Cashel Rock ASSI

Cashel Rock has been declared an ASSI because of its 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 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 normal 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.