A LATE CALEDONIAN MELANGE IN IRELAND - IMPLICATIONS FOR TECTONIC MODELS

The Clew Bay area in western Ireland contains the remnants of a Caledo nian terrane which separates the Dalradian of North Mayo from the Ordo vician of South Mayo to the south. Rocks of the area have been various ly 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 dis continuous outcrops of ultrabasic and basic rocks, together with quart zose and semipelitic schists on the south shore of Clew Bay. These are in contact to the north with a sedimentary and volcanic sequence expo sed both on the mainland and on Clare Island for which a detailed stra tigraphy had previously been established. We reinterpret this latter s equence as a melange containing blocks up to 500 m long of sandstone, conglomerate, chert and volcanics. Although microfossil evidence had p reviously shown that a chert block on Clare Island was of Late Llanvir n age, new fossil control shows that this date must be extended upward s. Microfossils extracted from a variety of lithologies within the mel ange show that the cherts range from Middle Ordovician to at least Car adoc. Further, spores extracted from the melange matrix show that the formation of the melange took place in the Silurian, probably in the W enlock or later. These data indicate that the melange is unrelated to any early Ordovician obduction event and in fact represents the effect s of a significant tectonic episode in the late Silurian of the Britis h and Irish Caledonides.