Cashel Rock
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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Ordovician · Rock Type: Breccia, Rhyolite
Landscape processes: Volcanic / Igneous
Description
Cashel Rock Caledonian igneous, Mineralogy Cashel Rock has been declared an ASSI because of it's important geology. This occurs as outcrops of tonalite and rhyolite, two types of igneous rock. The site also contains volcanogenic stock work mineralisation. The rocks are of Ordovician age, some 450 million years old and are part of the Tyrone Volcanic Group. The Tyrone Volcanic Group is comprised of rocks that record a phase during the closing of the Iapetus Ocean and the available evidence suggests they formed in an island arc setting. The main rock type exposed at Cashel Rock is the Laght Hill Tonalite. It is a coarse grained, grey-green rock, showing a high degree of variability across the site, due to different grades of deformation. The main minerals present are quartz, plagioclase feldspar weathered to mica and green hornblende crystals. In the least deformed outcrops the tonalite exhibits a nonnal igneous texture, but the most deformed areas show the constituent minerals have been stretched and broken down. A schistose fabric, giving a 'wavy' appearance, is developed in the most deformed rock. This fabric strikes north east-south west across the site. The second rock type exposed at the site is a pale pink-buff coloured, fine grained felsite of rhyolite-rhyodacite composition, found close to the summit of Cashel Rock. The rhyolite shows flow banding oriented to the north east-south west. Some defonnation is also observed in this rock, with localised fragmented areas forming a silica cemented breccia. This rhyolite is part of a series of outcrops 7km long extending between Slievemenagh, northeast of Cashel Hill and Brackagh to the southwest. These rhyolites are very similar to many of the acid igneous clasts which form a major component of the coarse volcanic breccias in the Tyrone Volcanic Group. Contact between the tonalite and the felsite is only exposed in a small area on the south western edge of the main crag. The rhyolite can be seen cutting across the tonalite and some partially consumed xenoliths oftonalite are found in the rhyolite. This shows the rhyolite intruded the tonalite and so post dates it i.e. it is the younger rock. Cashel Rock also includes a locality that exhibits volcanogenic stock work mineralization. The minerals are found in outcrop of silica-veined rhyolite and silicified, brecciated rhyolite. The mineral assemblage comprises sulphides, principally pyrite, chalcopyrite, covellite, galena and iron-rich sphalerite. The igneous rocks and vo1canogenic minerals found at Cashel formed in an island arc setting during the Ordovician as the Iapetus Ocean was closing. This arc formed as ancient ocean crust was being subducted in a north westerly direction under the ancient continent of Laurentia. The available evidence suggests the tonalite and rhyolite of Cashel Rock formed from the fractional crystallization of an igneous body of basic composition. The wider outcrop pattern of the Laght Hill-Cashel Rock-Slievemenagh Tonalite body indicates a lopolith type intrusion comprising three sill-like horizontal intrusions linked at depth to a common feeder. The fact that contact relationships indicate rhyolite intrudes and incorporates xenoliths of tonalite is an indication that tonalite emplacement was more or less contemporaneous with formation of the Tyrone Volcanic Group. The results of an isotopic analysis into the sulphide minerals show that the sulphur was produced by the reduction of sulphates in the Ordovician sea water. https://www.daera-ni.gov.uk/publications/cashel-rock-assi