The Clew Bay area in western Ireland contains the remnants of a Caledonian terrane which separates the Dalradian of North Mayo from the Ordovician of South Mayo to the south. Rocks of the area have been variously interpreted as representing part of the Dalradian succession, as a Cambrian to Ordovician rifted margin to subduction related basin, and as a shear carpet derived from an accretionary wedge overridden by an ophiolite in the early Ordovician. The lithologies present include discontinuous outcrops of ultrabasic and basic rocks, together with quartzose and semipelitic schists on the south shore of Clew Bay. These are in contact to the north with a sedimentary and volcanic sequence exposed both on the mainland and on Clare Island for which a detailed stratigraphy had previously been established. We reinterpret this latter sequence as a melange containing blocks up to 500 m long of sandstone, conglomerate, chert and volcanics. Although microfossil evidence had previously shown that a chert block on Clare Island was of Late Llanvirn age, new fossil control shows that this date must be extended upwards. Microfossils extracted from a variety of lithologies within the melange show that the cherts range from Middle Ordovician to at least Caradoc. Further, spores extracted from the melange matrix show that the formation of the melange took place in the Silurian, probably in the Wenlock or later. These data indicate that the melange is unrelated to any early Ordovician obduction event and in fact represents the effects of a significant tectonic episode in the late Silurian of the British and Irish Caledonides.
Abstrad:The supposed Laurentian margin in western Ireland is separated from an Ordovician arc-related basin by the Clew Bay Complex and two Silurian successions, informally termed the Croagh Patrick Silurian and the Louisburgh Silurian, whose relationships are enigmatic. Palynomorphs, tubular structures and sheets of cuticle from turbidites within the Clew Bay Complex indicate a Silurian (Wenlock) age. In the light of these new data and reappraisal of contacts, the Croagh Patrick Silurian is now considered to be in tectonic contact with the Clew Bay Complex. The Louisburgh Silurian may also be in tectonic contact with parts of the Clew Bay Complex and unconformable on other parts, but is probably younger than previously thought. The Silurian age of elements of the Clew Bay Complex, and the relationship of the complex with other Silurian successions in western Ireland, indicate that significant deformation occurred in this part of the Caledonides during mid to end Silurian times, and imply that significant Silurian-Devonian terrane movement took place.
The Clew Bay Complex of western Ireland contains a variety of rock types. These include schists possibly of Dalradian association, amphibolites and serpentinites of unknown age, cherts, volcanic rocks and graded gabbros of mid to upper Ordovician age and shales and sandstones of Silurian age. Intercalated within the cherts of the complex on Clare Island are chocolate-brown horizons of a porous rock, geochemically comparable to hydrothermal mid-ocean ridge metalliferous sediments (e.g. umbers). These cherts overlie volcanic rocks of MORB affinities with an island arc influence. The association exposed on Clare island of graded gabbros, MORB 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 ophiolite. The Clew Bay and Highland Border Complexes, both being parts of the Border Terrane, have many similarities including evidence of mid- to late Ordovician spreading, and it is suggested they formed in the same basin. However, they are dissimilar to both the South Mayo and Midland Valley Ordovician and Silurian rock associations, from which they are separated by a terrane boundary. The geochemical data and field associations do not indicate back-arc or intra-continental rifting. We suggest, therefore, that elements of older ocean crust became trapped inboard of a collided early Ordovician arc at re-entrants along the Laurentian margin. This trapped crust became the site of mid- to late Ordovician spreading. We also suggest that the site of this entrapment was to the northeast of the present positions of the Clew Bay and Highland Border Complexes and that they were emplaced into their present sites 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.
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