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.

Supplementary material:https://doi.org/10.6084/m9.figshare.c.4358243

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