ION MICROPROBE U-PB ZIRCON EVIDENCE FROM LOWER CRUSTAL XENOLITHS FOR PALAEOZOIC CRUST FORMATION AND CRUSTAL DEVELOPMENT WITHIN THE CALEDONIAN SUTURE ZONE IN CENTRAL IRELAND

Lower crustal xenoliths from central Ireland provide unique insight into crust formation in an oblique collision zone (the Iapetus Suture) within the Caledonian orogenic belt. They are predominantly metasedimentary (khondalitic granulites and psammites) with a minor component of acid orthogneiss and very rare mafic granulite. Thermobarometry and deep seismic data suggest entrainment of metasedimentary xenoliths from original depths of c. 22-28 km, and of the mafic xenoliths from deeper levels, during Lower Carboniferous volcanism. Ion microprobe U-Pb zircon dating yields Late Devonian ages (c. 360 - 380 Ma) for granulite-facies metamorphism, migmatisation and deformation. These lower crustal rocks from central Ireland are recording ductile strain, melting and metamorphism considerably later than the deformation seen in rocks at shallower levels, though the tectonic significance of these events is difficult to diagnose. Thermobarometry from an orthogneiss xenolith indicates that it originated at shallower crustal levels (< 20 km depth). It yields an Ordovician protolith age related to arc magmatism and zircon overgrowths record early Devonian metamorphism. Evidence of late Devonian metamorphism is lacking, possibly because these events were restricted to deeper levels. Detrital zircons from metasedimentary xenoliths yield mainly Mesoproterozoic to early Cambrian ages, suggesting derivation of the protolith from Avalonia rather than Laurentia in keeping with other geochemical constraints on provenance.