Earth Science Conservation Review
| Introduction to the Pre-Dalradian rocks of Northern Ireland | Fermanagh, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
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| Rock Age: | Precambrian (Proterozoic) |
| Interest | |
Description:
1. INTRODUCTION
The oldest rocks in Northern Ireland occur in two separate inliers, in County Fermanagh (part of the Lough Derg Inlier) and in County Tyrone (Central Tyrone Inlier). There remains some uncertainty regarding the absolute age of these rocks. However both inliers are considered to be older than the Dalradian and are believed to be of Moinian Age. The Lough Derg and Central Tyrone Inliers have undergone intense polyphase deformation and high-grade metamorphism (Grenvillian?). In all likelihood neither of these fault-bounded inliers can be regarded as autochthonous basement to the Dalradian in Northern Ireland. It appears more likely that they are allocthonous terranes which have undergone major transpressional displacement from their areas of origin and which "docked" in their present position as a result of late-Caledonian transpression.
Max & Long (1985) produced a Rb-Sr whole rock isochron indicating an 895+/-60Ma (Grenvillian?) age for the Lough Derg Inlier. They also pointed to similarities between the Lough Derg Inlier and rocks at Rosses Point and the Ox Mountains in the Republic of Ireland, introducing the concept of the Slishwood Division to encompass this pre-Dalradian basement.
2. THE LOUGH DERG INLIER (SLISHWOOD DIVISION)
LOCATION
Only a small part (approximately 35 Km2) of the Lough Derg Inlier lies within Northern Ireland. Outcrops are restricted to the area of west Fermanagh, east of Belleek, north of the River Erne and Lower Lough Erne and extending northwestwards to the remote uplands which adjoin the border with the Republic of Ireland.
GEOLOGICAL SETTING
The Lough Derg Inlier is bounded by faults along its south-eastern and northern sides and elsewhere is overlain unconformably by Carboniferous rocks. To the SE, the Pettigoe Fault downthrows to the east, juxtaposing the Inlier with Carboniferous rocks. Along the northern margin of the Inlier, Upper Dalradian rocks have been transported south-westwards across the Inlier along a curved low angle thrust fault (Lough Derg Slide).
LOUGH DERG PSAMMITE GROUP
Anderson (1948) described the Lough Derg Inlier (Lough Derg Psammite Group) as being characterised by a distinctive association of grey, pink and buff weathering, medium to coarse-grained banded siliceous, feldspathic and micaceous psammites and mica schist bands. Despite having undergone considerable recrystallization, the psammitic rocks retain evidence of original sedimentary structures including a variety of bedforms (bedding, cross bedding and slump bedding) which allow younging directions to be determined. Throughout the Lough Derg Psammite Group, there are thin micaceous partings parallel to bedding planes with spacing varying from centimetres to metres. Where micaceous partings are close spaced, the psammite is flaggy. Thin units of pelitic and semi-pelitic schist are also present throughout the Lough Derg Psammite Group. These vary in thickness from a few centimetres up to a couple of metres although overall they constitute only a small proportion of the succession.
In thin section, the psammitic rocks are seen to have strong granoblastic textures and consist of equigranular aggregates of irregularly sutured quartz and feldspar crystals and scattered mica flakes. In general, quartz predominates over feldspar by a ratio of about 3:1. The feldspars present are most commonly microcline and oligoclase with only minor orthoclase being recorded. Mica crystals, most commonly biotite but also muscovite, are randomly scattered with only a small proportion showing parallel orientation (Anderson, 1948). The subordinate pelitic and semi-pelitic schists show a strongly schistose structure in thin section. Muscovite is generally more abundant than biotite and chlorite is also present. The other main constituents are quartz and feldspar (microcline).
