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Earth Science Conservation Review
| Gransha | Down |
| Site number: | 573 | ||
| ESCC: | EO | ||
| Locality Type: | Inland exposure | Status: | ASSI |
| Grid Reference: | J160354 Centroid | ||
| Google maps: | 54.25477,-6.22012 | ||
| County: | Down | Council area: | Newry, Mourne & Down District Council |
| Period: | Devonian, Silurian | ||
| Stages: | Lower Devonian | ||
| Lithostrat: | Newry Igneous Complex | ||
| Site Description | |
|---|---|
| Highlights: | |
The site contains outcrops of strongly foliated granodiorite which forms part of the northeast pluton of the late Caledonian Newry Igneous Complex. The granodiorite at this site is more basic than granodiorite exposed towards the centre of the pluton indicating that the intrusion is "normally zoned. The granodiorite contains a range of igneous and sedimentary enclaves. | |
| Introduction: | |
The site covers an area of natural outcrops approximately 4.5km west-northwest of Rathfriland. Geologically it is located at the northwest margin of the northeast pluton which forms part of the Newry Igneous Complex. The site is in a roadside field and is accessible from the C class Gransha Road close to Gransha Gospel Hall. On the first edition of the Newry (Sheet 60) one-inch to the mile geological map (Geological Survey of Ireland, 1876) rocks at this site were mapped as granite. The rocks are rich in quartz, plagioclase feldspar, alkali feldspar, biotite mica and hornblende. Reynolds (1934; 1936) classified the rocks as granodiorite based on thin section petrology. This classification and terminology still stand within modern schemes such as Streckeisen (1976). Systematic sampling, thin section petrology and geochemistry (Grant, 1961; Meighan and Neeson, 1979; Neeson, 1984) have revealed that granodiorite at the margins of the northeast and southwest plutons are more basic than rocks at the centre of the intrusions. This pattern has been described as "normal zoning". The opposite situation is seen in the middle pluton and has been described as "reverse zoning". The rocks at Gransha, in common with granodiorites at the margins of plutons within the Newry Igneous Complex, have a foliation formed by the alignment of minerals particularly biotite mica, (Reynolds, 1934; Meighan & Neeson 1979). Fabrics are aligned parallel to contacts, a feature attributed to regional deformation and the shouldering aside of country rocks during diapiric emplacement (Murphy, 1987). A varied selection of enclaves (xenoliths), of both igneous and sedimentary origin, are found at this site. 'Sedimentary' enclaves are thought to represent metamorphosed Silurian country rock. Less evolved magma compositions are most likely represented by the igneous varieties. The enclaves are spectacularly flattened in the plane of the foliation. Meighan & Neeson (1979) published a Rb-Sr age of 399±3Ma for the granodiorite of the northeast pluton. Field relations indicate that this pluton was the earliest emplaced of the three major bodies that together account for most of the Newry Igneous Complex. | |
| Description: | |
Medium- to coarse-grained granodiorite, part of the northeast pluton is exposed at Gransha. The main mineral components of the rocks are quartz, plagioclase feldspar, alkali feldspar, biotite mica and hornblende. A strong mineral foliation, particularly of biotite, is present and strikes north-northeast to south-southwest sub-parallel to the main contact of granodiorite with Silurian country rock to the northwest. The foliation is steeply inclined to the northwest. The granodiorite at Gransha contains a selection of enclaves (xenoliths), of sedimentary and igneous origin, are found at this site location (Meighan, pers. comm.). Most have irregular rounded forms and in some cases show diffuse boundaries with enclosing granodiorite. In places, the enclaves are spectacularly flattened and aligned to the foliation in the granodiorite. Examples of igneous enclaves include ultramafic 'clots' (restite), microdiorite, and diorite, with included restite. 'Sedimentary' xenoliths appear to be metamorphosed Silurian greywackes and shales. | |
| Importance: | |
Gransha is a site of national importance offering access to high quality exposure of foliated granodiorite near to the margin of the northeast pluton of the Newry Igneous Complex. The granodiorite is more basic than that exposed towards the centre of the pluton indicating the intrusion is normally zoned. Such zoning could be evidence of fractional crystallization of magma after emplacement. Spectacularly flattened igneous and 'sedimentary' enclaves provide information concerning pluton emplacement and perhaps source magma compositions. | |
| Interpretation: | |
The compositional difference between granodiorites at this site and those towards the centre of the northeast pluton (see Key Site 572 - Rathfriland) indicates a "normal zoning" pattern. Such zoning could be interpreted as evidence of fractional crystallization of magma after emplacement. Alternatively, several injections of magma might have been involved in the formation of the northeast pluton; a model that could pertain to the formation of each of the plutons that together constitute the Newry Igneous Complex (Meighan and Neeson, 1979). The presence of foliation, and the flattening of enclaves, within the granodiorite is in keeping with the geological setting, i.e. a pluton margin close to the contact with country rock (Reynolds, 1934; Meighan & Neeson 1979). Foliation has been attributed to regional deformation (Cameron, 1981) and shouldering aside of country rock during emplacement of the Newry Igneous Complex. The regional curvature of foliation parallel to pluton margins lends weight to a diapiric intrusive model of formation (Murphy, 1987). 'Sedimentary' and igneous enclaves found at this site have been transported (to varying degrees) by the granodiorites. Diffuse boundaries indicate some assimilation of the enclaves into the granodiorite. 'Sedimentary' enclaves have almost certainly been derived from the surrounding Silurian country rock. Assimilation of the country rock is likely to have resulted from fracturing in response to regional deformation. The various igneous enclaves represent less evolved magma compositions. | |
| Conclusions: | |
The Gransha site is of national importance offering access to high quality exposure of foliated granodiorite in the northeast pluton. The granodiorite is more basic than that exposed towards the centre of the pluton indicating the intrusion is "normally zoned". Such zoning could be evidence of fractional crystallization of magma after emplacement. The presence of foliation and flattened enclaves in this area is attributed to regional deformation and shouldering aside of country rock during diapiric emplacement of the Newry Igneous Complex. | |
| Notes: | |
For information and references on other 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 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 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 | |
|---|---|
| Rocks: | Granodiorite |
| Structures: | Caledonian, pluton, enclave, normal zoning |
| Relations: | Intrusion |
| Access | |
|---|---|
| Approach: | The site is in a roadside field and is accessible from the C class Gransha Road close to Gransha Gospel Hall. |
| Management: | |
The site occupies an area of farmland currently used for crops and grazing of livestock; permission will be needed to gain access. Exposures are of a limited size; consequently there is a need to restrict hammering at this site. The rocks of the Newry Igneous complex are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration and production has previously been carried out in Counties Armagh and Down. The area which includes this site is likely to attract further exploration activity. | |
| Threats: | Sampling should be restricted due to limited size of outcrop. |
| Uses | |
| Uses: | Farmland; arable and grazing. |
| Map(s): | GEOLOGICAL SURVEY OF IRELAND, 1876. 1:63,360 Geological Sheet 60 (Newry). Dublin, Ordnance Survey for Geological Survey of Ireland. | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 03 MAR 2001 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Gransha, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=573. Accessed on 2026-11-06. |
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