It is found by the use of Pearce & Cann diagrams that basic magmatism during Dalradian sedimentation in Connemara was of a fundamentally different type to that which led to later synorogenic basic intrusions. The Dalradian magmatism is of a transitional within-plate to divergent plate margin type whereas the synorogenic intrusions are of volcanic arc type. It is suggested that the synorogenic basic intrusions of Connemara, Tyrone and NE Scotland represent the roots of a volcanic arc that developed by Arenig times over a northerly directed subduction zone.

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