METABASITES
The predominantly metasedimentary Lough Derg Group contains a significant number of fine- to medium-grained amphibolitic metabasite bodies in the form of massive pods and boudins. The metabasite masses have a weak schistosity along the margins of some intrusions. Otherwise they are massive and are not obviously deformed. They sometimes form discontinuous trains of boudins which are nearly always concordant or subconcordant with bedding in the metasedimentary rocks. These metabasite bodies represent basic or ultrabasic tholeiitic magmas emplaced as sills or dykes in the sedimentary pile prior to the main metamorphic event. Anderson (1948) described the metabasites as typically consisting of a very high percentage of hornblende (75%+) together with, turbid feldspar, epidote and sphene. The hornblende is a very fresh, bright pale green variety. Lemon (1971) described comparable metabasites in the Ox Mountains as containing garnet-clinopyroxene-plagioclase mineral assemblages.
LATE PEGMATITES
The Lough Derg Psammite Group is cut by numerous late-formed, coarse-grained pegmatite veins containing quartz, alkali-feldspar, biotite and muscovite. These irregular and often lenticular veins are both concordant and discordant. These veins are late stage features which cross-cut most other major structures and lithologies in the Inlier.
Individual pegmatite veins are up to 3m wide and many of the best examples of pegmatite veins outcrop within Northern Ireland in the south central part of the Lough Derg Inlier. Locally, the pegmatites are rich in potash feldspar (up to 64% modal K-feldspar, with 10% K2O in the whole rock). Up until the beginning of the 20th Century, the K2O rich pegmatites were mined locally and used for the manufacture of fine porcelain in the nearby Belleek Pottery (Boswell, 1918).
STRUCTURE OF THE LOUGH DERG INLIER
The Lough Derg Inlier is bounded on its eastern margin by the north-east to south-west trending Pettigoe Fault. Along its western and southern edges, it is overlain unconformably by Lower Carboniferous strata. The northern margin of the Lough Derg Inlier is defined by a northerly dipping tectonic slide (Lough Derg Slide) which juxtaposes the Lough Derg Inlier and inverted Upper Dalradian strata in south Donegal (Alsop, 1991).
There are no published modern structural maps or structural analyses of the Lough Derg Inlier. Anderson's map (1948) showed the Inlier as a large, north-easterly plunging asymmetrical anticline. Anderson interpreted this anticline as a comparatively late-formed (Caledonian?) structure which post-dates the main (Grenvillian?) tectono-metamorphic events. The rocks in the part of the Lough Derg Inlier exposed in Northern Ireland are located on the southern, more steeply dipping limb of the anticline and dip east-north-eastwards at about 60°-90°.
METAMORPHISM
The Lough Derg Inlier has undergone pre-Caledonian (possibly Grenvillian?) high pressure, high temperature (granulite facies) metamorphism followed by a later, low-grade metamorphic (amphibolite facies) event.
3. THE CENTRAL TYRONE INLIER (CORVANAGHAN FORMATION)
LOCATION
The Central Tyrone Inlier is a lenticular shaped area of pre-Caledonian (Moinian?) strata which form a series of rounded hills approximately 10km north-west of Cookstown, County Tyrone. The Inlier is a comparatively homogenous unit of highly deformed paragneiss, schist, and pelite together with chlorite epidote schists with possible green bed affinities.
GEOLOGICAL SETTING
The Central Tyrone Inlier is a fragment of pre-Caledonian Proterozoic continental crust which now forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Bluck et al., 1992). There are however still some doubts surrounding the exact status and origin of these rocks. The Inlier could conceivably represent in-situ autochthonous metamorphic basement to the Midland Valley Terrane. Alternatively, and more likely, the Inlier may be an allochthonous terrane, a slice of basement formed elsewhere and transposed to its present position by major sinistral strike slip or transpressional faulting (Hutton, 1987). In support of the latter interpretation, Hutton has pointed to lithological similarities between the Central Inlier and Grenvillian "greenstone" belts in North America. Hutton has also raised the possibility that the Central Inlier may be part of Gondwanaland Type Assemblage and therefore would have affinities with the basement of southern Britain.
