Dm. Williams et al., A LATE CALEDONIAN MELANGE IN IRELAND - IMPLICATIONS FOR TECTONIC MODELS, Journal of the Geological Society, 151, 1994, pp. 307-314
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.