Abstract

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.

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