INSIGHT INTO LATE-IAPETUS TECTONICS FROM NEW U-PB AND BIOSTRATIGRAPHIC AGE DATA FROM THE NAVAN AREA, EASTERN IRELAND

The Ordovician peri-Laurentian Grangegeeth volcanic arc terrane in eastern Ireland is a relic of the closing Iapetus Ocean. Volcanism in the Grangegeeth terrane has been documented at c. 465 Ma; a detrital zircon population includes abundant antecrysts to c. 480 Ma as well as Laurentian grains. The adjacent Ordovician-Silurian Rathkenny tract of mudstone and greywacke is considered a part of the Southern Uplands-Down-Longford (SUDL) terrane Laurentian accretionary prism; the contact between the two is cryptic. Two cores of Lower Paleozoic strata were obtained from boreholes situated along a buried projection of the Rathkenny - Grangegeeth outcrops. The core contains volcaniclastic debris-flow deposits within mudstone and siltstone. U-Pb LA-ICPMS dating of zircon from the volcanogenic horizons yielded a maximum depositional age of c. 450 Ma. Nearly 98% of zircon ages are within the range 445 – 480 Ma, indicating a proximal volcanic source and little mixing from other sources.

Volcaniclastic strata are overlain by mudstone that has yielded a diverse fauna of palaeocopid and podocopid ostracods. The fauna suggests a Katian (Ordovician) age, consistent with the c. 450 Ma age of the volcaniclastic deposits and within the previously published graptolite age range for the Rathkenny tract.

Core stratigraphy demonstrates that Rathkenny tract strata depositionally overlie those of the Grangegeeth arc. We propose a paired-subduction-zone model analogous to the present-day northern Philippine Sea. To the north, the SUDL accretionary prism was emplaced above a continent-directed subduction zone. The Rathkenny sequence was deposited on the oceanic plate, bounded to the south by a peri-Laurentian continental fragment on which the Grangegeeth arc was emplaced. Later stages of Grangegeeth volcanism contributed volcaniclastic detritus to the fine-grained Rathkenny sedimentary sequence, which was then incorporated into the advancing accretionary prism.