INTRA-ORDOVICIAN DEFORMATION IN SOUTHEAST IRELAND - EVIDENCE FROM THEGEOLOGICAL SETTING, GEOCHEMICAL AFFINITIES AND U-PB ZIRCON AGE OF THECROGHAN KINSHELAGH GRANITE

The Croghan Kinshelagh alkali granite intrudes a cleaved volcano-sedim entary sequence on the border of counties Wicklow and Wexford in south east Ireland. U-Pb dating of zircons from the granite indicate a mid-C aradoc emplacement age of 454 +/- 1 Ma. The Duncannon Group host rocks form the southwestern end of the Avoca Volcanic Belt, a Mid-Ordovicia n (Caradoc) sequence of acid and intermediate lavas and volcaniclastic s. Dolerite dykes intrude the granite; elsewhere in the region dolerit es are generally associated with volcanic rocks. The main, D1 deformat ion within the Duncannon Group rocks is manifest as a steep D1 cleavag e generally regarded as a product of Late Caledonian regional deformat ion in southeast Ireland. The Croghan Kinshelagh granite shows strong geochemical coherence with subalkaline varieties of the Caradoc volcan ic rocks; relatively high Th, Y, Nb and REE contents set it apart from any other known granite type in southeast Ireland. Together with the geochemical evidence, the age determination of 454 Ma indicates that t he Croghan Kinshelagh granite was generated and emplaced during Ordovi cian volcanism in southeast Ireland. Volcanism was closely followed by penetrative deformation and emplacement of the granite. The intra-Ord ovician deformation may have been a consequence of closure of the Iape tus Ocean or more localized events such as accretion on the hanging wa ll of the subduction zone. The age of the Croghan Kinshelagh granite p rovides an important datum for Orodovician volcanism and subduction in southeast Ireland.