Earth Science Conservation Review

Summary Full report
Larkhill - PreCambrian
Site number:127  
ESCC:ED
Locality Type:Pit Status: ASSI
Grid Reference: H012639 Centroid
County: FermanaghCouncil area:Fermanagh & Omagh District Council
Period:Precambrian
Stages:Moinian
Lithostrat:Lough Derg Group, Slishwood Division
Site Description

Abandoned feldspar trial pit, Larkhill, Co. Fermanagh.
Highlights:
The site consists of a pit located in a good example of an alkali feldspar rich vein in the Lough Derg Group (Slishwood Division). The vein was previously recorded as being worked (Boswell, 1918) and was possibly a source of raw materials for the nearby Belleek Pottery.
Introduction:
This site is located beside a minor road which runs north from the A47 (Kesh to Belleek Road) at Castle Caldwell towards the border with the Republic of Ireland.
The outcrop is severely restricted and is in the form of an abandoned trial pit located in open bogland.
Larkhill is situated close to the southern edge of the Lough Derg Inlier. Throughout this and the surrounding area, the rocks are predominantly coarse-grained, granulitic, siliceous psammites which form part of the Lough Derg Group (Slishwood Division).
On the first edition of the one inch to one mile Geological Map of the area (Sheet 32, Pettigoe, Geological Survey of Ireland, 1885) rocks at and around the site were referred to jointly as "gneiss & schist".
Geikie (1891) was first to indicate that the rocks of the Lough Derg Inlier could be "readily separated from the ordinary metamorphic rocks (Dalradian) of Donegal". Geikie assigned an Archaean age to the Inlier and both he and Symes (1891) drew attention to similarities with the "the more ancient gneisses (Lewisian) of the Scottish Highlands".
Boswell's (1918) description of the pegmatites in the area around Belleek is in relation to their use in the local Pottery. He referred specifically to a vein in the, "Townland of Larkhill, in the bog to the north of Croagh More".
Anderson (1948) published the first detailed account of the stratigraphy, structure and metamorphism of the Lough Derg Inlier. He also was first to suggest a Moinian age for the "Lough Derg Psammite Group". Anderson's account includes a description of the evolution of the Lough Derg Group and a detailed section on the pegmatites and quartz veins.
Max & Long (1984) published a Rb-Sr whole rock isochron indicating a 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.
The most recently published maps which cover this site include a second 1:50,000 scale edition of Sheet 32 (Kesh, Geological Survey of Northern Ireland, 1994) and the 1:250,000 scale Solid Geology Map of Northern Ireland (Geological Survey of Northern Ireland, 1997). These maps are based on a detailed resurveying in the area during the 1980's.
Description:
Pegmatite veins are ubiquitous throughout the outcrop of the Lough Derg Group
Detail of pegmatite vein, Larkhill, Co. Fermanagh.
and are particularly abundant within a circumscribed central area of the Inlier which includes most of the area within Northern Ireland (Anderson, 1948). Wherever they are seen, pegmatites are both cross-cutting and concordant features which post-date almost all other lithological and structural boundaries.
The pegmatite at Larkhill has been described by Boswell (1918) who probably saw an open excavation in the vein and about 20 tons of excavated "ore" at the side of the pit. Boswell described the vein "...running N.35E and dipping N.W. at 50 or 60 degrees", and estimated it to be "about 4 or 5 feet thick and was excavated to a depth of about 20 feet at the north-eastern end."
The most distinctive feature of the pegmatite is the large (up to 6cm long) interlocking crystals of pale pink and salmon-coloured potash (alkali) feldspar (predominantly microcline). This alkali feldspar is intergrown with varying amounts of grey and off-white quartz, plagioclase feldspar, biotite and muscovite. Quartz occurs in the pegmatite both as late-stage, cross-cutting veins and as intimate graphic intergrowths within the feldspar.
The approximate mineral composition of the Larkhill pegmatite is broadly comparable with pegmatites elsewhere in the vicinity as calculated from chemical analyses. Potash feldspar varies from 60% to 80% of the total rock composition with 10% to 20% sodic feldspar (albite) and 10% to 30% other constituent minerals including quartz and mica.
Importance:
Larkhill is a site of national importance which contains an example of pegmatite veining within the Lough Derg Group (Slishwood Division).
Interpretation:
The pegmatite at Larkhill represents one of the last phases in the complex geological evolution of the Lough Derg Group.
The Lough Derg Group is a complex association of ancient metasedimentary and basic igneous rocks which have undergone at least two major deformational and metamorphic events. Feldspar-rich sediments form the bulk of the Lough Derg Group. Max & Long (1985) have indicated that these rocks are in the region of 895+/-60Ma (Grenvillian?) and form part of the Pre-Dalradian basement in NW Ireland.
During the earlier (Grenvillian?) event, high-temperature, high-pressure Granulite Facies metamorphism resulted in widespread recrystallization and remobilization of these rocks. The later metamorphism during the early-Caledonian (Grampian) was by comparison a low-temperature and low-pressure (Amphibolite Facies) event which overprinted and retrogressed the earlier Granulite Facies mineral assemblages. This later event was accompanied by complex deformation which Anderson (1948) has associated with most of the folds in the Lough Derg Inlier, including the Lough Derg Anticline.
Anderson also indicated that in the Lough Derg Inlier, there may be at least two phases of pegmatite formation. He described the injection of veins of feldspar, quartz and fine-grained acid igneous material along schistosity in semipelites and somewhat less commonly along gneissic foliation in the coarser granulites, drawing a comparison with the Moinian injection complexes in Scotland. Anderson noted that these injection materials originated in pegmatite veins, and that in places, they in turn are sharply cross-cut by later pegmatite veins.
Conclusions:
Larkhill is a site of national importance allowing access to a pegmatite vein in the Lough Derg Group. From a geological point of view, knowledge of the composition and paragenesis of such pegmatites is important for the wider understanding of the evolution of the Lough Derg Group (Slishwood Division) which probably represents the pre-Dalradian basement in the western part of Northern Ireland.
Historically this and similar pegmatites represented an important economic resource which was used as one of the raw materials which helped establish the world famous Belleek Pottery.
Notes:

