Lurgylea ASSI

The area is of special scientific interest because of the rock exposures at the site. Radiometric dating (K-Ar) of the lavas exposed in a quarry gives an age of 375 million years (Ma) placing their age of extrusion at about the Middle-Upper Devonian boundary. Volcanic rocks of this age are uncommon in Ireland and in Northern Ireland these are the only Devonian lavas whose age is known accurately.

The section of the rock series known as the Fintona Group which occurs south of the Tempo-Sixmilecross Fault consists entirely of red-beds and contemporaneous lavas. Although no fossils have been recovered from these rocks, dating of the lavas has been carried out radiometrically and gives an age of 375 million years (Ma). This places their age of extrusion at about the Middle-Upper Devonian boundary. Volcanic rocks of this age are uncommon in Ireland and in Northern Ireland these are the only Devonian lavas where the age is accurately known.

Radiometric analysis of the basic igneous rocks at Lurgylea by the K-Ar whole-rock method gives results which fall into two well defined and distinct groups; one at 375 Ma and the other at 275 Ma. The older group of dates comes from relatively fresh lava and as such may well record a good age for their extrusion. The younger group of dates, which was obtained from lava samples that appear to have been altered, is less well defined yet still quite distinct and points to an event sometime in the middle Permian, at about 275 Ma. This probably relates to a ‘resetting’ event, which only affected parts of the lava pile.

The precise environment of origin of these volcanic rocks is uncertain primarily because their relationship with the surrounding sedimentary rocks is unclear. It is probable that they formed as extruded lavas originating from a volcanic centre within a Devonian desert environment.