THE TIMING OF TECTONIC MOVEMENTS DURING THE TACONIC OROGENY IN WESTERN IRELAND: EVIDENCE FROM THE ROSROE FORMATION

A stratigraphic record of Ordovician terrane accretion and amalgamation on the Laurentian margin during the Taconic orogeny is preserved in the long-lived South Mayo Trough (SMT) sedimentary basin in western Ireland. The Connemara terrane is an important and well-documented part of this assemblage, but its relations to adjoining terranes and the SMT are less well understood. Our previous work indicates that Connemara was in its present relative position, south of the SMT and outboard of Laurentia, by the time that ~464 Ma ignimbrites of the Mweelrea Formation were emplaced in the SMT, but continuous sedimentation from the southerly derived Rosroe Formation (Llanvirn) through at least the Mweelrea Formation allows us to test whether amalgamation occurred even earlier.

We analysed detrital zircons from the ‘type’ Rosroe Formation at and near Rosroe and from Rinavore southwest of Lough Nafooey, and the disputed correlative Maumtrasna Formation in the Partry Mountains where it underlies either Glenummera Formation or a slate member of the Mweelrea Fm. Detrital U/Pb spectra are dominated by a strong peak at ~468 Ma, with peaks at ~1000-1100 Ma and ~2700 Ma typical of Laurentian signatures; the Maumtrasna Fm sample has a significantly stronger early Proterozoic peak and fewer Ordovician grains than the Rosroe Fm grains.

Zircon age spectra from the Rosroe and Maumtrasna formations correspond well to spectra from the Connemara-derived Bunnacuneen Member of the Mweelrea Fm. The strong Ordovician peak matches the Bunnacuneen Member peak of 465-470 Ma and is coincident with the Notre Dame arc in Connemara and distinct from the earlier Taconic arc peaks preserved in northerly derived SMT sediments. The combined stratigraphic and zircon data strongly suggest that Connemara was an important source of detrital material for the southern SMT earlier than previously recognized.