The Ordovician peri-Laurentian Grangegeeth Terrane in eastern Ireland and the adjacent Katian to Wenlock Rathkenny Tract are together a relic 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. LA-ICPMS U-Pb dating of zircon from volcanogenic horizons yielded a maximum depositional age of c. 450 Ma. Nearly 98% of zircon ages are 445 – 480 Ma, indicating a proximal volcanic source. Trace element geochemistry of Ordovician zircon indicates host magma sourced in continental crust. A diverse ostracod fauna in mudstone suggests a Katian age, and includes species with Baltic, Laurentian and Avalonian affinities.
We propose a paired-subduction-zone model for 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 micro-continental 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.