The Central Inlier is completely surrounded by, and has faulted contacts with, a complex association of basic and intermediate, sub-volcanic and volcanic Ordovician rocks (Tyrone Plutonic Complex and Tyrone Volcanic Group) (Geological Survey of Northern Ireland, 1995). These Ordovician rocks form part of an ophiolite complex and back-arc volcanic sequence (about 470 Ma) which in parts have been tectonically overthrust by the older Central Tyrone Inlier (Hutton et al., 1985).
PREVIOUS GEOLOGICAL RESEARCH
Portlock (1843) mapped the lenticular form of the Central Inlier and distinguished it from the "Metamorphic Rocks (hornblendic type)" which surround it (Tyrone Volcanic Group and Tyrone Plutonic Complex, Geological Survey of Northern Ireland, 1997). However, Portlock failed to differentiate between the "Mica Schists" of the Inlier and the Dalradian rocks of the nearby southern part of the Sperrin Mountains.
On the first editions of the Geological Sheets 26 and 34 (Geological Survey of Ireland, 1882, 1877) the rocks of the Central Tyrone Inlier were described as "Schists and Gneisses". Although the metasedimentary lithologies of the Inlier were distinguished from the surrounding "Pyroxene Rocks" (Tyrone Plutonic Complex), both were categorised as "probably of Pre-Cambrian or Upper Laurentian age".
The Geological Memoir which accompanied Sheet 26 (Nolan, 1884) outlined key differences between the "schist and gneiss" of the central portion of the Tyrone Inlier and the chloritic schists elsewhere on the map (Tyrone Volcanic Group and Dalradian). Nolan also described the lithologies of the Central Inlier in some detail and noted what he believed was a gradational boundary between the quartzose schist and gneiss at the core of the Inlier and the hornblendic rocks (Tyrone Plutonic Complex) which surround it.
Cole (1900) presented a more detailed description of the petrology of what he referred to as the "central axis" in Tyrone. He suggested that "the gneissic core of eastern Tyrone appears to owe its characters to the intimate intrusion of ancient aplitic granite magma into pre-existing and fairly basic schists".
However, Cole's interpretation differed from those of Nolan and Portlock in one key respect. Cole suggested that the gabbros and the granite of Slieve Gallion were emplaced successively either side of, but did not seem to have penetrated, a pre-existing gneissic mass.
Hartley (1933) considered the Central Tyrone Inlier to be of Dalradian age. He mapped the Inlier in detail and described the rocks as "granulitic quartz-feldspar biotite schists and muscovite-biotite gneisses" which he believed most likely originated as "a recrystallised . . . and highly folded series of banded arenaceous and argillaceous sediments".
Hartley viewed the Central Inlier simply as a repetition of the "Sperrin Schists" and claimed that the higher (sillimanite) metamorphic grade of the Inlier was as a result of impregnation of the Central Inlier by granitic veins.
Following the Geological Survey of Northern Ireland resurvey of the southern part of the Inlier during the 1960's, Cobbing et al. (1965) proposed that the Central Inlier was of Lower Dalradian age and that the Ordovician Volcanics (Tyrone Volcanic Group) rested, in parts unconformably, upon it.
On the second edition of the Geological Survey Sheet 34 (Pomeroy) (Geological Survey of Northern Ireland, 1979), a clear distinction was made between "the siliceous granulite and quartzo-feldspathic gneiss" of the Central Inlier (referred to for the first time as the Corvanaghan Formation), and the surrounding Tyrone Basic Plutonic Complex. Both groups of rock were described as being of Dalradian age.
The second edition of Geological Sheet 26 (Draperstown) (Geological Survey of Northern Ireland, 1995) includes most of the outcrop of the Central Inlier (Corvanaghan Formation) which is shown as being of "Moinian?" age.
The most recent data available (Hutton, pers. comm., Earls et al., 1996) indicates that the age of the Tyrone Central Inlier is in the region of 650 Ma based on Rb-Sr dates for muscovite and feldspars in two late stage pegmatites which cross-cut most other structures in the Inlier. Furthermore, Hutton has also pointed to the presence of significant hornblende, biotite, chlorite (green bed?) assemblages which may indicate an affinity with the Grenville Terranes of Northern America rather than with other Northern British Moinian Terranes.
