Aughnavallog
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Geology & landscape
Geological Period: Silurian
Landscape processes: Volcanic / Igneous
Description
Aughnavallog Caledonian Igneous Aughnavallog 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 com-plexes emplaced within the Southern Uplands and Midland Valley Terranes in Ire-land and Scotland during Caledonian times. This period was characterised by moun-tain 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 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 Aughnavallog are medium to coarse-grained granodiorite, part of the northeast pluton. The main mineral components of the granodiorite are plagio-clase, alkali feldspar, hornblende, quartz and biotite mica. The granodiorite is free of foliation and alignment of biotite which is commonly seen close to the margins of the pluton. Systematic sampling, thin section analysis and geochemistry across the NIC have revealed compositional zoning within the granodiorite plutons. At the margins of the northeast pluton the rocks are more basic, but at Aughnavallog near the centre of the pluton, the granodiorite is more acidic indicating “normal zoning”. Such zoning could be evidence of fractional crystallisation of magma taking place after emplace-ment. An alternative explanation is that several injections of magma might have been involved in the formation of the northeast pluton. The absence of foliation within the granodiorites is consistent with the geological setting indicated by normal zoning, and would suggest that the site is at the centre of the northeast pluton, away from the marginal deformation caused by the regional deformation and the shouldering aside of the country rock. Aughnavallog offers important exposures of unfoliated granodiorite at the centre 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 within the granodiorite is at-tributed to regional deformation and shouldering of the country rock during emplacement of the NIC. https://www.daera-ni.gov.uk/publications/aughnavallog-assi