Insight into late Iapetus tectonics from new U-Pb zircon and micropalaeontological data from the Navan area, eastern Ireland.

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. [ABSTRACT FROM AUTHOR]