Shannaghan Hill
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Geology & landscape
Geological Period: Silurian · Rock Type: Shale
Landscape processes: Volcanic / Igneous
Description
Shannaghan Hill Caledonian Igneous Shannaghan Hill 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 de-rived 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. Located on the south-western slope of Shannaghan Hill, a series of natural exposures demonstrate the relationships between contact metamorphosed Silurian country rock, lamprophyre dykes and sheeted granodiorite of the north-east or Rathfriland pluton 413 to 411Ma. Metamorphosed country rock exposed at Shannaghan Hill is mainly biotite-hornfels and fused mobilised sediment. Heat from the north-east granodiorite pluton has caused contact metamorphism of the Silurian greywacke and shale country rock resulting in the hornfels and fused sediment. A complex association of lamprophyre and granodiorite sheets can be seen, with the lamprophyric dykes being found particularly associated with the north-east pluton. The granodiorite of the north-east pluton is seen as cross-cutting and in some cases partially remobilised, sheet-like intrusive bodies that show variable deformation. These complex rela-tionships indicate contemporaneous intrusion and regional deformation. The granodiorite sheets are considered to rep-resent the early stages of intrusion of the northeast pluton of the NIC. The granodiorite displays foliation, a feature generally restricted to the margins of the plutons of the NIC. Foliation is attributed to regional deformation and shouldering aside of country rock during emplacement of the NIC. This foliation is aligned parallel to the main contact between the pluton and the Silurian country rock. Shannaghan Hill offers an important series of exposures that demonstrate the complex relationships between contact metamorphosed Silurian country rock, lamprophyre dykes and sheeted granodiorites of the north-east pluton of the NIC. Shannaghan Hill demonstrates the complex early stages of intrusion of the north-east pluton and proves that there was an overlap between the timing of the intrusions and regional scale deformation. https://www.daera-ni.gov.uk/publications/shannaghan-hill-assi