Goraghwood Quarry
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Geology & landscape
Mining / Industrial: Quarry · Geological Period: Silurian · Rock Type: Shale
Landscape processes: Volcanic / Igneous
Description
Goraghwood Quarry Caledonian Igneous Goraghwood Quarry is of importance because of its geology. It is one of a series of sites that describes 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 building events following the closure of the Iapetus proto-Atlantic Ocean. The NIC intrudes 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 410Ma, 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 majority of the critical rock types in Goraghwood Quarry can be found in the southern half of the quarry. An unquarried steeply inclined 20m wide ‘screen’ of metamorphosed country rock can be found. This displays banded light green and dark grey rock and is composed of fine grained diopside-and biotite-hornfels respectively. The hornfels displays original sedimentary bedding that dips at approximately 70o to the northwest. This screen of Silurian country rock has been contact metamorphosed by granodio-rites of the central pluton of the NIC. The range of hornfels observed is related to lithological variations in the original sediment, with the main variation in the sedimentary protolith probably between greywacke biotite-hornfels and calcareous shale diopside-hornfels. Granodiorite of variable composition surrounds the hornfelsed screen and include dark-coloured porphyritic granodiorite and light-coloured biotite-granodiorite varieties. Granodioritic veins and pods of variable composition are found enclosed within the country rock. Field relationships between the range of granodiorite rocks demonstrates their relative ages.The variety of granodiorite observed is evidence that several injections of magma were involved during the formation of the central pluton 411 to 410Ma ; this is a feature regarded as common to the formation of each of the plutons that together constitute the NIC. The contacts between the metamorphosed country rock and the various varieties of granodiorite are generally sharp with no sign of gradational or chilled margins. This is consistent with a magmatic intrusive origin for the NIC. The rocks exposed at Goraghwood Quarry offer access to exposures that show the relationships between contact metamorphosed Silurian country rock of varying lithologies, and various grano-diorities of the central pluton of the NIC. It is possible to demonstrate the variety, and relative timing of intrusion, of igneous rock types that comprise the central pluton from research work at Goraghwood Quarry. https://www.daera-ni.gov.uk/publications/goraghwood-quarry-assi