A late Caledonian melange in Ireland: implications for tectonic models

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. 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. This latter sequence is reinterpreted 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. 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. -from Authors