Intra-Ordovician deformation in southeast Ireland: evidence from the geological setting, geochemical affinities and U—Pb zircon age of the Croghan Kinshelagh granite

Abstract

The Croghan Kinshelagh alkali granite intrudes a cleaved volcano-sedimentary sequence on the border of counties Wicklow and Wexford in southeast Ireland. U-Pb dating of zircons from the granite indicate a mid-Caradoc emplacement age of 454+ 1 Ma. The Duncannon Group host rocks form the southwestern end of the Avoca Volcanic Belt, a Mid-Ordovician (Caradoc) sequence of acid and intermediate lavas and volcaniclastics. Dolerite dykes intrude the granite; elsewhere in the region dolerites are generally associated with volcanic rocks. The main, Dl deformation within the Duncannon Group rocks is manifest as a steep Dl cleavage generally regarded as a product of Late Caledonian regional deformation in southeast Ireland. The Croghan Kinshelagh granite shows strong geochemical coherence with subalkaline varieties of the Caradoc volcanic 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 the Croghan Kinshelagh granite was generated and emplaced during Ordovician volcanism in southeast Ireland. Volcanism was closely followed by penetrative deformation and emplacement of the granite. The intra-Ordovician deformation may have been a consequence of closure of the Iapetus Ocean or more localized events such as accretion on the hanging wall of the subduction zone. The age of the Croghan Kinshelagh granite provides an important datum for Ordovician volcanism and subduction in southeast Ireland.

Integration of Detrital Gold Microchemistry, Heavy Mineral Distribution, and Sediment Geochemistry to Clarify Regional Metallogeny in Glaciated Terrains: Application in the Caledonides of Southeast Ireland

“…Chemical characteristics of the granites and volcanic rocks suggest that during their formation, the Leinster region was in a transitional position between a volcanic island arc that extended from County Waterford to the English Lake District and a back-arc basin in North Wexford -Wicklow. Gallagher et al (1994) demonstrated that the Croghan Kinshelagh Granite (Figs. 2 and 3; Croghan Complex of recent literature), cropping out west of the Goldmines River, is an older intrusion than the Leinster Granite.…”

“…In the lower Ordovician, thick laminated mudstones were deposited: the striped appearance of these gave rise to the name 'Ribband Group'. The Cambrian and lower Ordovician strata were deformed in mid-Ordovician time by an orogenic event, the Monian, which was localised to the Leinster region and affected the southeast of the region more intensely than the northwest (Tietzsch-Tyler et al, 1994;Gallagher et al, 1994). In the upper Ordovician, further sediments and volcanic formations were deposited on the submerging Monian landmass, forming the Duncannon Group.…”

“…The Leinster Granite batholith and satellite granitic plutons (Fig. 1) were emplaced into these actively deforming shear zones in the Early Devonian around 405 Ma (O'Connor and Brück, 1978;Gallagher et al, 1994). Gold, tungsten, tin and lead-zinc mineralization is associated with thermal metamorphism and hydrothermal fluid circulation accompanying batholith emplacement (McArdle et al, 1989).…”

Geological structure and tectonic evolution of the Lower Palaeozoic rocks of the Isle of Man

“…It is likely that the D1 in the Ordovician Manx Group, similar in all respects to that in the Niarbyl Formation, is the same age, although an older, possibly Ordovician, age cannot be ruled out (cf. Gallagher et al 1994;Tietzsch-Tyler 1996). The D1 structures are very similar to the main structures described from other parts of the southern Caledonides of Britain, folds having a 'Caledonoid' trend, generally upright axial planes and horizontal plunge.…”

Initiation of Iapetus subduction under Irish Avalonia

“…Ireland, 1990) are succeeded by early Ordovician greywacke siltstones, mudstones and minor volcanic units (the Ribband Group), late Ordovician acidic volcanic deposits and mudstones (the Duncannon Group), and Silurian greywacke sandstones (the Kilcullen Group) overlying the suture zone of the closed ocean (Hutton & Murphy, 1987). Ordovician tectonic deformations (Tietzsch-Tyler, 1989;Gallagher, O'Connor & Aftalion, 1994), possible accretion events, were succeeded by early Devonian continental collision and orogenesis (Hatcher, 1988), during which the Leinster Granite (404 ± 2 Ma; O'Connor, Aftalion & Kennan, 1989) was intruded. The Lower Palaeozoic inliers of Wales and the English Lake District have broadly similar geological histories, and plate tectonic models of a Caledonian-trending Leinster-Lake District volcanic arc with a Welsh back-arc basin have been proposed (e.g.…”