UMBERS, OCEAN CRUST AND THE IRISH CALEDONIDES - TERRANE TRANSPRESSIONAND THE MORPHOLOGY OF THE LAURENTIAN MARGIN

The Clew Bay Complex of western Ireland contains a variety of rock typ es. These include schists possibly of Dalradian association, amphiboli tes and serpentinites of unknown age, cherts, volcanic rocks and grade d gabbros of mid to upper Ordovician age and shales and sandstones of Silurian age. Intercalated within the cherts of the complex on Glare I sland are chocolate-brown horizons of a porous rock, geochemically com parable to hydrothermal mid-ocean ridge metalliferous sediments (e.g. umbers). These cherts overlie volcanic rocks of MORE affinities with a n island are influence. The association exposed on Glare island of gra ded gabbros, MORE volcanic rocks, discontinuous carbonaceous horizons, cherts, umbers and deep water shales suggest the presence of a mid-to late Ordovician spreading centre now preserved as a dismembered ophio lite. The Clew Bay and Highland Border Complexes, both being parts of the Border Terrane, have many similarities including evidence of mid-t o late Ordovician spreading, and it is suggested they formed in the sa me basin. However, they are dissimilar to both the South Mayo and Midl and Valley Ordovician and Silurian rock associations, from which they are separated by a terrane boundary. The geochemical data and field as sociations do not indicate back-are or intra-continental rifting. We s uggest, therefore, that elements of older ocean crust became trapped i nboard of a collided early Ordovician are at re-entrants along the Lau rentian margin. This trapped crust became the site of mid-to late Ordo vician spreading. We also suggest that the site of this entrapment was to the northeast of the present positions of the Clew Bay and Highlan d Border Complexes and that they were emplaced into their present site s by end-Silurian transpression. This Laurentian margin morphology is compatible with that demonstrated for parts of the Newfoundland-Quebec sectors of the Appalachian sector of the orogen.