SUPRA-SUBDUCTION ZONE OPHIOLITE COMPLEXES

Large ophiolite nappes (Oman, Newfoundland, Papua) clearly have a supra-subduction zone (SSZ) origin in extensional or transtensional arcs from their petrology, structure and chemistry. However, largely ultramafic-free ophiolite fragments in accretionary complexes such as the Annieopsquotch of Newfoundland and the South Connemara terrane of western Ireland are probably slices of clipped-off seamounts, oceanic plateaux, and transform-related upper oceanic crust. The Ordovician Bay of Island and Coastal Complexes in western Newfoundland and the related ophiolite complexes of Baie Verte, Mings Bight, Tilt Cove and the Notre Dame Bay Zone exhibit extremely complicated structural/magmatic relationship with a mid-Cambrian through early Ordovician primitive arc complex. The Sleepy Cove pillow lavas have an oblique transtensional constrictional fabric cut by the late Cambrian Twillingate Granodiorite cut in turn by the Mortons Harbour mafic dike complexes locally forming a sheeted complex, witnessing the progressive transtensional disruption of an oceanic island arc leading to the intra-arc development of oceanic crust and mantle, the whole supra-subduction zone package emplaced during the Ordovician as a high-level ophiolite sheet as the Laurentian margin collided with the arc. The Coastal Complex is probably a frontal arc assemblage nucleated in an intra-arc or fore-arc transform.

SSZ ophiolites occur through the Appalachian/Caledonian orogen where they appear to be the highest level nappes above metamorphic complexes, where sediments were deposited in rift complexes at the Laurentian margin and where deformation and metamorphism resulted from continental margin/arc collision. Two major unique periods of SSZ ophiolite obduction and blueschist metamorphism, following immediately-preceding ophiolite generation in arc settings, occurred during the Ordovician and the Cretaceous where they are associated with periods of high sea-level and appear to represent times of high intraplate compressional stress resulting from ridge push during continental dispersal.