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7. Volcanism |
Geological Survey, 14 Hume Street, Dublin 2
Basic volcanic rocks in the Dalradian Supergroup of Ireland are identified and listed here. They are widespread, though stratigraphically restricted, and provide useful marker horizons locally in Connemara, Achill Island, the Nephin Beg Mountains, the Ox Mountains inlier, and counties Donegal, Tyrone, Londonderry, and Antrim. A major volcanic unit is found in the Argyll Group (Easdale Subgroup) in the west of Ireland, whereas in N Ireland, including Donegal, several units are recorded in or immediately below the Southern Highland Group. Pillow lavas, lavas, agglomerates, hyaloclastites, tuffs and combinations of these occur. Minor striped amphibolites reminiscent of the tuffaceous or lava lithologies of the Lakes Marble Formation (Easdale Subgroup) are present throughout much of the Connemara succession. Incoming of the first substantial thickness of volcanics is associated with a change from stable, shallow-water, continental-shelf deposition to deposition of a deeper water, greywacke facies, associated with rifting and block faulting.