For information and references on other sites within the Pre-Dalradian Lough Derg Inlier see the following sites:

Key Site 125 - Croagh Mountain Key Site 126 - Gadalough
For information and references on sites within the Pre-Dalradian Central Tyrone Inlier see the following sites:
Key Site 128 - Oughtmore Mountain Key Site 129 - Eagles Rock
For general information on the Pre-Dalradian rocks of Northern Ireland see the following site:
Key Site 123 - The Pre-Dalradian Rocks of Northern Ireland
For information on the Dalradian rocks of Northern Ireland see the following site:
Key Site 124 - The Dalradian Rocks of Northern Ireland
For general information and an extensive reference list on the Precambrian of Northern Ireland see the following site:
Key Site 122 - An Introduction to the Precambrian of Northern Ireland
This site is also important for Mineralogy and Metallogenesis. See Key Site 542 - Larkhill, near Belleek.

Keywords
Minerals:Alkali feldspar, Microcline
Rocks:Pegmatite
Access
Approach:This site is located beside a minor road which runs north from the A47 (Kesh to Belleek Road) at Castle Caldwell towards the border with the Republic of Ireland.
Management:The site is located in partially cut bogland fenced from adjacent rough pasture and from the road. The partially open trench forms a small depression and is overgrown with only a few exposed rock surfaces remaining. Although the area of outcrop is very limited, it is not believed necessary to impose hammering or sampling restrictions at the site.
Threats:Possibly the greatest current threat will come from in-filling or tipping.
Uses
Uses:Bogland
References
Anderson, J.G.C. 1948: The occurrence of Moinian rocks in Ireland Quarterly Journal of the Geological Society of London, vol. 103, pp.171-
Boswell, P.G.H. 1918: British resources of sands and rocks used in glass-making , 2nd edition, London
Geikie, A. 1891: Anniversary Address to the President Quarterly Journal of the Geological Society of London, vol. 47, pp.63-
Max, M.D., O'Connor, P.J. and Long, C.B. 1984: New age data from the Pre-Caledonian basement of the NE Ox Mountains and Lough Derg Inliers, Ireland Bulletin of the Geological Survey of Ireland, vol. 3, pp.203-209
Max, M.D. and Long, C.B. 1985: Pre-Caledonian basement in Ireland and its cover relationships Geological Journal, vol. 20, pp.341-366
Symes, R.G. 1891: Explanatory memoir to accompany Sheets 31 (in part) and 32 of the maps of the Geological Survey of Ireland HMSO, Dublin
Map(s): GSI (1885). 1:63,360 Geological Sheet 32 (Pettigoe), Dublin OS for GSI / GSNI (1994). Kesh, Northern Ireland, Sheet 32, Solid Geology 1:50,000 (BGS) / GSNI (1997). Northern Ireland, Solid Geology (2nd edition) 1:250,000 (BGS)
Rec Type ESCR report Recorder: T.P. Johnston, GSNI.
Enterer: E M Porter
Updates: 19 Apr 2003 / 10 Apr 2003 / 13 MAR 00 / 27 FEB 00
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