The Iapetus suture in Ireland and Britain is that line which separates Caledonian rocks of the Laurentian and Avalonian continents. The suture is cryptic: nowhere is there an exposed fault-zone containing ophiolite remnants, blue-schist melanges, or trench deposits. Instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the Iapetus suture zone, which contains two or more tectonostratigraphic terranes. Several data sets are utilized to constrain this trace. The distribution of faunal provinces caused by oceanic separation can be used most reliably to define early Ordovician terranes. Fauna1 intermingling in the mid-Ordovician reduces provinciality and the confidence of terrane identification using faunal data. The late Ordovician and particularly Silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes, elimination of the Iapetus Ocean and development of the suture zone had begun by the Ashgill. Therefore the resolution of the suture trace becomes unreliable where Ordovician rocks do not crop out, because Silurian turbidite fans of Laurentian provenance may have dispersed across a significant width of Avalonian crust. Similarly, the precision of structural identification of the suture zone, by correlating the suture with a major fold-facing confrontation, may be weakened by late overthrusting. Tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop. Tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by Nd and Pb isotopic patterns; the Rb-Sr patterns of granites record a stronger distinction between north and south of the suture. Despite these difficulties, we depict a possible suture trace in Ireland that departs significantly from the traditionally placed line by following a NE-SW-aligned trace through the Slane fault and the Navan-Tipperary lineament, and thence through a dog-leg to pass north of the Dingle Peninsula.

Extensive and economically-important lithium pegmatites have been described from the eastern flank of the large S-type peraluminous Leinster Granite batholith in southeast Ireland. This flank area is also the locus of a major dip-slip ductile shear zone which was active during emplacement of the Tullow Lowlands pluton of the batholith. All the known bedrock occurrences of lithium pegmatite are contained within this marginal deformation zone.Most previous studies have suggested derivation of the lithium pegmatites by fractionation from the Leinster Granite itself, though none are conclusive. More recently, an anatectic model has been proposed whereby the pegmatites were derived by melting of Li-rich sediments adjacent to the batholithic contact. Our investigation has been concerned with a detailed study of the Rb-Sr isotope systematics and trace element geochemistry of the pegmatites and their host granitoids in an attempt to resolve the problem of pegmatite genesis.The results suggest a strong geochemical and temporal coherence between the Leinster Granite and the lithium pegmatites. A model is proposed in which Li enrichment in residual magmas may be linked to the development of muscovite instead of biotite in the Leinster Granite itself. The Li excluded from entry to biotite then accumulates in highly mobile, low-viscosity pegrnatitic fluids which migrate into the marginal shear zone of the Tullow Lowlands pluton where they crystallize at considerably lower temperatures.

-A more precise age for the synkinematic emplacement of the large S-type Leinster batholith (southeast Ireland) has been obtained by U-Pb dating of zircon and monazite separates from the two main granite varieties of the Northern Pluton. A concordant age of 405 + 2 Ma derived from monazite is considered to reflect both the age of intrusion and the main regional deformation event (Dl) in southeast Ireland. The monazite age is in good agreement with an earlier, less precise whole-rock Rb-Sr isochron determination. The zircon data indicate recent Pb loss effects and the existence of a small component of inherited radiogenic Pb derived from a crustal precursor during the generation of the batholith.

The lithostratigraphy of the Ribband Group of southeastern Ireland is revised through comparison to the equivalent Avalonian margin sequences of the Manx(Isle of Man) and Skiddaw (northern England) Groups. Four tracts are recognized in the Ribband Group, within which fossil age control and the ‘coticule package’ marker horizon are used to constrain lithofacies comparisons. Volcanic arc rocks in the Ribband and Manx Groups contrast with the passive margin provenance of the Skiddaw Group.

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