The oldest rocks in Northern Ireland occur in two separate inliers, in County Fermanagh (part of the Lough Derg Inlier) and in County Tyrone (Central Tyrone Inlier). There remains some uncertainty regarding the absolute age of these rocks. However both inliers are considered to be older than the Dalradian and are believed to be of Moinian Age. The Lough Derg and Central Tyrone Inliers have undergone intense polyphase deformation and high-grade metamorphism (Grenvillian?). In all likelihood neither of these fault-bounded inliers can be regarded as autochthonous basement to the Dalradian in Northern Ireland. It appears more likely that they are allocthonous terranes which have undergone major transpressional displacement from their areas of origin and which "docked" in their present position as a result of late-Caledonian transpression.
Max & Long (1985) produced a Rb-Sr whole rock isochron indicating an 895+/-60Ma (Grenvillian?) age for the Lough Derg Inlier. They also pointed to similarities between the Lough Derg Inlier and rocks at Rosses Point and the Ox Mountains in the Republic of Ireland, introducing the concept of the Slishwood Division to encompass this pre-Dalradian basement.
2. THE LOUGH DERG INLIER (SLISHWOOD DIVISION)
LOCATION
Only a small part (approximately 35 Km2) of the Lough Derg Inlier lies within Northern Ireland. Outcrops are restricted to the area of west Fermanagh, east of Belleek, north of the River Erne and Lower Lough Erne and extending northwestwards to the remote uplands which adjoin the border with the Republic of Ireland.
GEOLOGICAL SETTING
The Lough Derg Inlier is bounded by faults along its south-eastern and northern sides and elsewhere is overlain unconformably by Carboniferous rocks. To the SE, the Pettigoe Fault downthrows to the east, juxtaposing the Inlier with Carboniferous rocks. Along the northern margin of the Inlier, Upper Dalradian rocks have been transported south-westwards across the Inlier along a curved low angle thrust fault (Lough Derg Slide).
LOUGH DERG PSAMMITE GROUP
Anderson (1948) described the Lough Derg Inlier (Lough Derg Psammite Group) as being characterised by a distinctive association of grey, pink and buff weathering, medium to coarse-grained banded siliceous, feldspathic and micaceous psammites and mica schist bands. Despite having undergone considerable recrystallization, the psammitic rocks retain evidence of original sedimentary structures including a variety of bedforms (bedding, cross bedding and slump bedding) which allow younging directions to be determined. Throughout the Lough Derg Psammite Group, there are thin micaceous partings parallel to bedding planes with spacing varying from centimetres to metres. Where micaceous partings are close spaced, the psammite is flaggy. Thin units of pelitic and semi-pelitic schist are also present throughout the Lough Derg Psammite Group. These vary in thickness from a few centimetres up to a couple of metres although overall they constitute only a small proportion of the succession.
In thin section, the psammitic rocks are seen to have strong granoblastic textures and consist of equigranular aggregates of irregularly sutured quartz and feldspar crystals and scattered mica flakes. In general, quartz predominates over feldspar by a ratio of about 3:1. The feldspars present are most commonly microcline and oligoclase with only minor orthoclase being recorded. Mica crystals, most commonly biotite but also muscovite, are randomly scattered with only a small proportion showing parallel orientation (Anderson, 1948). The subordinate pelitic and semi-pelitic schists show a strongly schistose structure in thin section. Muscovite is generally more abundant than biotite and chlorite is also present. The other main constituents are quartz and feldspar (microcline).
METABASITES
The predominantly metasedimentary Lough Derg Group contains a significant number of fine- to medium-grained amphibolitic metabasite bodies in the form of massive pods and boudins. The metabasite masses have a weak schistosity along the margins of some intrusions. Otherwise they are massive and are not obviously deformed. They sometimes form discontinuous trains of boudins which are nearly always concordant or subconcordant with bedding in the metasedimentary rocks. These metabasite bodies represent basic or ultrabasic tholeiitic magmas emplaced as sills or dykes in the sedimentary pile prior to the main metamorphic event. Anderson (1948) described the metabasites as typically consisting of a very high percentage of hornblende (75%+) together with, turbid feldspar, epidote and sphene. The hornblende is a very fresh, bright pale green variety. Lemon (1971) described comparable metabasites in the Ox Mountains as containing garnet-clinopyroxene-plagioclase mineral assemblages.
