyry, Quartz-feldspar porphyry, Rhyolite, Shale, Tuff, Turbi
Fossil Groups:	Graptolite
Structures:	breccia, Caledonian, dyke, fiamme, mineral vein, tuff, turbidite, pluton, enclave, normal zoning, mineral layering, ophiolite, pillow lava, lopolith
Relations:	Extrusion, Intrusion


Map(s):	GEOLOGICAL SURVEY OF IRELAND, 1869. 1:63,360 Geological Sheet 38. Dublin, Ordnance Survey for Geological Survey of Ireland. GEOLOGICAL SURVEY OF IRELAND, 1869. 1:63,360 Geological Sheet 49 (Downpatrick). Dublin, Ordnance Survey for Geological Survey of Ireland. GEOLOGICAL SURVEY OF IRELAND, 1871. 1:63,360 Geological Sheet 48 (Banbridge). (Dublin, Ordnance Survey for Geological Survey of Ireland). GEOLOGICAL SURVEY OF IRELAND, 1875. 1:63,360 Geological Sheet 59 (Castleblayney). (Dublin, Ordnance Survey for Geological Survey of Ireland). GEOLOGICAL SURVEY OF IRELAND, 1876. 1:63,360 Geological Sheet 60 (Newry). Dublin, Ordnance Survey for Geological Survey of Ireland. GEOLOGICAL SURVEY OF IRELAND, 1877. 1:63,360 Geological Sheet 34 (Pomeroy). Dublin, Ordnance Survey for Geological Survey of Ireland. GEOLOGICAL SURVEY OF IRELAND, 1880. 1:63,360 Geological Sheet 27 (Cookstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF IRELAND, 1882. 1:63,360 Geological Sheet 26 (Draperstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF IRELAND, 1886. 1:63,360 Geological Sheet 14 (Cushendall). 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, 1963. Ballycastle, Northern Ireland Sheet 8. Solid Geology. 1:63,360. (Southampton: Ordnance Survey for the Geological Survey of Northern 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). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1983. Cookstown, Northern Ireland Sheet 27. Solid. 1:50,000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1995. Draperstown. Northern Ireland Sheet 26. Solid and Drift Geology. 1:50,000. (Keyworth, Nottingham: British Geological Survey). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1997. Northern Ireland. Solid Geology (2nd edition). 1:250,000. (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. Introduction to the Caledonian Igneous Complexes of Northern Ireland, County Antrim, Armagh, Down, Londonderry, Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=595. Accessed on 2026-11-06.