Aughnagon Quarry
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Geology & landscape
Mining / Industrial: Quarry · Geological Period: Silurian · Rock Type: Rhyolite, Shale
Landscape processes: Volcanic / Igneous
Description
Aughnagon Quarry Caledonian Igneous Aughnagon 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 Val-ley 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 northern face of Aughnagon Quarry exposes a zone of contact between igneous rocks belonging to the central pluton 411 to 410Ma of the NIC and metamorphosed Silurian country rock. Most of the section is in greywacke and shale country rock that has been altered to biotite-hornfels. Original sedimentary bedding is generally undisrupted, but some evidence of fused mobilised sediment can be observed close to the intrusive contact. The contact between the country rock and granodiorite is sharp with no signs of chilling. Heat from the central pluton of the NIC has caused metamorphism of the Silurian greywacke and shale country rock. The result is a biotite-hornfels which extends for up to 1km from the margin of the granodiorite pluton. At least two main intrusive rock types are represented at Aughnagon Quarry. The earlier intrusive is a porphyritic rhyolite that occurs at the main pluton contact. The main body of igneous rock is a medium to coarse grained, unfoliated granodiorite containing quartz, plagioclase and alkali feldspar, biotite and a small amount of hornblende. The absence of foliation is anomalous to comparable sites at the margin of the northeast pluton indicating a possible different emplacement mechanism for the central pluton. The granodiorite here is more acid in composition than the central part of the pluton, evidence of reverse zoning, suggesting that several injections of magma were involved in the formation of the central pluton, a fea-ture regarded as common to each of the plutons that make up the NIC. Aughnagon Quarry offers excellent exposures where the relationship between contact metamorphosed Silurian coun-try rock and granodiorite of the central pluton of the NIC can be seen. https://www.daera-ni.gov.uk/publications/aughnagon-quarry-assi