USE OF TH/U RATIOS TO ELUCIDATE TERRANE AFFINITY AND TIMING OF ORDOVICIAN UNROOFING OF SEDIMENT SOURCES TO THE SOUTH OF THE SOUTH MAYO TROUGH, WESTERN IRELAND

Sequential accretion of arcs and micro-terranes along the Ordovician southeastern Laurentian margin caused successive tectonomagmatic events that are elucidated by analysis of sedimentary detritus. We combine U-Pb dating of detrital zircon grains with assessment of Th/U ratios to consider the southern margin of the South Mayo Trough (SMT) in western Ireland, where much of the accretionary history is preserved.

We analysed zircons from the stratigraphic range from the uppermost Tourmakeady Group volcanic arc through the overlying Rosroe, Maumtrasna, and Derryveeny Formations. The Tourmakeady Group tuffaceous sandstone has more Proterozoic zircons than might be expected from subducted sediment, suggesting that the Tourmakeady arc may have formed on Laurentian/Dalradian crust. Most Ordovician zircons from the overlying units are ~465-480 Ma, and Th/U ratios are in a relatively narrow range from 0.7 – 1.5. This is a signature of the Connemara arc, the regional segment of the Notre Dame arc that intruded the Connemara Dalradian (Neoproterozoic) microterrane; zircons from a diorite from the Notre Dame arc in Connemara are the same age and have the same Th/U ratios. The Maumtrasna Formation includes a predominant Proterozoic detrital record typical of the Laurentian margin. The Derryveeny Fm contains both Rosroe-age zircons and ca. 450 Ma grains, indicating a younger age for this unit. Within the older Ordovician grain population (c. 470 Ma), however, is a small suite of grains with very low Th/U likely of metamorphic origin. Together, these data suggest that mid-Ordovician sediments on the southern side of the SMT were derived from the Notre Dame arc and from Neoproterozoic Dalradian rocks of Connemara that had experienced Grampian Barrovian metamorphism caused by earlier accretion onto the Laurentian margin (475-465 Ma). The Connemara crustal block and its segment of the Notre Dame arc were progressively unroofed to the south of the SMT in mid-Ordovician times.