Ophiolitic mélange separates ortho- and para-tectonic Caledonides in western Ireland

The nature of the contact between the orthotectonic (late Pre-cambrian to Cambrian) and the paratectonic (Ordovician and Silurian) Caledonides has long been a source of discussion. In western Ireland this junction occurs in a complex zone which crops out along the southern shores of Clew Bay, County Mayo, where Dalradian (Middle Cambrian) and Ordovician (Arenig) rocks are separated by a pre-Silurian rock unit termed the Deer Park Complex¹ (Fig. 1). The complex contains both meta-sedimentary and meta-igneous rocks and has been variously interpreted as representing a pre-Caledonian basement ridge1, an imbricate thrust zone² related to the development of a major fault system, the Fair Head-Clew Bay line³ or an Arenig ophiolite ⁴. We report here geological, geochemical and geophy-sical studies on the Deer Park Complex which indicate that it is a northerly dipping mélange unit containing the components of a dismembered ophiolite. These findings imply a source of ophiolitic rocks to the north possibly within the orthotectonic Caledonides.