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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Paleogene · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Lislea Tertiary igneous The area is of special scientific interest because of its geology, which is expressed in outcrop at Lislea. The Slieve Gullion volcanic complex, of which Lislea is a part, is the finest example of a Tertiary igneous centre in Ireland and is among the best topographic expression of a ring-dyke system in the British Isles. The rocks found here are of international geological importance, having played an important role in a number of theories related to the development of and interaction between igneous rocks. The Slieve Gullion complex developed in an area of crustal weakness, previously exploited by the much older Newry granite, around 400 million years old. In Palaeogene times, some 56-58 million years ago, it was the site of a major volcano of which there is now little evidence. Subsidence of this central unit was related to a ring fault, some 20 km in diameter, the latter providing a natural weakness exploited by intrusive igneous rocks. This produced the ring-dyke complex which has been exposed, through erosion of the softer surrounding rocks, to reveal the near-circular system known as the Ring of Gullion. The final phase saw activity retun to the central area with apparently layered igneous rocks, both acid and basic in nature. Past debate on the,fonnation of the central complex has been largely resolved. The layered structure apparent today appears to have developed as a series of igneous units intruded or injected into pre-existing rocks, rather than building up as a succession of extruded rock bodies. The Slieve Gullion complex is historically important as it has featured in a number of major geological debates on the nature of igneous rocks and the processes by which they can be formed. At Lislea, the inner and outer contacts of the ring dyke, composed of porphyritic granophyre, can be seen. The intruded relationship with the host Newry granodiorite can also be seen. Contacts within the granophyre show that it developed as a result of many intrusive pulses. Near the centre of the granophyre dyke, large, alien blocks of well rounded, fine-grained granitic rock, up to a metre across, can be seen. The origin of these blocks has been much debated, some geologists believing them to be altered Newry granodiorite, transformed by the more recent Palaeogene igneous activity. At the outer contact there is a zone of intensive crushing affecting both the granophyre and the Newry granodiorite of the ring wall. This late phase of movement produced both crush-banded rocks known as mylonites and shattered angular rocks known as the Camlough Breccias. This location provides a fuller understanding of the geological history of the Slieve Gullion Ring complex. https://www.daera-ni.gov.uk/publications/lislea-assi