Gransha
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Geology & landscape
Geological Period: Silurian
Landscape processes: Volcanic / Igneous
Description
Gransha Caledonian Igneous Gransha is of importance because of its geology. It is one of a series of sites that de-scribes the Newry Igneous Complex NIC, one of a number of granitic complexes emplaced within the Southern Uplands and Midland Valley Terranes in Ireland and Scotland during Caledonian times. This period was characterised by mountain build-ing events following the closure of the Iapetus proto-Atlantic Ocean. The NIC in-trudes older folded Silurian rocks and extends over an area of about 45km2, from Slieve Croob in the north-east to Forkhill in south Armagh. The NIC comprises a series of overlapping plutons consisting of a variety of acidic, intermediate and ultramafic rocks. U-Pb zircon dating method indicates that the age of the NIC is 414 to 407Ma million years old. The north-east or the Rathfriland Pluton is the oldest at 413 to 411Ma, the central or Newry pluton is 411 to 401Ma, and the south-west or Cloghoge Pluton is the youngest at 407Ma. The NIC featured in past international debates concerning the origin of granitic rocks; whether such rocks were derived ultimately from a magma source or whether they represent the end point of an alteration process affecting pre-existing rocks. The NIC continues to be of considerable importance for research. The rocks exposed at Gransha are medium to coarse-grained granodiorite, part of the northeast or Rathfriland pluton. The main mineral components of the granodiorite are plagioclase, alkali feldspar, hornblende, quartz and biotite mica. Systematic sam-pling, thin section analysis and geochemistry across the NIC have revealed composi-tional variation within the granodiorite plutons, with that in the margins of the north-east pluton being much more basic as is the case at Gransha than that exposed towards the centre of the pluton. This is described as normal zoning. The minerals within the granodiorite display a strong foliation, particularly biotite, and are aligned sub-parallel to the main contact of the granodiorite with the Silurian country rock to the northwest, thus indicating proximity to the pluton margin. A varied selection of enclaves xenoliths of sedimentary and igneous origin are found at this location, most of which have irregular rounded forms and in some cases have diffuse boundaries with the enclosing granodiorite. Sedimentary enclaves have undoubtedly been derived from the surrounding Silurian country rock and appear to be Silurian greywackes and shales. Igneous enclaves include restite, microdiorite and diorite. In places, the enclaves are spectacularly flattened and aligned with the plane of the foliation. Gransha offers important exposures of the foliated granodiorite close to the margin of the northeast pluton. The granodiorite is more basic than that exposed towards the centre of the pluton indicat-ing that the intrusion is “normally zoned”. The presence of foliation and the flattening of enclaves within the granodiorite indicate a pluton margin close to the contact with the country rock. Foliation and flattening of enclaves is attributed to regional deformation and shouldering of the country rock during emplacement of the NIC. https://www.daera-ni.gov.uk/publications/gransha-assi