LATE PEGMATITES
The Lough Derg Psammite Group is cut by numerous late-formed, coarse-grained pegmatite veins containing quartz, alkali-feldspar, biotite and muscovite. These irregular and often lenticular veins are both concordant and discordant. These veins are late stage features which cross-cut most other major structures and lithologies in the Inlier.
Individual pegmatite veins are up to 3m wide and many of the best examples of pegmatite veins outcrop within Northern Ireland in the south central part of the Lough Derg Inlier. Locally, the pegmatites are rich in potash feldspar (up to 64% modal K-feldspar, with 10% K2O in the whole rock). Up until the beginning of the 20th Century, the K2O rich pegmatites were mined locally and used for the manufacture of fine porcelain in the nearby Belleek Pottery (Boswell, 1918).
STRUCTURE OF THE LOUGH DERG INLIER
The Lough Derg Inlier is bounded on its eastern margin by the north-east to south-west trending Pettigoe Fault. Along its western and southern edges, it is overlain unconformably by Lower Carboniferous strata. The northern margin of the Lough Derg Inlier is defined by a northerly dipping tectonic slide (Lough Derg Slide) which juxtaposes the Lough Derg Inlier and inverted Upper Dalradian strata in south Donegal (Alsop, 1991).
There are no published modern structural maps or structural analyses of the Lough Derg Inlier. Anderson's map (1948) showed the Inlier as a large, north-easterly plunging asymmetrical anticline. Anderson interpreted this anticline as a comparatively late-formed (Caledonian?) structure which post-dates the main (Grenvillian?) tectono-metamorphic events. The rocks in the part of the Lough Derg Inlier exposed in Northern Ireland are located on the southern, more steeply dipping limb of the anticline and dip east-north-eastwards at about 60°-90°.
METAMORPHISM
The Lough Derg Inlier has undergone pre-Caledonian (possibly Grenvillian?) high pressure, high temperature (granulite facies) metamorphism followed by a later, low-grade metamorphic (amphibolite facies) event.
3. THE CENTRAL TYRONE INLIER (CORVANAGHAN FORMATION)
LOCATION
The Central Tyrone Inlier is a lenticular shaped area of pre-Caledonian (Moinian?) strata which form a series of rounded hills approximately 10km north-west of Cookstown, County Tyrone. The Inlier is a comparatively homogenous unit of highly deformed paragneiss, schist, and pelite together with chlorite epidote schists with possible green bed affinities.
GEOLOGICAL SETTING
The Central Tyrone Inlier is a fragment of pre-Caledonian Proterozoic continental crust which now forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Bluck et al., 1992). There are however still some doubts surrounding the exact status and origin of these rocks. The Inlier could conceivably represent in-situ autochthonous metamorphic basement to the Midland Valley Terrane. Alternatively, and more likely, the Inlier may be an allochthonous terrane, a slice of basement formed elsewhere and transposed to its present position by major sinistral strike slip or transpressional faulting (Hutton, 1987). In support of the latter interpretation, Hutton has pointed to lithological similarities between the Central Inlier and Grenvillian "greenstone" belts in North America. Hutton has also raised the possibility that the Central Inlier may be part of Gondwanaland Type Assemblage and therefore would have affinities with the basement of southern Britain.
The Central Inlier is completely surrounded by, and has faulted contacts with, a complex association of basic and intermediate, sub-volcanic and volcanic Ordovician rocks (Tyrone Plutonic Complex and Tyrone Volcanic Group) (Geological Survey of Northern Ireland, 1995). These Ordovician rocks form part of an ophiolite complex and back-arc volcanic sequence (about 470 Ma) which in parts have been tectonically overthrust by the older Central Tyrone Inlier (Hutton et al., 1985).
