The Ordovician peri-Laurentian Grangegeeth Terrane in eastern Ireland and the adjacent Katian to Wenlock Rathkenny Tract are together a relict of the closing Iapetus Ocean. The Rathkenny Tract succession is part of the Laurentian Southern Uplands–Down–Longford Terrane accretionary prism, but the contact between the Rathkenny and Grangegeeth terranes is cryptic. Two cores of Lower Paleozoic strata along a buried projection of the Rathkenny–Grangegeeth outcrops contain volcaniclastic units within a succession of mudstone and siltstone. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons from volcanogenic horizons yielded a maximum depositional age of c. 450 Ma. Nearly 98% of the zircon ages are 480–445 Ma, indicating a proximal volcanic source. The trace element geochemistry of the Ordovician zircons indicates a host magma sourced from continental crust. A diverse ostracod fauna in the mudstones suggests a Katian age and includes species with Baltic, Laurentian and Avalonian affinities.We propose a paired subduction zone model for the emplacement of the Grangegeeth–Rathkenny succession, with the Rathkenny strata deposited on an oceanic plate between the Laurentian margin accretionary prism and the 480–450 Ma Grangegeeth peri-Laurentian microcontinental arc. Volcaniclastic detritus fed into the Rathkenny basin was then incorporated into the accretionary prism. The mixed fauna indicates that the Iapetus Ocean was narrow by Katian time.
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