ORDOVICIAN/SILURIAN EVOLUTION OF THE LAURENTIAN MARGIN

Newfoundland provides a basic template for understanding the Appalachian/Caledonian Orogen because it appears to be less terraned than other segments of the orogen and because high structural levels are preserved. Newfoundland exhibits, superbly, the early to mid-Ordovician oceanic arc-continent collision/subduction flip model. The oceanic arc, from 510 my to about 475 my was transtensional; the Sleepy Cove pillow basalts are constrictionally rodded and invaded by the Twillingate Granodiorite, which is cut by mafic dyke swarms, locally sheeted, of the Moretons Harbour Group. This transtensional arc distension led to the development of the supra-subduction-zone (SSZ) ophiolites of the Bay of Islands, Baie Verte, Mings Bight and Nippers Harbour, which collided with the Laurentian Margin in the early to mid-Ordovician. In western Ireland, the fore-arc basin and accretionary prism of the SSZ ophiolite/arc complex is preserved in the South Mayo Trough (SMT) and Killadangan Complex. The SMT records, in its detrital heavy mineral assemblage, the unroofing of the sub-SSZ metamorphic complex of the Grampian Terrane with first ophiolite detritus, especially chromite, then garnet and staurolite as the Dalradian was unroofed. Subsequently, sinistral terraning both repeated the Dalradian/arc assemblage, with Connemara lying outboard of the SMT and also excized arc terranes in Scotland. Latest Silurian sinistral transpressional collision imposed clockwise-transecting cleavages across the Iapetus remnants.