PREVIOUS GEOLOGICAL RESEARCH
Portlock (1843) mapped the lenticular form of the Central Inlier and distinguished it from the "Metamorphic Rocks (hornblendic type)" which surround it (Tyrone Volcanic Group and Tyrone Plutonic Complex, Geological Survey of Northern Ireland, 1997). However, Portlock failed to differentiate between the "Mica Schists" of the Inlier and the Dalradian rocks of the nearby southern part of the Sperrin Mountains.
On the first editions of the Geological Sheets 26 and 34 (Geological Survey of Ireland, 1882, 1877) the rocks of the Central Tyrone Inlier were described as "Schists and Gneisses". Although the metasedimentary lithologies of the Inlier were distinguished from the surrounding "Pyroxene Rocks" (Tyrone Plutonic Complex), both were categorised as "probably of Pre-Cambrian or Upper Laurentian age".
The Geological Memoir which accompanied Sheet 26 (Nolan, 1884) outlined key differences between the "schist and gneiss" of the central portion of the Tyrone Inlier and the chloritic schists elsewhere on the map (Tyrone Volcanic Group and Dalradian). Nolan also described the lithologies of the Central Inlier in some detail and noted what he believed was a gradational boundary between the quartzose schist and gneiss at the core of the Inlier and the hornblendic rocks (Tyrone Plutonic Complex) which surround it.
Cole (1900) presented a more detailed description of the petrology of what he referred to as the "central axis" in Tyrone. He suggested that "the gneissic core of eastern Tyrone appears to owe its characters to the intimate intrusion of ancient aplitic granite magma into pre-existing and fairly basic schists".
However, Cole's interpretation differed from those of Nolan and Portlock in one key respect. Cole suggested that the gabbros and the granite of Slieve Gallion were emplaced successively either side of, but did not seem to have penetrated, a pre-existing gneissic mass.
Hartley (1933) considered the Central Tyrone Inlier to be of Dalradian age. He mapped the Inlier in detail and described the rocks as "granulitic quartz-feldspar biotite schists and muscovite-biotite gneisses" which he believed most likely originated as "a recrystallised . . . and highly folded series of banded arenaceous and argillaceous sediments".
Hartley viewed the Central Inlier simply as a repetition of the "Sperrin Schists" and claimed that the higher (sillimanite) metamorphic grade of the Inlier was as a result of impregnation of the Central Inlier by granitic veins.
Following the Geological Survey of Northern Ireland resurvey of the southern part of the Inlier during the 1960's, Cobbing et al. (1965) proposed that the Central Inlier was of Lower Dalradian age and that the Ordovician Volcanics (Tyrone Volcanic Group) rested, in parts unconformably, upon it.
On the second edition of the Geological Survey Sheet 34 (Pomeroy) (Geological Survey of Northern Ireland, 1979), a clear distinction was made between "the siliceous granulite and quartzo-feldspathic gneiss" of the Central Inlier (referred to for the first time as the Corvanaghan Formation), and the surrounding Tyrone Basic Plutonic Complex. Both groups of rock were described as being of Dalradian age.
The second edition of Geological Sheet 26 (Draperstown) (Geological Survey of Northern Ireland, 1995) includes most of the outcrop of the Central Inlier (Corvanaghan Formation) which is shown as being of "Moinian?" age.
The most recent data available (Hutton, pers. comm., Earls et al., 1996) indicates that the age of the Tyrone Central Inlier is in the region of 650 Ma based on Rb-Sr dates for muscovite and feldspars in two late stage pegmatites which cross-cut most other structures in the Inlier. Furthermore, Hutton has also pointed to the presence of significant hornblende, biotite, chlorite (green bed?) assemblages which may indicate an affinity with the Grenville Terranes of Northern America rather than with other Northern British Moinian Terranes.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Introduction to the Pre-Dalradian rocks of Northern Ireland, County Fermanagh, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=123. Accessed on 2024-12-26 |
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