Ardglass
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Geology & landscape
Geological Period: Silurian
Landscape processes: Volcanic / Igneous
Description
Ardglass Caledonian Igneous Ardglass 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 Ardglass are mainly coarse-grained, biotite-pyroxenite of ul-tramafic composition and meladiorites of intermediate composition. The main min-eral components of the biotite-pyroxenite are biotite, clinopyroxene and apatite, whilst the meladiorites are often heterogeneous with variable content of either biotite or clinopyroxene. The two rock types are found in close association, with the biotite-pyroxenite seen as alternations with, or lenses within, strongly laminated meladio-rite. Also present at Ardglass are light coloured granodiorite sheets and pegmatite veins. The granodiorite shows sharp and diffuse contacts with biotite-pyroxenite and me-ladiorite. This suggests that the rocks of the ultramafic-intermediate complex were emplaced before the main granodiorite bodies. The biotite-pyroxenite and associated meladiorites are likely to have been produced by a process of crystal accumulation from intermediate magma composition and pro-vide evidence that supports a magmatic origin for the ultramafic-intermediate com-plex and the NIC in its entirety. Some of the rocks are likely to approach the paren-tal magma compositions, the differentiation of which could have lead to the forma-tion of the granodiorites that make up most of the NIC and offers a unique opportunity to study such rock types in Northern Ireland. https://www.daera-ni.gov.uk/publications/ardglass-assi